Biology
Mitosis and The Cell Cycle
Both plant and animal cells contain a nucleus. The nucleus contains chromosomes which are
made of the molecule DNA. Body cells (humans) contain two of each chromosomes (they’re
paired), to be exact, human body cells contain 23 pairs of chromosomes. Gametes (sex cells)
do not have paired cells and only have 23 SINGLE chromosomes.
Chromosomes carry lots of GENES which decides many of our features. Human
chromosomes usually contain 100s of genes.
Cells have to be able to DIVIDE for growth and repair. This is called the cell cycle. They are
able to divide through mitosis and meiosis.
The Cell Cycle:
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The 1st stage: The DNA replicates to make two copies of each chromosome and they
remain attached. The cell also grows and copies its internal sub-cellular structures
like the mitochondria and ribosomes.
The 2nd Stage: Mitosis takes place. One set of chromosome is pulled to each end of
the cell. The nucleus also divides.
The 3rd Stage: The cytoplasm and the cell membrane divide to form two identical
cells.
Mitosis is essential for growth and development of multi cellular organisms (plants and
animals).
Mitosis takes place when an organism repairs itself, like when a broken bone heals.
Mitosis happens during ASEXUAL reproduction.
Stem Cells
In the early stages of an embryo, the cells have not differentiated, they are unspecialised
and we call these cells Stem Cells. These cells are capable of differentiating into any type of
body cell. These are EMBRYONIC STEM CELLS.
Stem Cell Definition: An undifferentiated cell which can give rise to more cells of the same
type and can differentiate to form other types of cells.
Stem cells can be found in adults too, in their bone marrow. Unlike embryonic stem cells,
adult stem cells cannot differentiate into ANY other type of body cell. These adult stem cells
form cells found in our blood like red blood cells, white blood cells and platelets.
Leukaemia is a cancer of the bone marrow. To treat it, the existing bone marrow is
destroyed using radiation. The patient receives a transplant of bone marrow from a donor,
the stem cells in this bone marrow divide and form new bone marrow for the patient. They
also differentiate to form blood cells.
There are two problems with bone marrow transplant:
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The donor has to be compatible with the patient or the white blood cells produced
by the donated bone marrow will attack the patients body.
There is a risk that viruses can be passed from the donor to the patient.
Therapeutic Cloning:
An embryo is produced with the same genes of the patient. This means that stem cells from
the embryo can be transplanted into the patient without rejection from the patients
immune system. These embryonic stem cells can differentiate to replace cells that have
stopped functioning.
This technique called therapeutic cloning can be used for a lot of medical conditions such as
diabetes or paralysis.
There are ethical considerations for therapeutic cloning, for example, because it involves the
deliberate creation of what ethical people deem a “human being” to be destroyed later on.
Plants Stem Cells:
Roots and buds of plants contain meristem tissue. These stem cells can differentiate into
any type of plant tissue and at any point in the life of a plant. We can use meristem tissue to
quickly produce clones of plants and it is also cheap. For example, we can produce disease
resistant plants for farmers to use.
Sexual and Asexual Reproduction:
Sexual reproduction includes the fusion of male and female gametes (sex cells) which are
the sperm & egg for animals, the pollen & egg cells for flowering plants.
In Sexual Reproduction, there is a mixing of genetic information (the two gametes fusing)
which leads to offspring variety. The formation of gametes is through MEIOSIS not mitosis.
In Asexual Reproduction, there is only one parent involved and there is no fusion of 2
gametes which means there is no offspring variety, the only thing being produced is
identical offspring (clones) and ASEXUAL REPRODUCTION ONLY INCLUDES MITOSIS.
Advantages and Disadvantages of Sexual Reproduction:
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Produces variation in the offspring, giving species survival advantage through natural
selection.
It gives a species a greater chance of survival if conditions become challenging.
It takes longer than asexual reproduction, as there is a need to find a mate.
Advantages and Disadvantages of Asexual Reproduction:
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Only one parent is needed so there is no need to find a mate, which is a greater
efficiency in the time and energy aspects.
Asexual reproduction is faster than sexual reproduction. It is extremely useful when
conditions are favourable.
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Allows an organism to produce many genetically identical offspring rapidly.
It is very risky, since they are all genetically identical, there is a risk they could all die
if the conditions become unfavourable.
