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2020-BIO-Module-5-Heredity-Syllabus-Notes

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Module 5: Heredity (COMPLETE)
Reproduction
Inquiry question​: H
​ ow does reproduction ensure the continuity of a species?
Syllabus Dot-point
Content
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:
Reproduction
● Fundamental evolutionary process that ensures the continuity of a species and the gene pool by producing offspring
● Reproductive success​ determined by an organism’s ability to produce offspring that survive to reproductive maturity. This can is
measured by ​biological fitness​ - relative likelihood that alleles (variants of a gene) will be represented in future generations
● The k
​ ey to successful fertilisation​ is that the single haploid gametes must meet and not dehydrate in the process
Sexual
Involves the combination of gametes which carry genetic information from both parents. G
​ enetic diversity a
​ nd ​variation ​are the
primary a
​ dvantages ​as the offspring contains a mix of parental genes and are not genetically identical to parents or other
offspring. This can also possess variations that become well-suited to a changing environment, allowing them to outcomplete
parents and gain a selective advantage. However, d
​ isadvantages ​include that the process demands greater expenditure of
energy: gamete production, finding a mate, mating etc.
Asexual
Involves reproduction of identical offspring (​clones​) from one parent through the process of mitosis, where each daughter cell
receives a copy of every chromosome of the parent cell. There is no production or fusion of gametes and no mixing of genetic
information to introduce variation. These are common in unicellular organisms due to lack of cell specialisation (no reproductive
organs or germ cells to produce gametes)
Advantages
● Allows individual organisms to reproduce in isolation when selective pressures (shortage of resources) are present
● Favourable, enabling rapid population expansion when environmental conditions are ideal as there is no disadvantage in
reproducing offspring with the same inherited traits as the parent. They will be well-suited to a stable and uniform
environment
Disadvantages
● Lack of genetic variation limits adaptability and evolutionary potential in long term and leads to overcrowding
● When environmental conditions are variable and subject to change - organism are not adaptable to new conditions
Hermaphroditism i​ s when an organism possesses both male and female reproductive organs. The process is advantageous for
species with low population density or low-motility.
Animals: advantages
and disadvantages of
external and internal
fertilisation
Internal fertilisation
Process where the female gamete is fertilised by the male gamete
inside the female reproductive tract
External fertilisation
Process where fertilisation occurs externally to the female
●
●
●
Most common in m
​ ammals, birds and reptiles​ (multicellular
land and terrestrial organisms)
During intercourse, a large number of male gametes are
released, however only a small number of female gametes
are available. This results in the production of a small
number of zygotes.
○ Eutherian (​ placentals) embryonic development
occurs in uterus where the p
​ lacenta ​organ develops
and supplies the young with nutrients and oxygen
Advantages
● Internal protection from immediate predation and
dehydration of gametes leads to a higher success rate
● Reptile oviparous​ eggs develop a shell are laid in the
external environment to complete its development
Disadvantages
● Seasonal and less frequent due to the need for greater
expenditure of energy and higher success rate
●
Male and female organisms do not have to be in contact for
fertilisation to occur. This results in the production large
numbers of zygotes that develop outside the male and
female parents.
Most common form of reproduction in a
​ quatic and moist
environments
○ Staghorn coral: c
​ olony of invertebrate marine ​polyps
that shed gametes and ​pheromones i​ nto the sea
which results in coordinated spawning
Advantages
● Rapid form of reproduction with higher breeding frequency
due to lower success rate
● Aquatic environments prevent gamete dehydration
Disadvantages
● Lack of protection means not all zygotes survive
● Limited parental investment
● Vulnerable to environmental elements: temperature,
predation, infection
Relative successes of fertilisation
Natural selection​ eliminates certain inherited characteristics if they fail to confer adaptations to suit their changing environment
Plants: asexual and
sexual reproduction
Diagram of sexual
Sexual reproduction
● Angiosperms​: vascular flowering plants where seeds are enclosed in an ovary (fruit)
● Gymnosperms​: vascular plants with unenclosed seeds often configured as cones
reproduction
1. Pollination
2. Fertilisation
3. Seed dispersal
4. Germination:
produces ​radicle
(root to absorb
water and
nutrients) and
plumule
(develops green
leaves for food
production)
Self-pollination
Requires less energy as there is no requirement to produce
structures to attract pollinators (petals, nectar). Favourable for
plants that live in isolation.
Cross-pollination
Rely on external pollinating abiotic (wind, water) and biotic (insects,
birds) agents to carry out fertilisation and transfer male gametes
from the anther to the stigma. This results in greater variation of
offspring.
Asexual reproduction
Vegetative propagation
The production of vegetative organs from which new plants can arise
● Perennating organs​: underground roots or stems that contain sufficient amounts of stored food to sustain plant in dormant state.
They develop into buds that begin to grow during adverse conditions.
