Module 5: Heredity
Reproduction
How does reproduction ensure the continuity of a species?
Explain the mechanisms of
reproduction that ensure the
continuity of a species, by
analysing sexual and asexual
methods of reproduction in a
variety of organisms, including
but not limited to:
➢ Animals: advantages of
external and internal
fertilisation
➢ Plants: asexual and
sexual reproduction
➢ Fungi: budding, spores
➢ Bacteria: binary fission
➢ Protists: binary fission,
budding
Sexual vs Asexual Reproduction
Sexual Reproduction
Asexual Reproduction
When the sperm from the male parent fertilises
an egg from the female parent, producing
offspring that is genetically different from both
parents.
The production of identical offspring from just
one parent. Produces new individuals or
offspring by mitosis.
Evolution
There is very little chance of variation with
asexual reproduction. Mutations in DNA can
still occur but not nearly as frequently as in
sexual reproduction.
Sexual reproduction leads to genetic variation
in new generations of offspring. This is
fundamental to evolution.
Involvement of sex cells
No formation or fusion of gametes
Formation and fusion of gametes
Found in
Lower organisms
Higher invertebrates and all vertebrates.
Unit of reproduction
Maybe the whole parent body or a bud or a
fragment or a single somatic cell.
Gamete
Time taken
Asexual reproduction is completed in a very
short period of time.
Sexual reproduction can take several months
to complete.
Advantages
★ Fertilisation is less risky and the young
are more likely to survive.
★ Unfavourable genetics are eliminated
from the population more efficiently.
★ Individuals do not need to find a mate.
★ Rapid population growth.
★ It uses mitosis, which is less
demanding on an organism.
Disadvantages
★ Slower reproductive rate.
★ Recombination during meiosis can
introduce unfavourable variation to
populations.
★ Energetically costly; parents have to
input a lot of energy.
★ Rapid population growth can lead to
overcrowding.
★ Lack of genetic variation; susceptibility
to changes in new environmental
conditions.
★ Increased competition for resources.
Asexual Reproduction
Asexual reproduction refers to the production of identical offspring from just one parent. This method involves the production
of new individuals or offsprings through mitosis.
Organism
Description
Bacteria
Bacteria are unicellular, microscopic prokaryotes that reproduce asexually.
– Due to the lack of organelles and a smaller amount of DNA, cell replication in prokaryotes occurs
more quickly than in eukaryotes.
– Due to the lack of a nucleus, bacteria undergo an asexual process called binary fission (similar to
mitosis) where a single parent cell splits into two equal daughter cells.
Protists
Protists are eukaryotes that live in aquatic or moist environments. Their primary mode of reproduction is
asexual binary fission or budding.
– The binary fission of protists is different to that of bacteria because protists have a membrane-bound
nucleus that needs to be replicated.
– Budding occurs when a new identical organism grows from the body of the parent. Sometimes it
detaches to live on its own, other times they form a colony.
Fungi
Fungi are composed of eukaryotic cells that secrete enzymes over the surface of their food and absorb the
breakdown products directly. They utilise two forms of reproduction: budding and spores.
– Budding: In the budding process the parent yeast cell produces a small outgrowth that grows larger
and forms a bud. The nucleus of the parent cell splits off a smaller daughter nucleus which migrates
into the “bud”.
– Spores: Spores that are produced asexually are called mitospores and spores produced sexually are
called meiospores.
Plants
Plants are multicellular and include small and structurally simple forms, such as mosses and liverworts, and
large, more complex forms such as ferns and seed plants.
Rhizomes:
Underground stem that branches and facilitates new roots and shoots. Includes the ginger
plant.
Runner:
Similar to rhizomes, but they grow above the ground. Includes the strawberry plant.
Tuber:
Underground stems with buds that easily grow into new plants, seen in potatoes.
Bulb:
Observed in the garlic plant, they produce lateral buds that develop into new plants.
Sucker:
New shoots that arise from roots.
Sexual reproduction
Sexual reproduction requires both a male and a female and produces genetically different offspring. Life cycles of sexually
reproducing organisms follow a pattern called alternation of generation.
1. Haploid (n) stage: The production of gametes in animals and gametophytes in plants.
2. Diploid (2n) stage: The production of cells for everyday body structure and function, sporophytes in animals
Organism
Fungi
Description
–
–
Plants
–
–
–
–
Animals
–
–
The hyphae are the vegetative feeding state of the fungus and absorb the food digested by
secreted enzymes. The nuclei of the mycelium cells are haploid (n).
The mycelium of a fungus is a mass of branching, thread-like hyphae.
External agents of animals, winds, and water are commonly used as pollinating agents.
The fertilised ovule (seed) develops and is protected by the ovary, becoming the fruit.
The two structures most commonly observed for sexual reproduction in seed-producing plants
are the seed cones of the gymnosperm plants and flowers of the angiosperm plants.
The other plant types, grouped as mosses and fems, reproduce sexually with spores formed using
less prominent structures.
In animals sexual reproduction varies depending on the organism; internal or external fertilisation,
and where the daughter organism forms.
Hermaphrodite: A sexually reproducing organism that produces both male and female gametes.
Plants
Sexual reproduction in plants relies on the successful fusion of male and female gametes. This is more difficult because
plants grow in the ground and cannot move.
Angiosperms: Plants that produce their seeds through flowers and bear their seeds in fruits.
1. The male gametes inside pollen are carried from the anther to the stigma. Process is called pollination.
2. Once pollen has been deposited on the stigma, a pollen tube germinates and grows down the style, carrying the
male gamete to an ovule in the ovary.
Structure
Stamen/Carpel
Function
Anther
Stamen
Where pollen grains (male gametophytes) are stored
Filament
Stamen
Stalk that carries the anther. The length determines whether the anthers are contained inside
the petals for insect pollination or hang outside for wind pollination
Stigma
Carpel
Sticky top surface of the flower, where pollen attached. May be relatively small and smooth
(in insect-pollinated plants) or large and feathered (wind-pollinated plants)
Style
Carpel
Joins the stigma to the ovary
Ovary
Carpel
Where ovules are formed; eggs are held inside the ovule.
Petals
N/A
The petals are arranged in a circle or cylinder around the reproductive organs.
Gymnosperms: Vascular, non-flowering seed plants. The seeds of gymnosperms are produced by cones instead of flowers
and when mature they are exposed rather than surrounded by a fruit.
1. A conifer has a haploid stage with separate female and male cones, called the seed cone and pollen cone
respectively.
2. Small pollen grains (called microspores) develop in the male cones and when released they are transported by wind
to the female cone which contains the megaspores.
Ferns and mosses
Structure:
Dominant stage:
Reproductive
system:
FERNS
MOSSES
Ferns have vascular tissues that transport water
and soluble nutrients. Characterised by;
1. The absence of flowers and fruit.
2. The production of tiny spores instead of
seeds.
Mosses are one of the only plant groups without
vascular systems, due to this they lack leaves.
Sporophyte
Gametophyte
Ferns consist of alternating generations of
diploid plants and haploid plants.
★ Ferns are different from other land
plants in that both the gametophyte and
the sporophyte phases are free-living.