Malaria – in human host, the malaria parasite reproduces asexually. In the mosquito, it
reproduces sexually.
Fungi – reproduces asexually using spores. They can also reproduce sexually.
Flowering plants – can reproduce sexually to produce seeds. Can reproduce asexually and
strawberry is a good example. It does this by sending out “runners”. When the runner
touches the soil it can produce a new plant which is genetically identical.
Meiosis:
Gametes such as sperm and egg cells contain single chromosomes. They are not paired
chromosomes, only single. A human gamete contains 23 single chromosomes while a
human body cell has 46 total chromosomes or 23 PAIRS.
Meiosis only takes place in reproductive organs, like testes in males or ovaries in females.
Meiosis Stages: (starts with a regular human body cell)
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All the chromosomes are copied and cloned from the body cell and the cell divides
into two cells and these two cells divide one more time forming gametes, and the
gametes have 23 SINGLE chromosomes so meiosis has halved the amount of
chromosomes.
Meiosis produces 4 gametes from one regular body cell and these gametes are
genetically different from each other because they have different alleles.
After the gametes are produced through MEIOSIS, they join together (female and male)
through sexual reproduction and this is called fertilisation. The two gametes join their 23
single chromosomes to have 23 PAIRS of chromosomes, or 46 total. The new cell produced
by fertilisation (zygote) divides by mitosis to make identical cells and this is called an
EMBRYO.
The cells differentiate to form different cell types as the embryo develop, for example nerve
and muscle cells in an animal.
DNA and the Genome + Structure:
Chromosomes contain polymers called DNA, DNA is the genetic material in a nucleus. DNA is
a double helix structure, which is two strands of a polymer wrapping around each other.
A Gene is a small section of DNA on a chromosome and for example, 1 gene could
determine blood type.
Proteins are made by joining amino acids together and each gene contains a specific
instruction for a specific sequence of amino acids to make a specific protein. For example
the blood type gene has the sequence of amino acids for the protein that determines blood
type.
The GENOME is the entire genetic material of an organism. For example, the human
genome is the genetical material that makes a human. Scientists have studied the whole
human genome.
This comes with its benefits, for example:
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it helps us search for genes that are linked to a disease like genes that increase
cancer development risk or Alzheimer Disease.
It helps us understand and treat inherited disorders like cystic fibrosis. Cystic fibrosis
being the genetic disease that causes persistent lung infections and limits the ability
to breathe over time.
We can use the human genome to trace human migration patterns from the past.
This helps people discover their ancestry.
DNA Structure
DNA is a polymer made of molecules called NUCLEOTIDES. The structure of nucleotides is a
Sugar, a Phosphate Group and a Base (Adenine or Cytosine or Guanine or Thymine).
The phosphate group is attached to the sugar and the sugar is attached to the base, so the
sugar is in the middle of both. The phosphate group and sugar molecule never change in
DNA. DNA contains four different nucleotides, the nucleotides are only different through
their bases either being A, C, G or T.
DNA strands are complementary, this means that the same bases always pair to the
opposite strands. A always links with T and C always links to G.
A sequence of 3 BASES is the code for a particular amino acid. The order of these bases
controls the order in which amino acids are assembled to produce a specific protein.
A mutation in the gene can change the order in which amino acids are assembled therefore
a different protein than planned is made which can have severe or non severe effects on the
human.
Proteins are made on ribosomes and carrier molecules called transfer RNA bring the amino
acids using the instructions given by the messenger RNA and assemble the protein. When
the protein chain is complete it folds up to make a unique shape and this shape allows the
proteins to do their assigned task as either enzymes, hormones or forming structures in the
body such as collagen.
Mutations occur all the time but most do not alter the protein or they do not alter it much
which means that the function is not changed. However, there are cases in which mutations
alter the shape of the protein which means its function is altered for example, a structural
protein that is meant to be hard and tough like COLLAGEN may be made soft and weak.
Non coding parts of DNA can switch genes on and off so basically they tell the genes when
to produce proteins. Mutations in these non coding proteins can affect how these genes are
switched on or off. A gene may be turned on when it should be off and the effect of this is
the cell will produce a protein that it is not meant to have at that time and could trigger
things like uncontrolled mitosis which leads to CANCER.