○ Runners →
​ modified stems that grow on surface of soil and produce new leaves and roots at each n
​ ode
○ Rhizomes →
​ modified stems that can propagate new shoots at each node
○ Suckers →
​ modified roots where shoots can grow and develop into new plants
○ Apomixis ​→ generative tissues that give rise to plantlets that produce seeds asexually
Artificial propagation of plants
Used in agriculture to produce perennial plants in commercial quantities. Favourable when seeds are unavailable or difficult to germinate or
when farmers want to perpetuate features of a desirable plant. The technique also used in research and recovery programs for endangered
plant species (round-leaved sundew, Wollemi pine).
Fungi: budding, spores
Eukaryotes that secrete
enzymes over the
surface of their food and
absorb the breakdown
products directly
Budding​: adult organism gives rise to a small bud which develops as an outgrowth on the parent cell. The bud is smaller but genetically
identical to the parent. Repeated budding forms a chain of connected but independent cells. Most y
​ easts ​reproduce asexually by asymmetric
budding
Process
1. Parent cell forms small outgrowth that grows larger and forms bud
2. Nucleus splits into a smaller nuclei which migrates to daughter cells
3. Bud pinches inward at the base to detach from parent cell. Although in some organisms, it may
remain in contact
Spore formation​ ​(typical)​: spores are tiny haploid unicellular reproductive cells capable of developing into an adult without fusion with a
second cell. Daughter cells are genetically identical to parent fungus and often arise from structures called s
​ porangia, ​which produce spores in
large numbers. Spores are encased in a protective coating, which turns black as it ripens, and its light mass enables widespread air dispersal.
If a spore lands in a favourable environment, it germinates, forming a new fungus.
●
●
Bacteria: binary fission
Asexual ​→​ spores are called m
​ itospores ​and are produced by mitosis
Sexual​ → ​spores are called​ meiospores ​and are produced by meiosis
Binary fission​ is the main method of asexual reproduction in unicellular organisms where a newly divided cell replicates its genetic material
and splits into two genetically identical cells.
Process
1. A dividing bacterium copies its DNA (single circular chromosomes with no nucleus) in the o
​ rigin of replication
located on chromosome by r​ eplication enzymes. ​The bacterial chromosome is found in the nucleoid.
2. Two split origins move towards opposite ends of the cell, pulling the rest of the chromosome along with them.
3. The cell elongates and a ​septum ​forms down the middle of the cell (division of the cytoplasm)
4. The cell pinches in two to form two identical bacteria cells
Differences to mitosis
● No mitotic spindle (strings that keep chromosomes organised) forms in the bacteria during cell division
● DNA replication and separation occurs in tandem
○ Unlike in mitosis, DNA is copied during the S phase, prior to its separation in the M phase
Protists: binary fission,
budding
Asexual binary fission​: typical mode of reproduction
Process differs to bacteria due to the need to ​replicate a membrane-bound nucleus​.​ ​The parent cell no longer
exists but neither has it died. Rather, it has redistributed itself to genetically identical daughter cells. The
length of time needed for fission varies between organisms and with environmental conditions.
Binary fission can be along a l​ ongitudinal ​axis (splits along its longest axis) or along a ​transverse
(shortest axis)
- Paramecium →
​ p
​ rotist of ciliate group
Multiple fission​ can occur in ​parasitic protists​ where the nucleus in parent cell divides repeatedly and at a rapid rate to produce large
numbers of daughter nuclei for the formation of multiple progeny
Plasmodium falciparum​ → causes malaria in humans
Budding: o
​ ccurs on outside of a cell from which it detaches to live independently or in contact to form a colony. Unlike fission, the cytoplasm
is unequal to parent cell as it is smaller in size.
Analyse the features of
fertilisation,
implantation and
hormonal control of
pregnancy and birth in
Mammals have several reproductive mechanisms to maximise reproductive success:
Hormonal
control
Hormones​: chemical substances that act as messengers in the body. Coordinates bodily processes so that actions within
the body are synchronised.
● Sex hormones​: specifically affects the growth and functioning of reproductive organs. Part of endocrine system
mammals
and are produced in special tissues in the ovaries, testes, and pituitary gland.
○ Pituitary gland​ (located at base of brain) secretes hormones that stimulate/inhibit other endocrine glands
and regulates the release of their hormones for growth, metabolism and reproduction.
○ In humans, reproductive organs (​gonads​) mature and begin their function when stimulated by hormones
during puberty
Reproductive processes that hormones regulate
● Breeding seasons​: regulate sexual behaviour by limiting ability of some mammals to reproduce at certain periods
○ Seasonal breeders are ​“in oestrus”​ during periods of female fertility
● Gametogenesis​: production of male and female gametes in respective gonads
● Male and female reproductive cycles
○ In females, the pituitary gland secretes​ gonadotropic hormones​ (FSH and LH) for the maturation of follicles
in ovaries as well as a l​ actogenic hormone called ​prolactin f​ or the preparation of breast tissue for milk
production.