Alternation of generations and production of
haploid meiospores by meiosis from a diploid plant.
★ The meiospores grow into gametophytes
that form male and female gametophytes,
Which fertilise to form a sporophyte again.
CROSS-POLLINATION: One flower to another. Increases variation.
SELF-POLLINATION: Pollinates itself. Requires less energy and no need to have structures to attract pollinators.
Wind-pollinated flowers
Bird-pollinated flowers
Insect-pollinated flowers
Petals:
Small and inconspicuous, usually
green or dull in colour
Usually large and colourful, red or
orange, often a tubular shape,
sometimes no petals at all.
Usually large and colourful with
specific shapes to encourage
specific pollinators.
Scent:
Usually absent
Rarely fragrant because birds
have little sense of smell.
Often present because insects
are highly attracted to scents.
None
Large amounts of nectar are
Sometimes produced at the base
Nectar:
produced in nectar at the base of
flowers.
of petals so insects must enter
the flower to reach the nectar
Anthers:
Anthers protrude outside the
flower, so pollen is easily blown
off by the wind. Abundant pollen
is produced.
Anthers are commonly lower than
stigma, colourful and may not be
enclosed by petals.
Enclosed within flowers,
commonly lower than stigma.
Stigma:
Stigma produces from the flower,
often long, feathery and sticky, to
create surface area for trapping
wind-borne pollen.
Higher than the antlers,
sometimes not enclosed and
often colourful.
Enclosed within flowers, sticky
and commonly higher than
anther.
Very small grains, light and
powdery, large amounts
produced.
Sticky or powdery pollen, small
amounts produced.
Relatively large grains and often
small amounts are produced.
Pollen:
WHAT ARE THE TWO IMPORTANT FACTORS OF POLLINATION?
★ Seed dispersal: Seeds need to be dispersed over a wider distance to prevent overcrowding and competition for
resources. The widespread distribution also increases the chances of continuity of the species in other locations in
a sudden change in environmental conditions (diseases, fire).
★ Germination: The plant's embryo inside the seed is in a dehydrated form and dormant, allowing the seed to
survive adverse conditions. If in a suitable area (soil, water, oxygen and light/warmth) the seed will germinate
producing a radicle (small root) to absorb water and a plumule (stem) to develop leaves for photosynthesis.
Animals
EXTERNAL FERTILISATION
INTERNAL FERTILISATION
Meaning:
The process of fusion of male and female gamete
(sperm and egg) taking place in the external
environment (in water bodies).
The process of fusion of male and female gamete
(sperm and egg) taking place inside the body of the
female, is called internal fertilisation.
Gametes
released:
Numerous gametes (sperm and egg) are released
into the environment. These gametes are male as
well as female.
Less number of gametes (sperms)are released,
which get deposited inside the female body.
Processes:
Examples:
Analyse the features of
fertilisation, implantation and
hormonal control of pregnancy
and birth in mammals
1. Both male and female release or discharge
their gametes in their external surroundings.
2. Further process of development (syngamy)
occurs outside the body.
3. It occurs only in an external environment.
Amphibians, algae, fish, crustaceans
1. Only male gametes are released into the
female genital tract.
2. Further process of development (syngamy)
occurs inside the body.
3. There are 3 types of internal fertilisation
Mammals, reptiles, birds, bryophytes and
tracheophytes.
Fertilisation
Fertilisation refers to the fusion between male and female gametes during sexual reproduction, forming a zygote. There are
two main types of fertilisation; internal and external.
External fertilisation
·
Usually occurs in aquatic environments where both egg and sperm are released into the water. Cannot occur on dry
land, as the gametes would desiccate (dry out). Concentration gradient maintained.
·
After the sperm reaches the egg, fertilisation takes place.
ADVANTAGES
★ The offspring produced are at a much larger
magnitude and quicker.
★ A low amount of energy from both parties as there
is no need to find a mate.
★ Due to the aquatic environment and flow of water,
this process results in less competition between
offspring and their parents.
DISADVANTAGES
★ Many organisms consume these gametes for
nutrients, resulting in fewer chances of survival of
the offspring.
★ They are exposed to more disease.
★ More gametes need to be produced.
★ Many gametes are not fertilised. This process can
largely only occur in moist environments only.
Internal fertilisation
Internal fertilisation occurs most often in land-based animals, although some aquatic animals also use this method. There are
three ways that offspring are produced following internal fertilisation.
1. Oviparity: Fertilised eggs are laid outside the female’s body and develop there, receiving nourishment from the yolk
that is part of the egg (e.g. reptiles and birds).
2. Viviparous: In most mammals, the fertilised egg becomes an embryo that is nurtured inside the female parent’s
body, obtaining nutrients through a placenta and being born alive.
3. Ovo-Viviparous: In rare instances, a combination of the above occurs and eggs with yolk for nourishment are
retained inside the mother’s body until they are ready to hatch. Newly hatched young are born alive.
ADVANTAGES
DISADVANTAGES
★ There are more chances of the survival of the
offspring due to the larger amount of care that is
given to the offspring and embryo.
★ The rates of successful fertilisation are substantially
higher as well as the chances of the offspring
surviving.
★ Fewer gametes have to be produced. Gametes are
protected from predation and disease while inside.
★ The process requires large amounts of energy and
care, including finding the mate, contributing
nutrients to the offspring etc.
★ The amount of offspring produced is at a smaller
magnitude and there is a larger contribution from
the maternal parent.
★ Potential for the spread of sexually transmitted
diseases throughout the population.
Male reproductive system
Structure
Function
Penis
Protects the urethra
Urethra
The urethra tube passes through the penis, delivering urine or semen out of the body but not at the
same time.
Scrotum
Protective loose sac is divided into two compartments for the internal organs.
Paired testes
Produce and store mature sperm continuously during mating periods; the main structures are the
seminiferous tubules, where sperm cells are formed. Testosterone stimulates the testes to produce
sperm.
Epididymis
Stores the sperm cells.
Vas deferens
Transports sperm from the testes and epididymis into the urethra.
Seminal vesicle
Produces most of the seminal fluid (cum).
Prostrate
Produces some of the seminal fluid; also produces a thin, white fluid that mixes with seminal fluid to
neutralise the urethra and vagina to maintain sperm viability.
Female reproductive system
There are significant differences (no shit), between the male and female reproductive systems.
Structure
Function
Ovaries
Female reproductive organs that produce and hold oocytes (immature egg cells), produce hormones.
Uterus
If an egg is fertilised it implants in the uterine wall. Place of implantation, placenta formation, and foetal
development. NOTE: THE ENDOMETRIUM IS THE TISSUE THAT LINES THE UTERUS
Fallopian tubes
Connect the ovary to the uterus. The open end of each tube has fringe-like structures called fimbriae
(singular fimbria) that surround the ovary to catch the eggs. Fertilisation takes place high in the tubes.
Cervix
A narrow muscular canal lined with mucus, that connects the uterus and vagina. During monthly
menstruation, it is controlled by the hormone oestrogen to become softer and more open.
Vagina
A muscle-lined canal from the cervix to the genitals, receives the male penis during sexual intercourse.
Monthly menstrual blood flow from the uterus exits the body through the vagina.