Photosynthesis
Plants use light for their source of energy. The reaction used is called Photosynthesis. Since
it takes in energy, it is an endothermic reaction.
Photosynthesis takes place in the leaves of a plant and the leaves contain chlorophyll in the
chloroplasts. The chlorophyll can absorb light energy.
Equation: [Carbon Dioxide + Water = Glucose + Oxygen]
Light energy is used to convert the carbon dioxide and water into glucose. Oxygen is also
produced.
Things that affect the rate of photosynthesis:
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Light intensity: Increasing the amount of light will increase the rate of
photosynthesis because more light energy is available to be used to convert carbon
dioxide and water into glucose so the reaction gets faster. Light intensity can be a
limiting factor if there is not enough light intensity. If there is too much light
intensity, there is a point at which photosynthesis stops increasing and at this point
light intensity is no longer the limiting factor, something else is in short supply like
carbon dioxide concentration.
Carbon Dioxide Concentration: increasing the amount of carbon dioxide increases
the rate of photosynthesis. But at a certain point the rate of photosynthesis no
longer increases.
Amount of chlorophyll: lower chlorophyll means less light is absorbed so there is less
energy; this causes a lower rate of photosynthesis.
Temperature: as we increase the temperature, the enzymes involved in
photosynthesis work faster so rate of photosynthesis increases. However, if the
temperature is too high, the enzymes denature and this causes the rate to fall.
Uses of Glucose:
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It releases energy in respiration which takes place in the mitochondria.
Produce the insoluble storage molecule called starch. The starch can be converted
back to glucose when it is needed.
Produce fats and oils and are a storage form of energy.
Produce cellulose (for the cell wall in a plant) and provides the plant strength.
Produce amino acids which is used to make proteins.
Genetic Inheritance:
DNA is found in chromosomes, and humans have 23 pairs. One chromosome in the pair
comes from the father, and the other comes from the mother. There are genes on every
chromosome.
Genes often come in different versions, and these different versions are called alleles. Since
chromosomes come in pairs, there are two copies of every gene.
E – black hair
e – brown hair
The genotype of a person tells us the alleles present. For example, a person’s genotype can
be “ee”, where lowercase ‘e’ can resemble brown hair. Since both alleles are lowercase ‘e’,
the person is said to be homozygous where the person has two copies of the same allele.
The phenotype of a person tells us the characteristics caused by the person’s alleles. So for
the example above, the person having the genotype “ee”, their phenotype is having brown
hair.
However, if someone’s genotype is “Ee” for example, where they have two different alleles,
it is called heterozygous. The phenotype for the genotype “Ee” will be black hair because it
is the dominant allele, a dominant allele can show characteristics even if there is only one
present. The allele for brown hair will be recessive and the phenotype for brown hair can
only show if there are two recessive alleles present, not one.
Inherited Disorders
Cystic fibrosis is a disorder of cell membranes. It is controlled by a single gene that is
recessive.
The allele for normal cell membrane function is dominant with the symbol C and the allele
for defective cell membranes is recessive with the symbol c. In order to have cystic fibrosis,
the person has to have two copies of the defective allele, which means they have to inherit
a defective allele from both parents.
A carrier of the cystic fibrosis allele is a person that has a Cc genotype, where the allele for
cystic fibrosis is recessive but they still have the allele present and can pass it on.
If a father who Is a carrier (Cc genotype) reproduces with a mother who is homozygous with
normal membrane function (CC genotype), then 50% of the offspring will have homozygous
genotype of CC, and the other 50% will have carrier genotypes for cystic fibrosis which is Cc.
This simply is a probability, meaning on average 50% of the offspring will be not affected,
and 50% will be carriers. However, since they are probabilities, there is a chance that all
offspring become carriers, or all offspring are unaffected.
Polydactyly is when people have extra fingers or toes. It is caused by a dominant allele.
You cannot be a carrier of polydactyly. If you have a dominant allele then you will have that
characteristic.
Solution to Inherited Disorder:
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Embryo Screening: the embryos are tested to see if they have the allele for inherited
disorders. Embryos which don't have the defective allele are implanted into the
woman. This can develop into healthy offspring.
Embryo Screening is expensive and some people think that the money should be
spent elsewhere in the health service.