Types of
hormones
involved in
mammalian
reproduction
1. Androgens​: male hormones that regulate male sex organs and secondary sex characteristics (deepening voice,
increase in growth etc.) Cells in testes secrete a
​ ndrogen testosterone​ during spermatogenesis. While present in
both sexes, levels are higher in males. They are also precursors of oestrogens.
2. Oestrogens​: female hormones ​(with the converse of male characteristics). M
​ ain functions are ​ovarian functioning
and ​fertility ​in females
3. Progestogens​: second main group of female hormones responsible for the regulation of pregnancy, secretion of
milk in mammary glands (​lactation​) and menstruation.
Fertilisation
Occurs when the haploid nucleus of an egg fuses with that of a sperm, forming a diploid zygote.
● Sperm are attracted to the egg by r​ heotaxis (movement through a fluid) ​where they are held in storage in the
oviduct. The presence of p
​ rogesterone ​and a
​ lkaline pH​ allow sperm to mature so they can penetrate the 3 layers of
the egg: f​ irst membrane​ which contains fragments of follicle cells, z​ ona pellucida​, and ​plasma membrane​. Surface
proteins allow only one sperm to penetrate the final barrier, which triggers enzymes that destroy the glycoproteins
in the second layer and prevent other sperm from entering.
● Fertilised egg travels along oviduct and begins embryonic development
Implantation
Implantation of embryo into the uterine wall for internal development
● Oestrogen ​and p
​ rogesterone​, produced by the ovaries, prepares the uterus for the implantation of a fertilised egg
each menstrual cycle. If fertilisation does not take place, the levels of both hormones decrease and the lining of the
uterus tears away.
● Once implantation has occurred, the main role of progesterone is suppressing uterine activity, thus supporting
foetal development and reducing risk of foetus being disturbed by uterine contractions
Evaluate the impact of
scientific knowledge on
the manipulation of
plant and animal
reproduction in
agriculture
Pregnancy
Pregnancy begins when fertilised egg implants on uterine wall
● Corpus luteum​ in ovary secretes hormones for first three months of pregnancy. Once it begins to degenerate, the
placenta d
​ evelops and takes over the role of producing hormones to maintain pregnancy for the latter six months
● Levels of oestrogen and progesterone are optimised during ovulation to create ideal conditions for implantation
○ Oestrogen ​→ promotes growth of endometrium
○ Progesterone ​→ stimulates secretion of mucus by cells lining endometrium and growth of blood vessels.
Also reduces mother’s immune response to foetal ​antigens. A
​ dequate progesterone production by corpus
luteum is essential in maintaining pregnancy until placenta takes over at 7-9 gestational weeks.
○ Placenta ​→ connected to foetus by umbilical cord where blood vessels pass to and fro to transport oxygen
and nutrients and remove wastes
○ Other hormones prevent foetal overgrowth and regulate nutrients that cross placenta
Birth
Prostaglandins s
​ ecreted by the wall of the uterus initiates labour, softening the tissue of the cervix to allow the passage of
the baby. Secretion of o
​ xytocin ​promotes coordinated contraction of the smooth muscle of the uterus, and secretions
continue after the birth to expel the placenta and limit blood flow to the uterus. P
​ rolactin ​promotes the enlargement of
mammary glands for milk production.
Reproductive technologies​ applies to the use of any technology to assist and improve reproduction.
Agriculture -​ breeding and cultivation of animals, plants, fungi for food, biofuels and other products used to enhance human life
Human intervention in necessary to improve the quality and yield of production
(info already written in notes)
Cell Replication
Inquiry question​:​ How important is it for genetic material to be replicated exactly?
Model the processes
involved in cell
replication:
- Mitosis and meiosis
- DNA replication using
the Watson and Crick
DNA model, including
Mitosis
Meiosis
Form of nuclear division that ensures daughter cells receive exact
copies of chromosomes
‘Reduction’ cell division that takes place in reproductive organs and
results in the formation of gametes with haploid number of
chromosomes.
nucleotide composition,
pairing and bonding
Role and importance
Growth of multicellular organisms - mitotic division followed
by c
​ ell assimilation, enlargement ​and ​differentiation
Repair of damaged tissue and replacement of worn-out cells
Asexual reproduction - cloning, budding
Genetic stability - ensures equal distribution of
chromosomes
Process​:
SPMAT + Cytokinesis
-
-
-
Diploid zygote arises from the fusion of haploid gametes
that were formed respectively by maternal and paternal
chromosomes
Ensures chromosome number of species is maintained
Genetic variation is introduced
Process​:
DNA replication occurs before division
1. Meiosis I - d
​ iploid cell divides into two haploid cells
2. Meiosis II - t​ wo cells divide again to form four haploid
daughter cells (​tetrad​)
Telomeres ​- DNA-protein regions on the ends of
chromosome that shortens with age, preventing further
division and leading to c
​ ell senescence​/death
DNA Replication (​ info already written in notes)
Relatively recent discovery of its structure (60 years) XRD- X-ray Diffraction
- DNA is the SAME in every cell (different sections are turned on and some are turned off)
- Each of 46 chromosomes has one large DNA molecule
Assess the effect of the
cell replication
processes on the
continuity of species
(info already written in notes)
DNA and Polypeptide Synthesis
Inquiry question​:​ W
​ hy is polypeptide synthesis important?