Urethra
Opening for the excretion of urine.
Menstrual cycle
Stage
Time span (days)
Events
Menstruation
1-4
Uterine bleeding, accompanied by shedding of the endometrium.
Pre-ovulation
(Follicular phase)
5-12
Endometrial repair begins; development of ovarian follicle; uterine lining gradually
thickens.
Ovulation
13-15
Rupture of the mature follicle, releasing the egg.
Secretion
Pre-menstruation
16-20
Secretion of watery mucus by glands of the endometrium, cervix and uterine
tubes; movement and breakdown of the unfertilised egg; development of corpus
luteum.
21-28
Degeneration of corpus luteum; deterioration of endometrium
WHAT IS HAPPENING HORMONALLY?
1. Under the influence of follicle-stimulating hormone (FSH), one or more of the oocytes will resume its meiotic
division up to metaphase Il and mature within a group of nutritive cells called a follicle. Also triggers to make
oestrogen.
2. Follicles containing a maturing egg release the hormone oestrogen which causes thickening to the lining of the
uterus (endometrium) and also acts on the anterior pituitary gland.
3. Ovulation is the release of a mature egg and is triggered by a surge of luteinising hormone (LH) released from the
anterior pituitary gland in the brain. → dip in progesterone.
4. The burst follicle, now without its egg, is called the corpus luteum. The corpus luteum, stimulated by LH, secretes
large amounts of both oestrogen and progesterone. These create a further thickening of the uterine lining.
5. If it is not fertilised, the egg simply passes out of the reproductive tract. The corpus luteum slowly disintegrates
and stops releasing its hormones. As a result, the thickened uterine lining breaks down and menstruation (monthly
bleeding) occurs.
Implantation
In the case of fertilisation, two gametes fuse to form a zygote, which developmentally processes as it passes down the
fallopian tube.
– The head of a sperm contains the nucleus with a haploid set of chromosomes and a cap called the acrosome that
contains enzymes used for penetrating the outer layers of the female egg.
Stage
Cleavage
Time span (days)
1
Events
The first stage of development of the new zygote is cleavage, which commences
following the fusion of sperm and egg. During this period the cell divides into
hundreds of smaller cells by mitosis.
Morula
3-4
The early embryo continues to divide until, three to four days later, it consists of 16
cells and then enters the uterus.
Blastocyst
4-5
Cells begin to differentiate. The blastocyst attaches to the wall of the uterus. The
outer layer of cells sends out finger-like projections into a part of the wall of the uterus
(endometrium) and this area develops into the placenta.
Gastrula
14
After the blastocyst is implanted, gastrulation occurs over approximately five days,
and the blastocyst becomes a gastrula that has three different layers of cells.
YOLK SAC: The yolk sac which surrounds the egg yolk has a well-developed vascular system that transports nutrients
from the egg yolk to the developing embryo.
UMBILICAL CORD: Develops from the remnants of the yolk sac. It replaces the yolk sac as the source of nutrients for the
embryo, acting as a conduit for embryonic blood vessels to reach the placenta. The umbilical cord remains attached to
the foetus until after birth.
★ There is no direct exchange of blood, rather nutrients and oxygen from the mother diffuses across into the blood
of the umbilical vein and move to the foetus.
★ The reverse happens for the removal of waste products and circulation of depleted blood through the umbilical
arteries from the foetus back to the placenta.
Hormone control
Hormones are signalling chemical substances that act as messengers in the body to coordinate functions including
metabolism and reproduction.
Hormone
Gland
Target
Function
Follicle stimulating
hormone (FSH)
Anterior
pituitary
Ovaries
Promotes development of follicle and secretion of
oestrogen. Controls when eggs in the ovaries
ripen and cause the ovaries to release the
hormone oestrogen.
Luteinising
hormone (LH)
Anterior
pituitary
Ovaries (female), Testes
(male)
Promotes ovulation, development of corpus
luteum, and secretion of progesterone, along with
when eggs are released into the fallopian tubes
(female); stimulates the secretion of the steroid
hormone testosterone in the testes (male)
Oestrogen
Ovary
Reproductive tract, whole
body
Promotes menstrual cycle, development of female
features and behaviour. Controls when the lining
of the womb thickens and when it breaks down.
Sends signals to the pituitary gland when an egg
is ripe and when an egg is fertilised-this stops the
pituitary hormones from releasing more eggs.
Progesterone
Ovary
Uterus
Prepares the uterus for and maintains pregnancy.
Make sure more FSH and LH aren't produced so
no more eggs are matured.
Oxytocin
Posterior
pituitary
Mammary glands
Cause release of milk; stretch cervix for birth.
NOTE: POSITIVE FEEDBACK LOOP
Testosterone
Testes
Reproductive tract
Development of masculine features and
behaviour.
Human chorionic
gonadotropin
(hCG)
Placenta
Ovaries
Maintains corpus luteum for production of
progesterone; stops ovulation → happens post
implantation.
POSITIVE FEEDBACK LOOP: In the period just before a human birth the balance of two hormones, oestrogen and
progesterone, changes.
1. The natural level of prostaglandins increases which in turn increases the sensitivity of the cervix and uterus to
oxytocin.
2. Oxytocin is the hormone that causes uterine contractions. The hormonal changes create irregular uterine
tightening or contraction.
3. The foetus has usually moved with its head low in the pelvis, putting pressure on the cervix, this pressure
stimulates further release of oxytocin and so labour begins.
4. When the cervix reaches full dilation, oxytocin and adrenaline hormones work together to start the final series of
muscular contractions. After the baby is delivered the uterine contractions are maintained by oxytocin until the
placenta is pushed out and the uterus starts shrinking back to normal size.
Evaluate the impact of
scientific knowledge on the
manipulation of plant and
animal reproduction in
agriculture
Selective breeding
The process by which humans decide which individuals may breed and leave offspring to the next generation is called
selective breeding or artificial selection.
RULES OF SELECTIVE BREEDING:
1. Determine the desired trait.
2. Interbreed parents who show the desired trait.
3. Select the offspring with the best form of the trait and breed these offspring.
4. Continue this process until the population reliably reproduces the desired trait.
A common problem that arises with selective breeding is gene linkage, meaning that it is not only the desired trait selected.
– Genes that are located close to one another on a chromosome may be inadvertently selected because they travel
linked together during cell division and into the offspring.
Plants: Selective breeding in plants is done to produce higher-quality food.
1. This process is carried out by collecting seeds from the individuals with the largest, most attractive or numerous
grains, fruits, or other parts of the plant that will be eaten.
2. These are planted and cross-pollinated in a controlled environment with individuals yielding similar traits.
3. Once a desirable plant has been bred, artificial pollination or cloning methods can be used to mass-produce
identical plants.
There are several significant techniques and terms to be aware of when studying selective breeding in plants.
TERM
EXPLANATION
Polyploidy
This is the condition where the cell nucleus has more than two sets of chromosomes.
– Polyploidy can emerge from natural errors in meiosis where gametes may end up being
diploid rather than haploid.
– The chemical colchicine has been discovered to induce polyploidy, when exposed to
this chemical during cell division the paired chromosomes are prevented from pulling
apart.