Often a large number of embryos are created but only a small number are used. This
means that some healthy embryos are destroyed and some people think that that is
unethical.
We may be able to screen embryos to produce offspring with desirable features, like
taller, and many people find this unethical.
Gene therapy: correcting faulty alleles and use this to treat inherited disorders.
Evolution:
Nearly 9 million different species of animals and plants on Earth.
Scientists believe that life first developed on Earth more than 3 billion years ago. These first
life forms were very simple, like single cells. All living things have evolved from these simple
life forms. Scientists call this process evolution by natural selection.
For example:
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If the environment gets a lot colder, rabbits with the alleles for thicker fur that they
have inherited will be able to survive the colder temperatures far better than the
rabbits that did not inherit the thicker fur allele. Because the rabbits with thicker fur
can survive better in the cold, they are more likely to reproduce and their offspring
could inherit the allele for thicker fur and these offspring are also more likely to
survive the cold and reproduce.
This causes the allele for thicker fur to become more common among the population
of rabbits.
Definition of Evolution: The change in the inherited characteristics of a population over
time through a process of natural selection.
Sometimes, two populations of one species can become so different in phenotype that they
can no longer interbreed to produce fertile offspring. These two populations have become
two separate species.
Cloning
Animal Cloning Stages:
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Start with sperm and egg cell from horses with desirable characteristics. Fertilisation
produces a fertilised egg with the sperm and egg cell.
We then allow the fertilised egg to develop into an early stage embryo. The cells in
this embryo must not have started to specialise.
Use a glass rod to split the embryo into two. Transplant the two embryos into host
mothers. The embryos will grow and develop then will be born. When they are born,
we will get two identical offspring (clones).
We cannot be certain that the offspring will have the desired characteristics.
Adult Cell Cloning Stages:
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Remove a cell from the animal we want to clone like a skin cell. Remove the nucleus
from this cell.
Take an unfertilised egg cell from the same species. Remove the nucleus from the
egg and throw it away.
Insert the nucleus from the skin cell into the empty egg cell. The egg is given an
electric shock which makes the egg cell divide to form an embryo. The embryo
contains the same genetic information as the adult skin cell.
When the embryo develops into a ball of cells it is inserted into the adult female
womb to continue development. The host mother gives birth to the clone. The clone
will not look like the mother, but will look as the same as the adult skin cell host.
Theory of Evolution:
Charles Darwin is the author of the Theory of Evolution.
Animals with the most favourable characteristics suited to the environment that they have
inherited from their parents are more likely to survive and have offspring with the same
characteristics.
1859 – Darwin published the theory in a book called “on the origin of species”.
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It was controversial and only gradually started to be accepted. This is because at that
time, religion was a big thing and they believed God was the reason everything came
into being. Darwin’s theory challenged that idea.
Scientists also believed that Darwin did not have enough evidence for his theory and
scientists did not understood how characteristics were inherited.
Lamarck – suggested that when a characteristic is regularly used it becomes more
developed, and this strengthened characteristic is passed on. There is one problem, we now
know that changes that occur in an organism’s lifetime cannot be passed onto offspring.
Lamarck is incorrect.
Gregor Mendel – heavily developed our idea on genetics. Mendel’s units were renamed
genes.
Classification:
Carl Linnaeus Classification System
Animal Kingdom and Plant Kingdom were the two Kingdoms created by Carl.
These kingdoms were divided into:
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Phylum
Class
Order
Family
Genus
Species
King Philip Came Over For Good Soup
Binomial Systm – The genus and species put together as a name.
The Human Nervous System
Consists of two parts:
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Central nervous system: brain and spinal cord
Nerves that run to and from the central nervous system.
Neurone = nerve cell
Receptors detect a stimulus and send electrical impulses down neurones to the central
nervous system. The Central nervous system sends electrical impulses down other
neurones to effectors and the effectors bring about a response. Effectors are usually
muscles that contract or glands which secretes a hormone.
The central nervous system enables humans to react to their surroundings and coordinate
their behaviour. One way to do this is by the reflex arc.
Reflex Arc when touching hot object:
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Stimulus is detected by a receptor, and the stimulus in this case is heat and the
receptor is in the skin.