Construct appropriate
representations to
model and compare the
forms in which DNA
exists in eukaryotes and
prokaryotes
The genetic code is u
​ niversal​ → the same nucleotide base-pairing code is used in ​all​ ​living organisms for protein synthesis.
Prokaryotes
Circular chromosomal DNA​ is contained in one chromosome that
holds two circles of single-stranded DNA twisted around each other.
The chromosome is found in the nucleoid and measures about
Eukaryotes
DNA is ​linear​ and not circular. DNA molecules are organised into
chromosomes and are located in nucleus.
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
phenotypic expression
1300µm.
In most eukaryotic cells, a large proportion of DNA is non-coding
DNA, and are not used directly to make proteins or RNA. Only 3% of
DNA is coding DNA.
May have one or more small rings of non-chromosomal DNA
(​plasmids​), which replicate independently of chromosome. These
plasmids do not necessarily code for the essential features required
for survival, but provides bacteria with a selective advantage
(resistance to antibiotics).
Non-nuclear DNA is found in the mitochondria and chloroplasts of
cells. These replicate and are inherited independently of nuclear
DNA.
Mitochondrial DNA​ is linked to maternal inheritance: used to
study evolutionary relatedness and trace a direct genetic
line, as mtDNA are only passed down from the mother.
DNA ​supercoils​ around a central protein (​scaffold​) to form nucleoid.
DNA ​winds​ around proteins (​histones​) to form n
​ ucleosomes​. ​There
are 5 main histones which are responsible for the packaging of DNA.
Polypeptides ​are chains consisting of amino acids held together by ​peptide bonds​ and fold in 3D conformations to form proteins.
The sequence and arrangement of amino acids determines the configuration of a protein. Therefore, any change in the sequence may
change the shape of the protein and hinder its ability to carry out its function.
Polypeptide synthesis
Transcription​: a section of DNA is transcribed by mRNA
1. RNA polymerase​ enzyme binds to a part of the DNA ‘​ sense/non-coding’ strand​ (promoter). The DNA helix
unwinds and unzips. The sense strand acts as a template for RNA nucleotides to attach to its
complementary base pair on the DNA strand (HOWEVER,​ A b
​ inds with ​U [uracil]​).
a. In eukaryotes, ‘editing’ or splicing of pre-mRNA may take place:
RNA processing​ - Eukaryotic cells
mRNA transcribed from DNA is termed ​pre-mRNA a
​ s it requires further editing before it acts as a template
for translation. pre-mRNa contains coding sequences of nucleotides (exons) with sequences of noncoding
nucleotides (introns) Introns code for their own removal, forming mature-mRNA.
Splicing and rearrangement of blocks of mRNA ​is important for gene regulation in complex
organisms as it can give rise to different versions of the same protein (​isoproteins​) - can produce
antibodies specific to a pathogen/invader
2. The mRNA molecule exits the nucleus via the nuclear pores and enters the cytoplasm, where it will attach to
a ribosome. (Usually, the mRNA template is read by many ribosomes so that the same polypeptide product is
produced in larger quantities)
Translation​: mRNA binds with tRNA on the ribosome to synthesise a polypeptide chain
1. Ribosomes move along the mRNA strand. tRNA molecules bind with amino acids in the cytoplasm and
subsequently attaches to mRNA by temporarily pairing the bases of the tRNA anticodon to its
complementary codon on mRNA. The ​‘start’ codon​ initiates the synthesis of the polypeptide.
2. A chain of amino acids form, held together by an enzyme
3. After the amino acid on a tRNA molecule binds with the growing polypeptide chain, tRNA moves away from
the mRNA and back into the cytoplasm to attach another amino acid
4. Polypeptide chain is processed in the cell where it folds to form protein in a 3D configuration
5. Once the ​‘stop’ codon​ is reached, the mRNA breaks down into its individual nucleotides, which can be reused
Assessing the importance of mRNA and tRNA in transcription and translation
mRNA (messenger)
Single stranded and are a few thousand
bases long, shorter than DNA. Functions as
the intermediate molecule that carries info
from DNA to ribosomes in cytoplasm.
tRNA (transfer)
Each molecule is 75 nucleotides long and twisted in the shape of a
clover-leaf. At one end are three unpaired bases (​anticodon​) which
attach to its complementary bases (​codon​) on mRNA.
Formed from DNA
Other end holds a site of attachment for amino acids. The sequence
of the three bases determines the amino acid carried.