Hybridisation
The crossing of different varieties within one species to produce new varieties with different
combinations of characteristics is one kind of hybridisation.
– Hybrid plants are higher yielding and sometimes more disease-resistant.
– When hybrid offspring are produced artificially, they are designed to be cultivated or
reared under controlled conditions of intensive agriculture, horticulture, or farming and
may not be suited to wild conditions
Heirloom plants
A heirloom plant is a traditional cultivated plant that is maintained by small-scale gardeners and
farmers.
– In modern agriculture most food crops are now grown using limited varieties in large,
keeping the product to a consistent standard.
– It provides food security for the future in the face of climate change, new pests, and
diseases, and other issues that may make current monoculture unavailable.
Cloning
The production of new individuals that contain the same genetic information as the parent
organism.
– Natural clones are produced by asexual reproduction when a single parent cell divides to
produce two new identical daughter cells.
– The term cloning is used to refer to the artificial methods of producing genetically
identical organisms.
Example
Evidence
Maize
Modern maize has significantly larger cobs with many more rows of much larger kernels compared to
the ancestral teosinte.
Wheat
Modern wheat has become polypoid with strains that are tetraploid (4n, two sets of chromosomes)
and hexaploid (6n, three sets).
Orange sweet
potatoes
Orange-fleshed sweet potatoes were bred and introduced by a group of scientists whose project
recognised the importance of provitamin A.
Animals: When a selected species has a variety of traits, different traits may be useful in different situations. A single wild
species can be the original source of a great variety of different breeds.
Example
Evidence
Domestic dogs
Today’s domestic dogs were selectively bred from a wolf species, some of which would be unlikely to
survive in the wild.
– This has led to inbreeding with questionable outcomes for the welfare of the animal, such as
the huge head and narrow hips of the bulldog which means pups must be born by
Caesarean section.
Poultry
Geneticists select birds with the best characteristics for egg or meat production.
Prawns
The Australian black tiger prawn was developed through collaboration between CSIRO and industry
partners.
– The new breed has improved growth and survival rates, boosting bond yields to more than
50%.
Selective breeding and genetic modifications have brought considerable benefits to humankind, however, these
improvements have not come without costs and controversy.
ISSUE/CONCERN
IMPACT
Health of the animal
or plant
Transgenic animals may experience adverse effects from transgenes that affect growth rates.
Uncontrollable pest
plant species
Crops that have been modified for herbicide or insecticide resistance may breed with other plants,
producing hybrid pest species that farmers may not be able to control.
GM animals can be
competitors
If there are wild relatives of genetically modified animals, there is a selective advantage to the
presence of a transgene in a wild population.
– Due to the presence of the transgene they can pose a threat to the native species.
Loss of biodiversity
The increase in selective breeding and genetic modification results in fewer crops which vary from
these, reducing the genetic variation of a population.
– If disease or environmental change occurs, there could be widespread and catastrophic
effects on food production.
– This is already an issue in modern agriculture where monocultures and commercial
practices have reduced genetic variation due to selective breeding over many centuries.
Evolution through
artificial selection
Selective breeding, hybridisation, polyploidy, and genetic modification can all lead to speciation,
which may be induced artificially by human pressures.
Cell Replication
How important is it for genetic material to be replicated exactly?
Model the processes involved
in cell replication, including but
not limited to:
➢ Mitosis and meiosis
➢ DNA replication using
the Watson and Crick
DNA model, including
nucleotide
composition, pairing
and bonding
Replication: Multicellular organisms replicate for various reasons, but primarily for the purpose of growth and repair.
Mitosis vs Meiosis
Mitosis
Meiosis
Number of cells
Mitosis produces two genetically identical
daughter cells.
Meiosis produces four genetically unique
daughter cells.
Genetic
Mitosis does not change genetic information
Meiosis rearranges genetic information between
recombination
(chromosomes do not cross-over).
chromosome pairs, creating unique genetic
variation.
Number of
chromosomes
The daughter cells produced from mitosis have
the same number of chromosomes (diploid, 2n) as
the
parent
The daughter cells produced from meiosis have
half the number of chromosomes (haploid, n) of
the parent.
Location
Occurs in all parts of the body to
replicate somatic cells.
Only occurs in the gonads to produce gametes
from germ cells
CELL CYCLE: The cell cycle is the repetitive sequence where cell division and enlargement occur. Mitosis is only one part
of this cycle taking an hour or two. The cell spends most of its time in interphase, where the cell doubles its mass and
duplicates its entire components. The cell cycle can be summarised into the following steps;
1. G1: Gap phase for cell growth before DNA replication. Metabolic changes prepare the cell for division and the cell
reaches a point at which it is committed to division.
2. S: Synthesis phase during which DNA is replicated.
3. G2: Second gap phase after replication, when enzymes in the cell check the duplicated chromosomes for any
errors and correct these, and cytoplasmic materials accumulate in preparation for division.
4. Mitosis (a division of the nucleus) followed by cytokinesis (a division of the cytoplasm), marking the division of
the cell into two.
Mitosis
Mitosis occurs in somatic cells for the purpose of growth and repair, referring to the division of the nucleus into two
genetically identical daughter cells.
Stage
Prophase
Description
1. Chromosomes condense (shorten and thicken) and
become visible; each chromosome can be seen as two
chromatids held together at the centromere.
2. Centrioles move to opposite sides of the nucleus and
form poles.
3. Nuclear membrane breaks down.
4. Centrioles form spindle fibres between the two poles.
Diagram
1. Chromosomes align at the equatorial plane of the cell (in
the centre).
2. Spindle fibres attach to centromeres of chromosomes.
Metaphase
1. Spindle fibres contract, splitting the centromeres and
separating the sister chromatids.
2. The separated chromosomes are pulled to opposite
poles, assuring that daughter cells receive the same
genetic information.
Anaphase
1. Nuclear membrane reforms around the two sets of
chromosomes.
2. Spindle fibres disappear.
3. Chromosomes become longer and thinner.
Telophase
Meiosis
Meiosis is the process of nuclear division required for the formation of new sex cells or gametes.
·
Meiosis is called a reduction division because, unlike mitosis, it reduces the number of chromosomes in gametes
(daughter cells) to half (n) of that in somatic cells (2n).
Meiosis I
Stage
Description
Prophase I
1. Genetic material condenses into chromosomes.
2. Nuclear envelope and nucleolus disappear and spindle
fibres spread across the cell.
3. Homologous chromosomes (maternal and paternal) are
paired together, called synapsis.
4. Crossing-over may occur; homologous chromosomes
overlap and sections of genetic material may be
swapped.
Metaphase I
1. Tetrads (pairs of homologous chromosomes) move to the
equatorial plane.
2. Spindle fibres attach to the centromeres of each
chromosome.
3. This differs from mitosis as the homologous pairs line up
next to each other across the equator, not on top of each
other.
4. They line up randomly, meaning the maternal and paternal
chromosomes do not line up on the same side of the
midline.
Anaphase I
1. Cells begin to elongate and sister chromatids from
homologous chromosomes separate together, moving to
opposite poles of the cell.
2. Metaphase and anaphase result in the random
assortment of maternal and paternal chromosomes and
their alleles in the gametes.