Impulses are sent from the receptor along a sensory neurone to the CNS. Sensory
neurones are connected to receptors. At the end of the sensory neurone there is a
junction called a synapse and at the synapse, a chemical is released and this diffuses
across to a relay neurone in the CNS where it triggers an electrical impulse.
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The impulse is passed across the relay neurone and reaches another synapse. A
chemical is released and this triggers an electrical impulse in a motor neurone. The
electrical impulse passes down the motor neurone to an effector. The effector is a
muscle which contracts and pulls the hand away from the heat.
Pulling the hand away is the response.
The brain makes decisions about what action to take. In the case of reflexes though, there is
no decision making by the conscious part of the brain which makes reflexes automatic and
rapid which help to protect us from danger.
The Brain:
The brain plays a critical role in the CNS. It controls complex behaviour and to do that it
contains billions of interconnected neurones. Different parts of the brain carry out different
functions.
Parts:
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Cerebral Cortex – Highly folded outer part of the brain
Functions include language, memory and consciousness.
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Cerebellum
Controls our balance and co-ordinates our movement.
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Medulla
Controls our heart rate and breathing rate.
---Since the brain is protected by the skull, it is tricky to access and the structures of the brain
are extremely complex. The brain is also delicate and easy to damage.
Three ways scientists use to investigate the brain:
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Trying to see where the damage (in the case of brain damage) has taken place to link
that part of the brain to its function.
Electrically stimulating parts of the brain and look at the effects on person’s
behaviour. This allows us to narrow down specific regions to their functions.
MRI scanning to look at which parts of the brain are most active during different
activities.
The Eye:
The eye is a sense organ and it contains receptors that are sensitive to both light intensity
and the colour of light.
Parts of the Eye and the Functions:
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Cornea: Light rays pass through the transparent front of the eye, and it starts the
focussing of light rays.
Pupil: The light rays pass through the pupil
Lens: the rays pass through the lens and it focuses the light rays onto the back of the
eye, the lens can change its shapes allowing us to focus on distant or near objects.
Retina: When the light rays focus on the back of the eye, the retina which contains
receptor cells for light that detects light intensity and colour. The retina sends
electrical impulses down the optic nerve to the brain through its receptor cells.
Sclera: the white part of the eye which protects the eye with its tough outer
structure.
Ciliary muscles and Suspensory Ligaments: works with the lens to allow us to focus
on distant and near objects.
Iris: coloured part of the eye which has a space in the centre called the pupil where
light passes through. The iris controls the size of the pupil.
When entering a dark room, the amount of light entering the eye is low, the drop in light
intensity is sensed by light receptors in the retina and it sends electrical impulses to the
brain. The brain sends impulses to muscles in the iris which contract, causing the pupil to
become larger. This allows more light to enter the eye. This is a reflex action.
When entering a bright room, the reflex causes the pupil to become smaller which reduces
the amount of light entering the eye and protects it from damage.
How the eye focuses:
The lens changes its shape to focus on objects. This is caused accommodation.
The lens is surrounded by ciliary muscle which is connected to lens through suspensory
ligaments. By contracting or relaxing, the ciliary muscle can change the thickness of the lens.
When the ciliary muscle contracts, the suspensory ligaments loosen, and the lens becomes
thicker and refracts light rays more strongly.
When the ciliary muscle relaxes, the suspensory ligaments are pulled tightly, and the lens is
pulled thin and only slightly refracts light rays.
Distant Objects:
Light from distant objects needs to be focuses only a relatively small amount. So for this
case, the ciliary muscle relaxes and the suspensory ligaments tighten, which causes the lens
to become thin. Since the lens becomes thin, the light rays are only slightly refracted, and
the light rays are focused to a point on the retina.
Near Objects:
Light from near objects needs to be focused a large amount. So for this case the ciliary
muscle contracts and the suspensory ligaments are loosened. The lens is now thicker and
refracts light rays more strongly. The light rays are now focused to a point on the retina.
What is the reason why we can only see black and white in the dark?
This is because we have two types of receptor cells making up the Retina. These are called
the:
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Rod Cells – sensitive to light intensity but only works in black and white.
Cone Cells – sensitive to the colour of light and allows us to see in colour. They
don’t work well in low light conditions.
In dark conditions, only rod cells are available to respond to the brain which means that
we are not able to see colour since cone cells do not work well in low light.