Importance
● mRNA allows for the genetic code in
DNA to be translated into proteins.
This is because DNA is held in the
nucleus of a eukaryotic cell and is
too large of a molecule to penetrate
the nuclear membrane. The
single-stranded mRNA is able to pass
Importance
● tRNA is an important molecule in ​transcription​ that
attaches to mRNA and gives rise to a specific sequence of
amino acids that synthesise into a protein
● Important for the attachment of amino acids to form a
growing polypeptide chain. It maintains the sequential
synthesis of the protein after it detaches from the mRNA to
bind with another amino acid that will contribute to the
rRNA (ribosomal)
Made in the
nucleolus and act as
enzymes that aid in
protein synthesis
and forms a
structural
component of
ribosomes.
through and thus translate the
genetic instructions to the protein.
Role
●
Responsible for the carrying of
genetic information contained in DNA
to the ribosomes in the cytoplasm.
making of the protein.
Role
●
The main function of tRNA is to transport amino acids to
the ribosome. The other end of a tRNa molecule contains
the anticodon triplet that will bind to its complementary
codon on mRNA.
Analysing the function and importance of polypeptide synthesis
Importance
● DNA unzipping​ is essential for the formation of mRNA. A section of DNA containing the gene required for the synthesis of a specific
protein unwinds and unzips where RNA nucleotides can attach to the exposed “sense” strand and form an mRNA molecule that carries
transcribed genetic information from the DNA. This section provides the c
​ ode for the production of a protein​.
● The processes of transcription and translation are carried out in tandem in cells for the creation of proteins. Proteins are essential
biological molecules that cells and tissues require for their specific function.
● It is crucial that the genetic information being translated via RNA contains full and accurate instructions for the correct functioning of
a cell. If any errors occur in the creation of mRNA or the pairing of nucleotides, the end-products of synthesis may change and hinder
the survival of an organism, especially if the required protein is not being produced.
● Gene expression is regulated by proteins, hence, it is important that these proteins undergo accurate synthesis according to DNA
instructions to produce an overall phenotype for an organism
Assessing how genes and environment affect phenotypic expression
Genotype →
​ all the alleles present on a chromosome for a particular trait
Phenotype →
​ physical expression of an organism, based on its dominant genotype
➔ Some variations can be genetically-determined or influenced by environment
➔ Genes that ARE expressed dictate the types of proteins in cells and thus, the overall phenotype of organism
Genes
●
Dependent on the accurate synthesis of
Environment
The environment does not chemically modify the genome (sequence of bases), but
proteins that are coded for by specific
sections of genes on DNA
influences the phenotype via chemical markers or tags being added to DNA.
● Temperature can cause organisms to express the phenotype of mutant
alleles through colour of fur etc.
● Hydrangeas ​→ the pH of soil influences the phenotype and causes a change
in the colour of the flower
Regulation of genes
● Modifying DNA for transcription​: ​methylation​ represses gene expression. It increases the density of binding between DNA and
histones, restricting access by RNA polymerase.
● Post-transcription (modifying and processing RNA)​: includes splicing (removal of introns) and regulation of the length of time for
which mRNA remains active and stable.
○ MOST COMMON REGULATION IN ​EUKARYOTES
● Post-translational regulation​: activation of protein by adding or removing a chemical group
Investigate the structure
and function of proteins
in living things
Chemical structure
● Contains carbon, hydrogen, oxygen and nitrogen (sometimes sulfur). Proteins are composed of chains of amino acids bonded
together by ​peptide bonds​. Folds into a particular shape that is crucial to its functioning.
○ Protein ​→ single chain of 40-50+ amino acids folded in a specific manner
○ Polypeptide →
​ chain shorter than 40-50 amino acids and combines with other chains to fold into functional protein
Physical structure
Primary ​→ polymers containing an amino acid sequence arranged in linear chains called polypeptides. This structure determines the
secondary and tertiary structures, and any incorrect amino acid may hinder the shape of the protein and therefore, its function.
Secondary →
​ three-dimensional arrangement of polypeptides by twisting + hydrogen bonds. Can fold into a
​ lpha helices​ ​or p
​ leated
sheets​. F
​ ibrous proteins ​have a secondary physical structure.
Tertiary ​→ further folding into complex 3D conformations caused by interactions between polypeptide and immediate environment.
Typical of ​globular proteins​.
Quaternary →
​ proteins made up of more than one polypeptide chain
Types of protein groups
Fibrous
Forms structural components of cells and tissues. Are long and insoluble in water.
E.g. keratin, collagen, elastin
Globular
More complex proteins with tertiary structure and are spherical in shape. Compact and soluble.