3. The centromeres do not split, it’s the homologous
chromosomes that separate, not the chromatids.
Telophase I
1. Chromosomes reach opposite poles.
2. The chromosomes decondense and a nuclear envelope
begins to form around them.
Meiosis II
Diagram
Similar to mitosis; no crossing over, and no need for DNA replication because if this occurs the final daughter cells will
become diploid.
Prophase II
1. The nuclear envelope and nucleoli disappear.
2. Spindle fibres form and spread across the two daughter cells.
Metaphase II
1. The chromosomes line up on the equatorial plane.
2. The chromosomal alignment differs from metaphase I as the chromosomes line up on top of each
other, similar to mitosis. This results in the separation of sister chromatids of each chromosome
pair.
Anaphase II
1. Spindle fibres contract, splitting the centromeres and separating the sister chromatids.
2. The sister chromatids of each chromosome pair separate and move to opposite poles of the cell.
Telophase II
1. The sister chromatids (now called individual chromosomes) are now at opposite poles of the cells.
2. They decondensed and a nuclear envelope reforms around them.
DNA Replication
DNA (deoxyribonucleic acid) is one of the two types of nucleic acids, the molecule which contains the genetic code for
proteins that enable cells to undergo growth, repair, and other specialised functions.
·
It is a long double-stranded helix found within the nucleus of eukaryotic cells (sometimes found in the mitochondria
or chloroplasts.)
·
In prokaryotic cells DNA is stored in single-looped chromosomes within a region called the nucleoid. There are small
loops of DNA called plasmids in the cytoplasm.
Genes: Sequences of DNA that contain specific information for making polypeptides.
DNA STRUCTURE: DNA is a biopolymer made up of repeating subunits called nucleotides. Each nucleotide consists of
three basic components.
1. A phosphate group.
2. A nitrogenous base - adenine (A), thymine (T), guanine (G) and cytosine (C)
a. Due to their chemical structure, adenine always pairs with thymine and cytosine always pairs with
guanine.
b. The nitrogenous bases are joined together by hydrogen bonding, forming the ‘rungs’ of the double helix
ladder.
3. A five-carbon sugar (in the case of DNA, the sugar is deoxyribose). The five carbon atoms are number 1’-5’. In an
individual nucleotide, a phosphate is attached to the 5’ carbon, and a base is attached to the 1’ carbon.
a. Each end of a DNA or RNA strand is often labelled as 5’ or 3’, meaning five prime or three prime. This
refers to the numbered carbon atoms in the sugar part of the backbone and gives the strand a direction.
RNA STRUCTURE: RNA (ribonucleic acid) is the other nucleic acid molecule and plays an important role in polypeptide
synthesis. RNA differs from DNA in three main ways;
1. RNA contains the nitrogenous base uracil (U), instead of thymine (T)
2. RNA is single-stranded rather than having a double-stranded structure.
3. RNA also contains ribose sugar instead of deoxyribose sugar in its backbone.
DNA replication: DNA replication refers to the process where a molecule of DNA is synthesised and replicated into two
other strands.
★ During replication each strand of the parental DNA molecule acts as a template strand on which the new strand is
synthesised.
★ The process begins with the unwinding of a portion of the double helix, due to the action of the enzyme helicase,
which breaks hydrogen bonds between nitrogenous bases.
★ On one strand (the leading strand), an enzyme called DNA polymerase binds to the DNA and continuously begins
moving along its ‘reading’ the bases to assemble a complementary strand of nucleotides.
○ DNA polymerase makes very few errors; and any of these errors are quickly corrected by DNA polymerase
and other enzymes that ‘proofread’ the new nucleotides.
★ On the other strand (the lagging strand), discontinuous segments of nucleotides called Okazaki fragments are joined
to the waiting bases on the DNA template. Another enzyme, DNA ligase, acts like a glue and stitches these
fragments together.
○ DNA polymerase can only add DNA bases from 5’ to 3’ in, one direction only, while the leading strand is
replicated continuously, the lagging strand runs in the opposite direction hence it must be replicated in
chunks or fragments, each fragment starts with a primer. Enzymes involved include RNA polymerase, DNA
polymerase, Exonuclease and DNA ligase.
Enzyme
Description
Helicase
Unwinds the double helix, breaks hydrogen bonds between nitrogenous bases.
DNA polymerase
Binds to the DNA and continuously moves along, forming a complementary strand of nucleotides.
DNA ligase
Sticks the Okazaki fragments onto the lagging strand of DNA.
Exonuclease
Can act as proofreaders during DNA polymerisation in DNA replication, to remove unusual DNA
structures that arise from problems with DNA replication fork progression.
DNA REPLICATION: The process can be summarised into the following steps;
1. The DNA double helix unwinds - helicase and topoisomerase cause the DNA to progressively unwind and the
strands to separate, creating a replication fork.
2. Single-stranded binding proteins bind to and stabilise the newly separated strand of DNA.
3. There is a bank of free nucleotide molecules stored in the nucleus. For synthesis to be initiated a short strand of
RNA needs to be made and attached to the DNA primer, this is initiated by the enzyme primase.
4. Nucleotides are added against every single strand with DNA polymerase. As each new complementary strand
forms, the replication strand moves along and the completed pairs begin to rewind into double helices.
Assess the effect of the cell
replication processes on the
continuity of species
Genetic continuity: The ability of individuals to pass on identical genetic information to the daughter cells. It is important for
the survival and thus the continuity of a species via reproduction.
EFFECT OF MITOSIS: Cell replication maintains a species continuity by copying the same genetic make-up of cells for
any individual from fertilisation to death.
– For sexual reproduction, they are going to be compatible with other members of the same species because their
chromosomes will form homologous pairs. This allows the species to move from one generation to the next.
– For mitosis and asexual reproduction it is very important that the DNA is copied correctly. If there's a mistake it
may result in cell death or cancer
Despite the need for genetic continuity in cells and species, there is enough variation to give rise to new species.
EFFECT OF MEIOSIS: The introduction of variation is introduced within the process of meiosis due to these processes
being essential to the continuity of a species.
1. Crossing Over: Crossing over refers to the exchange of genetic material between homologous chromosomes
during synapsis, resulting in new combinations of characteristics in the offspring.
2. Random Segregation: Chromosomes are distributed randomly among different gametes, depending on the way
in which the chromosomes line up during meiosis. This results in many possible haploid gametes, each containing
a unique set of chromosomes.
3. Independent assortment: The law of independent assortment states that pairs of genes separate independently
of others during meiosis into different gametes. That is, the inheritance of one character does not depend on
another because the genes controlling these traits are found on separate chromosomes, eg; hair colour is not
reliant on height.
4. Random fertilisation: Each sperm and egg has undergone segregation and independent assortment and crossing
over during meiosis. Which means each sperm or egg is unique. Fertilisation is a random process - involving any
of the 8.4 million chromosome combinations of a sperm cell pairing up with any of the 8.4 million possible
chromosome combinations of an egg, resulting in 64 trillion possible combinations of chromosomes.
DNA and Polypeptide Synthesis
Why is polypeptide synthesis important?
Construct appropriate
representations to model and
compare the forms in which
DNA exists in eukaryotes and
prokaryotes
There are several aspects which can summarise the unique differences between DNA in both prokaryotes and eukaryotes.
​Model the process of
polypeptide synthesis,
including:
➢ Transcription and
translation
➢ Assessing the
importance of mRNA
and tRNA in
transcription and
translation
➢ Analysing the function
and importance of
polypeptide synthesis
➢ Assessing how genes
and environment affect
Polypeptide Synthesis
PROKARYOTE
EUKARYOTE
★ One chromosome per cell
★ Circular chromosome
★ Chromosomal DNA is in a region of the cytoplasm
called the nucleoid, lacking a membrane.
★ Contain plasmids, small circular DNA
★ There are fewer introns (non-coding DNA) (greater
number of genes per number of bases).
★ Multiple chromosomes
★ Linear thread-like chromosomes
★ DNA stored in the nucleus, separated from the
cytoplasm by a double-layered membrane.
★ No plasmids but there are other sources of DNA mitochondrial DNA and chloroplast DNA
★ There is more non-coding DNA (introns) than in
prokaryotes (fewer genes per number of bases).
Genes are sections of DNA that encode for a particular polypeptide. One or more polypeptides form proteins. This process
of producing a polypeptide (and eventually a protein) from a gene is called protein synthesis.
Gene expression: The process by which information encoded in a gene is used to direct the formation of a polypeptide. It is
about what controls which genes are expressed and when.
– This is required because DNA is identical in somatic cells, and certain genes need to be correctly switched on in the
correct cell at the correct time to allow for differentiation.
There are three primary forms of RNA: messenger RNA (mRNA), transfer RNA (tRNA) and ribosomal RNA (rRNA). Each has
its own role in polypeptide synthesis.
Form
Function
phenotypic expression
Messenger RNA
(mRNA)
Messenger RNA (mRNA) is formed in the nucleus by the process of transcription. The mRNA carries a
complementary copy of the nucleotide sequence of DNA that specifies the amino acid sequence for a
particular polypeptide.
– During transcription, the primary transcript (pre-mRNA) goes through a process known as
RNA splicing to remove non-coding DNA.
– Some non-coding DNA exists in smaller chunks within genes. A chunk of this type of
non-coding DNA is referred to as an intron. These introns help regulate gene expression.
– The coding DNA sequences (that are used to produce the amino acid sequence) surrounding
introns within a gene are referred to as exons.
Ribosomal RNA
(rRNA)
Ribosomal RNA (rRNA) is synthesised in the nucleolus of the cell nucleus and is based on the
nucleotide sequence of the DNA. Together with proteins rRNA forms an organelle called the
ribosome. Ribosomes are the sites where mRNA is translated into a chain of amino acids.
Transfer RNA
(tRNA)
Transfer RNA (tRNA) molecules transfer amino acids to the cytoplasm to the ribosomes, where they
are joined to form a polypeptide chain based on the sequence of nucleotides in the mRNA.
– There are three places for tRNA to bind to the ribosome: the exit site (E), the peptidyl site (P)
and the aminoacyl (A).
– At the other end of the tRNA molecule, there is a sequence of nucleotides known as the
anticodon. The anticodon recognises a particular sequence of nucleotides in the mRNA,
enabling an amino acid to be positioned in the correct place on a polypeptide chain.
– The genetic code is said to be degenerate because more than one RNA codon can code for
the same amino acid.
Polypeptide synthesis is split into two main steps:
1. Transcription: Copying of a gene into a temporary portable copy called an mRNA transcript. This takes place within
the nucleus.
2. Translation: Deciphering of the information in the mRNA transcript into a chain of amino acids (a polypeptide) with
the help of a ribosome and tRNA. This takes place in the cytoplasm.
Guide to Polypeptide Synthesis:
Transcription: Transcription is the process by which the DNA base sequence is read by an enzyme called RNA
polymerase and copied onto a single strand of mRNA.
1. A specific section of DNA containing the gene of interest unwinds, exposing the DNA base sequence template to
the enzyme, RNA polymerase.
2. Free-floating mRNA nucleotides undergo complementary base pairing with exposed DNA bases on the template
strand.
3. The RNA transcript goes through a process known as RNA splicing to remove the non-coding sequences (introns)
so that only the coding sequences (exons) are carried to the ribosome for translation. These forms mature mRNA.
4. A cap is added to the 5 prime ends and a poly-A tail is added to the 3 prime ends. These help the mRNA leave the
nucleus and protect it from being broken up by exonuclease enzymes.
5. When transcription has been completed, the complementary mRNA-DNA complex is unwound and a singular
mRNA strand leaves the nucleus, to the cytoplasm through to the rough endoplasmic reticulum.
6. The mRNA transcript attaches to a ribosome in preparation for translation. Transcription differs from DNA
replication as only one strand of DNA is copied.
Translation: The process where the mRNA transcript is read by the cell in order to produce a polypeptide. This process
can be split into three stages: initiation, elongation, and termination.
1. Initiation
– The mRNA transcript attaches to a ribosome in the cytoplasm of the cell. The ribosome acts as the ‘protein
manufacturing site’ that will bring together all the necessary components to produce a polypeptide.
– The ribosome then reads the mRNA strand three bases at a time. These groups of three bases are called codons.
Each codon (and anticodon) matches a specific amino acid.
2.
–
–
–
Elongation
As the ribosome continues to read the codons, it recruits tRNA molecules.
Codons on the mRNA strand are matched with anticodons carried by tRNA molecules.
The tRNA molecules bring the respective amino acid to the site. Each amino acid that is dropped off at the
ribosome tRNA is covalently bonded to the previous amino acid in line. This process continues to create a long
chain of amino acids.
–
Many amino acids correspond to more than one codon. This is because if a mutation occurs, the base can be
more effectively swapped and still code for the same acid.
3. Termination
– Growth of the amino acid chain will cease once a stop codon is reached. Leaving us with a polypeptide.
– After this final step, the polypeptide chain is folded and joined with other polypeptides/molecules to form a
protein.
Investigate the structure and
function of proteins in living
things
Proteins
Proteins are large biomolecules that vary in size, from less than 10 amino acids to thousands of amino acids, and may be
synthesised as one or several polypeptide chains.
·
Polypeptide chains are folded and organised into specific shapes that are vital to the correct function of the protein.
AMINO ACIDS: Amino acids are composed of four elements; carbon (C), hydrogen (H), oxygen (O), and nitrogen (N). All
amino acids consist of three parts linked together through peptide bonds;
1. An amine group - NH2
2. A carboxylic acid group - COOH
3. A -R functional group; provides a wide range of functionality for the protein.
There are 20 common amino acids that are incorporated into polypeptide chains to form a protein. 11 of these are
‘non-essential’ meaning the body can make them. 9 of them are ‘essential’ meaning we obtain them through our diet.
There are four different levels of the organisation when describing protein structure.