E.g. transport proteins, enzymes, immunoglobulin, hormones
Functions of main group proteins
Structural
Promotes structural support and movement of cells (​Tubulin i​ n microtubules are responsible for forming the cytoplasm,
Actin a
​ llows for the contraction of the cytoplasm during cytokinesis and promotes the crawling movement of protists)
Enzymes
Is involved in all biochemical aspects of cell metabolism - anabolic and catabolic reactions. The shape of the active site
determines its binding specificity. Important molecule in gene functioning, replicating, repairing and protein synthesis.
Cell
communication,
signalling and
biological
recognition
Proteins embedded in membranes regulate the movement of ions and molecules between internal and external
environments. These include ​transport a
​ nd ​storage p
​ roteins.
Sensory
Responds to stimuli (changes in environment) by changing its shape or biochemical activity
Hormones a
​ nd ​neurotransmitters a
​ ct as chemical messengers between cells - biological recognition between chemical
messengers and their target cell is essential. ​Receptor proteins i​ n cell membranes are responsible for receiving these
messages and must have a shape that is ​reciprocal t​ o the molecule with which it binds.
Genetic Variation
Inquiry question​: H
​ ow 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
(Info on meiosis in above dot point)
How is genetic variation introduced?
Mendel’s laws of genetics
● Random segregation (1st law) → ​pair of alleles segregate and each gamete receives one allele for a gene
● Independent assortment (2nd law) → a
​ lleles for each different ​trait​ separate independently of other alleles
●
●
Model the formation of
new combinations of
○ There are 2 23 possible combinations of chromosomes in the formation of gamete from independent assortment alone
Fertilisation →
​ random fusion of two gametes ensures further mixing of genetic material, producing variations in phenotype that may
be acted upon through natural selection in process of evolution
○ Offspring arising from gametes of u
​ nisexual animals​ will produce greater genetic variability than those from ​hermaphroditic
animals
○ Offspring arising from ​cross-fertilisation ​between plants will have greater genetic variability than those from s
​ elf-fertilisation
Mutation ​→ occurs during DNA replication prior to cell division
Mendel’s model of inheritance: Autosomal recessive inheritance
● Alleles pass from one generation to the next according to s
​ et ratios
genotypes produced
during meiosis:
- Interpreting examples
of autosomal,
sex-linkage,
codominance,
incomplete dominance
and multiple alleles
- Constructing and
interpreting information
and data from pedigrees
and Punnett squares
●
●
●
The alleles in an individual:
○ Are the same ​in homozygous/pure-breeding offspring
○ Differ in h
​ eterozygous/hybrid offspring
■ In heterozygous individuals, the trait that is expressed is the d
​ ominant allele​ and the one that is hidden is the
recessive allele. ​For a recessive allele to be expressed, both alleles need to be recessive.
■ These alleles are located on one of the non-sex chromosomes (autosomes). Humans have 22 pairs. The one sex
chromosome determines gender.
Diploid individuals inherit two alleles of the same gene (one from each parent) and haploid cells have only one allele of each gene
However, despite there being multiple alleles of a gene in a population, an individual can only carry two alleles of one gene. These are
dependent on the alleles passed down from parental chromosomes.
___
Techniques
Phenotype does not necessarily tell us what the genotype of an organism is. A ​testcross​ c
​ an be used to determine whether an organism is
homozygous or heterozygous → the tested organism is crossbred with an organism that is homozygous for the recessive gene. (INCLUDE
PUNNETT SQUARES)
Pedigree charts​ are used to identify inheritance patterns of a particular trait in a family lineage and make predictions about the expected
phenotypes and genotypes of future offspring. (INCLUDE TABLE)
Sex-linkage
● In humans, genes on sex chromosomes code for the production of sexual reproductive organs and the development of secondary
sexual characteristics that define whether an individual is phenotypically male or female
○ Y carries testis-determining gene
Sex linkage​ occurs when genes on sex chromosomes code for characteristics other than gender.
● Genes on X = females have two alleles for that gene and males will have only one = recessive disorders appear frequently in males
○ Haemophilia is a bleeding disorder and X-linked. Males who inherit this mutant allele on X chromosome will suffer from the
disorder as they have no equivalent allele to mask the gene on the Y chromosome.
○ If a female inherits one copy of the allele from haemophilia, she will not suffer from disorder if her other allele is dominant.
She is termed a ​carrier →
​ disorder may be passed on to her sons (affected) or daughters (carriers or affected)
○ If a daughter inherits pair of defective alleles, the condition is lethal
Rest of info written in notes
Collect, record and
present data to
represent frequencies of
Population genetics​ → study of how the gene pool of a population changes over time, leading to a species evolving. Combines Mendelian
genetics and Darwinian evolution to explain how changes in allele frequencies arise and how these can result in micro or macroevolution.
Scientists conduct frequency studies to predict the potential of populations to adapt, as well as future resilience, stability and survival.
characteristics in a
population, in order to
identify trends, patterns,
relationships and
limitations in data, for
example:
- Examining frequency
data
- Analysing single
nucleotide
polymorphism
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●
Gene pool is the sum total of all genes and their alleles present in a population
Genetic diversity is the total of all genetic characteristics in the genetic makeup of a species and is dependent on genetic variability
(tendency of individual traits to vary). Species that have a greater degree of variability is more likely to adapt and survive.
Allele frequency​ ​→ measure of how common an allele is in a population
F requency of allele A =
N umber of copies of allele A in population
T otal number of copies of the gene (A+a) in population
Factors affecting allele frequency​: selective environmental pressures, natural selection, other external (gene flow, genetic drift)
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●
To conduct a scientifically valid study, pop. geneticists use a model based on the allele frequencies derived from a stable population
with Mendelian inheritance (population in equilibrium) and compare this to populations exposed to selective pressures.
They study mathematical changes in frequencies to develop quantitative ways of exploring evolutionary hypotheses.
Single nucleotide polymorphism
● SNPs are mutations where one nucleotide has been incorrectly inserted during DNA replication, creating an error in the base sequence
at a particular location on a chromosome (loci on chromosomes where alleles differ at a single base). They are often termed ‘genetic
markers’ because they are used to compare genetic similarities and differences between individuals without sequencing the genome.
○ Genetic markers → identified sequence of DNA at a known site on a chromosome
● The rarer allele must have a frequency of 1% in a certain population to be termed a SNP.
● SNPs usually occur in non-coding regions (introns) of DNA
GWAS (genome-wide association studies)
● Bioinformatics technology (DNA manipulation techniques) is used to scan the genomes of people and find genetic variations
associated with a particular disease or phenotypic trait. SNPs are studied to identify disease susceptibility and evolutionary
relatedness.
● SNPs that occur in higher frequencies in people with a particular disease are said to be associated with such disease
● Computer technology allow hundreds or thousands of SNPs to be analysed at the same time → genotyping (identifying genetic
variations in individuals) is faster and cheaper than sequencing whole genomes.
● GWAS are based on the presence of a g
​ roup o
​ f SNP markers (​haplotype​) associated with a trait, rather than individual SNPs
Limitations​:
● Data from GWAS is reliable as long as the regions selected are evenly distributed throughout the genome.Genetic markers that are
more closer together give more accurate data → for haplotype studies, SNPs inherited from one parent are studied, thus, crossing
over during meiosis mean the SNPs on a chromosome might not all be inherited together.
Inheritance Patterns in a Population
Inquiry question​:​ C
​ an population genetic patterns be predicted with any accuracy?
Investigate the use of
technologies to
determine inheritance
patterns in a population
using:
- DNA sequencing and
profiling
DNA sequencing​ → exact nucleotide sequence (order of bases) of a gene on a chromosome is determined
● Sanger chain method​ (dideoxy DNA sequencing - d
​ dDNA​)
○ DNA is isolated from cells and replicated by the polymerase chain reaction (PCR)
■ SEQUENCING REACTIONS:
○ Double-stranded DNA is separated into single chains by heating
○ A small piece of DNA called a p
​ rimer ​binds to the start of template DNA strands and acts as a template to build the
complementary strand using free nucleotides
○ After a chain-terminating nucleotide (​dideoxy nucleotide triphosphate​)​ h
​ as attached to their complementary base on template
strand, they prevent other nucleotides from attaching (dd​A​TP, dd​TT
​ P, dd​GT
​ P, dd​CT
​ P) Each of these are labelled with
fluorochromes (fluorescent dye).
○ Chain termination occurs at different positions leading to varying fragment lengths of DNA
○ Process continues until every position on template strand has been identified with a ddNTP
○ DNA fragments are placed into a tiny capillary tube containing a gel where an electric current is used to pull the strands
through. When strands emerge, they pass through a laser beam which causes them to glow at a particular wavelength specific
to the base defined by the ddNTP at the end of the fragment.
○ Computer analyses colours and displays chromatogram of base sequence in original DNA sample (order of complementary
bases on different lengths of dye-stranded DNA allows sequence to be determined).
●
●
Maxim-Gilbert method​ → sequencing of DNA by modifying the chemical conditions suited to a specific base, allowing them to be
removed from the ribose sugar it is attached to and creating fragments of different lengths where the DNA has been cleaved.
Fragments undergo gel electrophoresis and all patterns derived from each base are compared to determine final sequence.
○ DNA is radioactively labelled on one end by adding phosphorus atom to the phosphate molecule prior to modification of
conditions
○ Chemical reactions are specific to two groups of bases: pyrimidines (C and T) and purines (A and G)
Nanopore sequencing​ → propelling a DNA molecule with a motor protein through a protein nanopore. Pore in a membrane separates
two compartments both containing a buffered KCl solution. Differences in current when passing through is dependent on the identity
of the DNA sequence.