1. Primary: Linear sequence of amino acids.
2. Secondary: The folding/coiling of the polypeptide chain.
3. Tertiary: Combination of several secondary structures.
4. Quaternary: The combination of several polypeptide chains.
Proteins require optimal conditions. Enzymes are proteins and all proteins are biological molecules that require certain
conditions for optimal functioning. This includes
1. Temperature: When a protein is heated beyond its optimal temperature, bonds holding it together are disrupted,
causing it to denature (lose its structure)
2. Changing pH: The pH (power of hydrogen) refers to the concentration of hydrogen ions in the solution. The higher
concentration of hydrogen ions, the more acidic it is. Each protein works best at a particular pH level, depending
on where it is found.
3. Cofactors: Even once the transcription and translation processes are complete and the polypeptide has been
folded into its tertiary shape, some proteins still need a cofactor molecule before they can perform effectively. E.g.
Haemoglobin requires iron before it can transport oxygen. DNA polymerase requires magnesium and zinc to
function.
Genetic Variation
How can the genetic similarities and differences within and between species be compared?
Conduct practical
investigations to predict
variations in the genotype of
offspring by modelling meiosis,
including the crossing over of
homologous chromosomes,
fertilisation and mutations
Variation
Genotype: The set of alleles present in the DNA of an individual organism, the result of inheritance
ALLELE: Alternative form of a gene. Each individual usually only has two alleles for each trait: one inherited from their
mother and one inherited from their father. But one gene can have multiple alleles, leading to variation in the population.
★ When they are both the same allele it is referred to as homozygous, but when they are two different alleles they
are referred to as heterozygous.
★ Dominant phenotypes are expressed if the individual carries at least one allele for the dominant trait, recessive
phenotypes are expressed only if the individual carries two alleles for the recessive trait.
Phenotype: All of an organism's observable characteristics. It is the result of inheritance and the effect of the organism’s
environment.
– Environmental conditions can affect the phenotype of certain genotypes. The genotype determines the possible
range of phenotypes for a particular characteristic and the environment influences where in that range the actual
phenotype will be.
Example
Evidence
Phenylketonuria
The inherited disorder phenylketonuria (PKU) is a consequence of the buildup of an amino acid
called phenylalanine in the blood.
– PKU is caused by a mutation in the PAH gene, which codes for an enzyme that converts
phenylalanine into another amino acid, tyrosine.
– The development of the symptoms can be prevented by modifying the diet for babies that
test positive for PKU.
Fur colour in the
Himalayan rabbit
The Himalayan rabbit is homozygous for a mutant allele that encodes a heat-sensitive tyrosinase.
– This is inactive at normal body temperature, but at low temperatures melanin is activated,
causing black fur to form.
Flower colour in
hydrangeas
If cuttings of a single hydrangea plant are grown in very acidic soil the flowers produced are blue,
however, if they are grown in alkaline soil, the flowers are pink. The effect is caused by the
relationship between soil pH, a pigment called anthocyanin, and the availability of aluminium in the
soil for uptake by the plant.
– At a soil pH of 5.5 or less, aluminium is free to be taken into the plant. Anthocyanin is
normally red, but it binds to aluminium in the plant to form a blue pigment called
metalloanthocyanin, resulting in blue flowers.
– At a soil pH of 6 or more the aluminium binds to soil particles and is less available to the
plants. This leaves most of the anthocyanin in the plant in its red form, resulting in pink
flowers.
INDEPENDENT ASSORTMENT
Mendel’s law of independent assortment states that the alleles of a gene controlling one trait assort independently of
alleles of another gene controlling a different trait.
★ The exceptions occur when two or more genes are located on a single chromosome and are inherited together.
This is known as linkage and is another key principle of inheritance.
★ The closer the genes are, the more likely they are to be inherited together, but the linkage is never complete
because of crossing over.
Polygenic inheritance: For some traits, more than one gene contributes to the phenotype of an individual. This is known as
polygenic inheritance and results in a much greater range of phenotypes.
– Human traits which are referred to as polygenic inheritance include; height, skin colour, and eye colour.
Model the formation of new
combinations of genotypes
produced during meiosis,
including but not limited to:
➢ Interpreting examples
of autosomal,
sex-linkage,
codominance,
incomplete dominance
and multiple alleles
➢ Constructing and
interpreting information
and data from
pedigrees and Punnett
Combinations of genotypes
Mendel demonstrated that traits are passed from parents to offspring and that these traits form specific patterns over
generations of crossbreeding.
MONOHYBRID CROSS:
A cross of traits being studied can be carried out to determine which trait is dominant. A monohybrid cross is a cross
between two individuals with different alleles at a single locus.
– The ratio of genotypes in the offspring is written in the following order: homozygous dominant: heterozygous:
homozygous recessive.
– The ratio of phenotypes observed in the offspring is written as dominant phenotype: recessive phenotype.
squares
Collect, record and present
data to represent frequencies
of characteristics in a
population, in order to identify
trends, patterns, relationships
and limitations in data, for
example:
➢ Examining frequency
data
➢ Analysing single
nucleotide
polymorphism
TERM
DEFINITION
Autosomal dominant
Referred to as complete dominance, this inheritance is the passing down of a dominant gene to
offspring via an autosomal gene. An autosome is any gene that is not a sex chromosome. Only
one copy of an autosomal gene is needed to express a dominant phenotype.
Autosomal incomplete
dominance
This refers to when neither phenotype is completely dominant, so the intermediate phenotype
between the two appears in the offspring.
Autosomal codominant
This refers to when both alleles are expressed to varying degrees in the phenotype of
heterozygous individuals. Human blood is an example of autosomal codominant inheritance;
– There are three alleles for blood type at the same locus, and individuals can have A, B,
AB, or O phenotypes.
– In this mix, A and B phenotypes are co-dominant, while the O phenotype is recessive.
Sex-linked inheritance
Sex-linked inheritance refers to the phenotypes inherited through genes on sex chromosomes.
– In humans, X-linked recessive traits are predominantly expressed in males, because
males carry only one X chromosome.
– X-linked disorder may also display a dominant phenotype. When a father is affected and
a mother is normal, all female children will be affected.
Allele frequencies
When a gene has different alleles in a population, it is referred to as polymorphic. These different alleles are more common
in some populations than others.
– The rate at which alleles occur in a population is known as the allele frequency.
Genetic marker: Any piece of DNA that can be reliably analysed using sequencing or genotyping. Genetic markers can be
in coding regions (exons) or non-coding regions (introns). The ideal genetic marker has the following properties.
1. It is polymorphic within the population of interest, showing variation. If a genetic marker is the same for all
individuals, it will not provide any information.
2. It is usually a neutral marker, meaning it is not currently under selection, this allows for probability-based models
to be applied.
3. It is easy and affordable to work with. Some genetic markers are very expensive to isolate and analyse from the
organism.
Single nucleotide polymorphisms (SNPs) are another way of examining genetic variation. These are single base changes,
but unlike the examination of a single DNA sequence, they are scattered through the genome across multiple chromosomes.
–
–
–
The utilisation of SNPs implies that many points of variation can be examined simultaneously.
SNPs refer to the replacement of one nucleotide by another, a form of typing error that usually appears during DNA
replication.
They are important genetic markers that distinguish individuals and their susceptibility in individuals.
Short tandem repeats (STRs) are another example of looking at genetic variation. STRs are usually non-coding pieces of
DNA that contain a string of repeating nucleotides.