DNA profiling (​ DNA fingerprint analysis) → organism’s unique DNA profile is determined and represented as a distinct series of bands
● Whilst 99.9% of DNA is common to all humans, ​STRs (​ Short tandem repeats) are unique. These are sections of non-coding DNA
following a repetitive sequence (e.g. TATATATA)
○ Number of repeats in non-coding regions of DNA varies between individuals and gives rise to different profiles
○ Process​: DNA is isolated and the PCR is used to increase the amount of DNA under study. Strand undergos gel
electrophoresis and fragments will migrate different lengths in the gel depending on the amount of repeats.
Investigate the use of
data analysis from a
Population genetics​:
● Genetic differences between species can be analysed to determine the evolutionary history of populations - those with similar gene
large-scale collaborative
project to identify trends,
patterns and
relationships:
- The use of population
genetics data in
conservation
management
- Population genetics
studies used to
determine the
inheritance of a disease
or disorder
- Population genetics
relating to human
evolution
pools are most closely related
Conservation management
● Aims to avoid extinction of a species by employing conservation methods that ensure biodiversity is maintained. Involves gathering
genetic data that will aid in identifying conservation strategies to increase the chance of saving and protecting endangered
populations.
● Methods​:
○ Field observation by sampling and statistical analysis - distribution and abundance of a species
○ DNA analysis (SNPs, GWAS, haplotypes) → determines kinship lineages, improved scientific understanding of microevolution
by selection and mutation
■ Enabled scientists to identify sections of the genome that are essential for adaptation to the environment and to
identify deleterious alleles and any mutations that can enhance biological functions
● Scientists study past extinction events to develop models that can assist in modern-day conservation of endangered species
Wooly mammoth extinction​ → mammoths in isolation on Wrangel Island suffered a ‘genomic meltdown’ with various detrimental mutations
that hindered the potential for the species to survive.
● Mutations primarily affected olfactory processes and reduced number of urinary proteins which reduced their ability to mark and
recognise territory, hunt and mate.
● Others include a mutation for fur (cream coloured, satiny coat) which reduced its insulating properties which were essential in ice age
climates. Resultant inbreeding and loss of genetic diversity (precursor to disease susceptibility) led to their extinction.
Modern koala populations​ → two biogeographic barriers that have emerged in the last ice age (20 000 years ago) led to a split in the koala
population. Further habitat fragmentation and other selective pressures including those instigated by human activity (fur trade, habitat
clearing, disease) have reduced the distribution of koalas over time. Current research is being implemented on local scales to collect DNA
samples and analyse genetic variation in koala populations.
Disease
New-generation gene-sequencing technologies have allowed scientists to identify genes associated with genetic diseases and disorders and
individualise diagnosis of specific diseases and predict the possibility of offspring inheriting conditions quickly and accurately. Large-scale
screening and DNA analysis provide opportunities for early detection and improved treatment options.
SNPs unique to a particular disease can be detected in newborns → NSW newborn screening program provides free genetic tests for
SNPs associated with various congenital diseases (cystic fibrosis, phenylketonuria etc.)
Human Gene Mutation Database stores info on germline mutations associated with human-inherited diseases
Human evolution
Anthropological genetics aims to explain the causes of human diversity over time. Scientists study the human genome to gain a greater
breadth of understanding of human evolution.
Human migration theories
● Multiregional hypothesis (MRE)​ → relies mainly on fossil evidence and suggests all human populations can be traced back to when
Homo erectus f​ irst left Africa about 2 million years ago.
○ Suggests ​gene flow​ (change in allele frequency from individuals leaving and entering) between neighbouring populations
● Replacement hypothesis (Out of Africa)​ → suggests archaic ​Homo sapiens ​left Africa and proposes a second migration happened
about 100 000 years ago. Modern humans of African origin conquered archaic groups and replaced them by interbreeding and
out-competing them.
○ Modern genetic studies have shown that if MRE were correct, modern populations would contain ancient alleles scattered in
different regions of the world. Mitochondrial DNA was used due to a relatively uninterrupted lineage.
■ Humans were grouped according to mutations in mtDNA: members that share the same mutations must be
descendents of a common ancestor (haplogroups). Phylogenetic trees were produced from mtDNA haplogroups.
○ However, genetic evidence favours the Replacement hypothesis as it was discovered that most of the variation in mtDNA
sequences were shown only in African populations. The mtDNA of other ethnic groups represent just a subset of total human
mtDNA diversity (M and N haplogroups).
■ 2 surviving haplogroups (M and N) that colonised other continents are most closely related to the African L3
haplogroup
○ Evidence for the Out of Africa hypothesis suggests that two other species of humans (Neanderthal and Denisovans) had also
migrated out of Africa, interbreeding with our human ancestors in Eurasia. A small amount of Neanderthal DNA was
introduced into the human genome and persists today → most Europeans and Asians have 2% Neanderthal DNA.
Notes for later modules​:
- Occasionally, n
​ ondisjunction ​of chromosomes may occur during meiosis, where sister chromatids do not separate. This results in an incorrect number of
chromosomes in the offspring.
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