– If the number of repeats for the STR is the same on each chromosome the individual is homozygous for that STR.
– If the number of repeats is different on each chromosome the individual is heterozygous for that STR.
Inheritance Patterns in a Population
Can population genetic patterns be predicted with any accuracy?
Investigate the use of
technologies to determine
inheritance patterns in a
population using, for example:
➢ DNA sequencing and
profiling
DNA sequencing
Refers to the process of determining the exact nucleotide sequence (the order of the bases) of a gene on a chromosome.
– Used in scientific research to identify new alleles, diseases genes, and evolution patterns & relationships.
– Involves many computer-aided technologies (e.g. Sanger method), in combination with Polymerase Chain Reaction
(PCR) to amplify short tandem repeats.
SANGER SEQUENCING METHOD:
1. The DNA sample to be sequenced is combined in a tube with primer, DNA polymerase, and DNA nucleotides.
2. Four dye-labelled, chain termination dideoxy nucleotides are added as well, but in much smaller amounts than the
ordinary nucleotides.
3. The mixture is first heated to denature the template DNA (separate the strands), then cooled so that the primer can
bind to the single-stranded template.
4. Once the primer has bound, the temperature is raised again, allowing DNA polymerase to synthesise new DNA
starting from the primer.
5. DNA polymerase will continue adding nucleotides to the chain until it happens to add a dideoxy nucleotide instead
of a normal one. At that point, no further nucleotides can be added, so the strand will end with the dideoxy
nucleotide.
6. The ends of the fragments will be labelled with dyes that indicate their final nucleotide.
7. After the reaction is done, the fragments are run through a long, thin tube containing a gel matrix in a process
called capillary gel electrophoresis.
8. Short fragments move quickly through the pores of the gel, while long fragments move more slowly.
9. As each fragment crosses the finish line at the end of the tube, it’s illuminated by a laser, allowing the attached
dye to be detected.
10. The smallest fragment (ending just one nucleotide after the primer) crosses the finish line first, followed by the next
smallest fragment (ending two nucleotides after the primer), and so forth.
DNA profiling
DNA profiling relies on an individual’s unique DNA. The non-coding sections of the DNA, those that do not code for proteins,
can vary wildly between individuals.
– DNA profiling uses multiple genetic markers to identify individuals.
– STRs are examples of commonly used genetic markers, they are short sections of DNA with repeating nucleotides.
Variation in the number of repeating units within an STR is common between individuals, making them valuable for
identification purposes.
– DNA profiles have been used to identify key features of an individual's appearance. The identification of alleles for
eye, skin, and hair colour allows investigators to narrow down the list of suspects based on these characteristics.
DNA PROFILING:
1. Small amount of blood, semen, or another sample containing DNA is found at a crime scene.
2. A small amount of DNA is added to a PCR, which contains specific primers for each STR.
3. The STRs are amplified using PCR, producing a much larger sample for testing, even from a very small amount of
DNA.
4. Differences in the size of the STRs can be detected by standard gel electrophoresis or by capillary electrophoresis,
a rapid, automated method. ln capillary electrophoresis, the DNA fragments move in a thin tube under the
influence of an electric field.
a. The smaller the size of the fragment, the faster it moves through the capillary tube. As each fragment
moves through the tube a laser detector registers a peak on a graph.
5. The STR analysis of DNA from the crime scene is compared with the STR analysis of DNA from a suspect.
Investigate the use of data
analysis from a large-scale
collaborative project to identify
trends, patterns and
relationships, for example:
➢ The use of population
genetics data in
conservation
management
➢ Population genetics
studies used to
Human Genome Project
In the human genome there are 23 pairs of chromosomes. Between 1990 and 2003, all 23 chromosome pairs were fully
sequenced through an international research effort known as the Human Genome Project (HGP) led by an American Institute.
– The goal of HGP was to determine the sequence of the complete human genome - the precise order of nucleotides
within a DNA molecule and the number of genes in one human individual.
– Through the Human Genome Project it was found that each haploid set of human chromosomes consists of
approximately 3.2 billion DNA base pairs. Diploid cells contain twice as many base pairs- approximately 6.4 billion.
The sequence of bases tells scientists where particular genes are located on the chromosomes. The position is
known as the locus (plural loci).
determine the
inheritance of a
disease or disorder
➢ Population genetics
relating to human
evolution
Conservation management
Conservation genetics relies on gathering genetic data, for biodiversity conservation and making informed decisions about
protecting populations that are endangered or nearing extinction.
– It is a useful tool for scientists to use when determining current and future strategies for the conservation of
populations.
– Individuals with alleles that provide resistance to disease are more likely to survive and reproduce than individuals
that do not have these alleles.
– In populations with low genetic variation, alleles that provide disease resistance are less likely to occur, leaving the
entire population vulnerable in the event of a disease outbreak.
– Population genetic researchers can measure the genetic variation of a population by sampling multiple individuals
and analysing multiple genetic markers.
– Another way in which population genetics can assist in conservation is by identifying invasive species. When these
appear in a new environment and begin to cause damage to the local ecosystem, population genetics can be used
to find out where they came from.
– These methods also enable scientists to identify segments of the genome that are essential for the organism’s
adaptation to the environment.
– They can determine relationships and identify individuals that could be reintroduced into a population for recovery.
– Any harmful alleles can be detected early in a population, as well as any mutations that may enhance these
functions.
– These all together are essential in identifying conservation strategies to increase the chance of saving endangered
species and maintaining biodiversity.
Disease inheritance
Isolated populations are a particularly rich source of data when it comes to genetic disorders. This is because isolation often
results in inbreeding and the genetic characteristics of the population become concentrated over multiple generations.
– The overall genetic variation in isolated populations is reduced, and traits that are rare in the wider population are
more likely to occur.
– The study of genetically identical twins in a population is another way to monitor disease inheritance. Identical twins
offer an opportunity to separate those factors due to their identical DNA.
Alkaptonuria (black urine disease): A disease in which the body cannot process the amino acids phenylalanine and
tyrosine.
1. This disease causes many problems but the first symptom is urine with an unusually dark colour, which turns
black if left exposed to air.
2. With the advancement of DNA sequencing, it was found that a mutation in the HGD (homogentisate 1,
2-dioxygenase) gene, Alkaptonuria is an example of a disease that results from a mutation in a single gene in all
cells of the body and is known as a monogenic disease.
Evolution of modern humans
One of the first ways that researchers used DNA to look at human origins was by using mitochondrial DNA (mtDNA). Since
mtDNA does not recombine and is only maternally inherited (i.e. from mother to offspring), it is much simpler to analyse.
– In 1987, researchers learnt that the most recent common ancestor could be traced to Africa around 200,000 years
ago. In the science media of the time, this woman was referred to as 'Eve', but it is important to remember that she
is only the mitochondrial Eve, telling us about the mitochondrial genome of lineages.
– Because the Y chromosome is paternally inherited the ancestor is a male who lived in Africa approximately 90,000
years ago. In some ways, the migratory patterns shown by the analysis of nuclear genes are very similar to those of
the mitochondrial analyses, but there are distinct differences.