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Cell Division – Revision Pack (B3)
Multicellular:
Advantages of being multicellular:
• allows organism to be larger
• allows for cell differentiation
• allows organism to be more complex.
Becoming multi-cellular requires the development of specialised organ systems,
limited to:
• communication between cells (nervous system)
• supplying the cells with nutrients (digestive system)
• controlling exchanges with the environment (respiratory and excretory
system)
Mitosis
New cells for growth are produced by mitosis, new cells are genetically identical
because they contain the same genetic information – it is a copied cell. Body cells
are DIPLOID (contain two copies of each chromosome).
The first step, before cells can divide is to replicate the DNA. This is done by:
-
‘unzipping’ the chromosome to make it into single strands
New strands forming by complementary base pairing
The following steps happen in mitosis:
1) The chromosomes line up in the centre
of the cell
2) They then divide; or are divided by an
enzyme
3) The two copies move to two poles of
the cell
These cells are genetically identical.
Meiosis:
Cell Division – Revision Pack (B3)
Gametes are produced by meiosis.
Gametes are haploid (contain one
chromosome from each pair). In
meiosis, the chromosome number is
halved and each cell is genetically
different because:
• One chromosome from
each pair separate to
opposite poles of the cell in
the first division
• Chromosomes divide and
the copies move to opposite
poles of the cell in the
second division.
Gametes and Fertilisation:
Fertilisation results in genetic
variation because:
-
-
The two gametes (egg
and sperm cells) form a
diploid zygote
Genes from the two
chromosomes combine
to determine the
characteristics of that
zygote
The structure of a sperm cell allows it to
be adapted to its function because:
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It has a lot of mitochondria which
supplies energy
It has an acrosome releases
enzymes that help digest (enter)
the egg membrane
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Cloning Animals:
Dolly the sheep was produced by a technique called nuclear transfer. This process
follows six simple steps:
STEP 1: The egg cell from one sheep (sheep A) is removed
STEP 2: A cell was taken from the udder of another sheep (sheep B)
STEP 3: The nucleus from sheep B is put into the egg of sheep A
STEP 4: This embryo is then given an electric shock to encourage it to divide
STEP 5: The embryo is then implanted into a surrogate mother sheep
STEP 6: The embryo then grew into a clone of sheep B
This diagram
is not linked
to the text
above;
sheep A and
B are
different in
both cases
Animals could be cloned to:
-
Mass-produce animals with desired characteristics
Provide human products through producing GM animals
Produce human embryos to support stem cells for therapy
Cloning Plants:
Cloning plants is done by a technique called Tissue Culture; this is very easy to do
and only takes three steps:
STEP 1: Choose a plant with the desired characteristics
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STEP 2: Cut a large number of small pieces of tissue from the plant
STEP 3: Grow these small pieces of tissue in test tubes or dishes containing a growth
medium
(NOTE – aseptic methods are used to stop the plants being infected by microbes)
Advantages of cloning plants
Can be sure of the characteristics
of the plant since all plants will be
genetically
identical
It is possible to mass produce plants
that may be difficult to grow from seed
Disadvantages of cloning plants
If plants become susceptible to
disease or to change in environmental
conditions
then all plants will be affected
Lack of genetic variation
Additional Notes:
There are ethical issues concerning human cloning because people think that the
clones will not be ‘true individual’.
Cloning animals is useful in a number of ways, but it always carries risks. For example,
there is suggestion that GM animals could supply replacement organisms for
humans. Some people think that this could lead to disease being spread from
animals to humans.
Cloning animals is harder than cloning plants because animals lose their ability to
differentiate early on in life, while plants retain the ability to differentiate. In other
words, animals become specialised for one purpose and cannot change into
different types of cells early on in their lives; it is hard to change this.
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Different types of cells:
Plant cell
Animal cell
Bacterial cell
Cell wall
Chromosomes.
presence of a nucleus
NO cell wall
Chromosomes.
presence of a nucleus
Sometimes has a cell wall
single circular strand of DNA
Absence of a nucleus,
mitochondria and chloroplasts
Measuring Growth:
There are two phases of rapid growth in humans; the first is just after birth and the
second is during adolescence.
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Girls start their second growth
spurt at about 10 years old
and are normally finished by
14.
Boys start at about 11 and
finish by the time they are 18.
In both cases as the years go
on the cm grown per year
decreases after the initial
spurt.
There are different ways of measuring growth:
Method
Wet
mass
Disadvantages
Hard to do for some organisms;
water content of organisms can
vary
Dry Mass Can only be measured by
driving off the water in an
organism and thus killing it
Length
Only measures in 1 direction
Differentiation:
Advantages
Easy to do with organisms like
humans
However, it does measure the true
growth of the whole organism
Easy to do
When a cell in ‘undifferentiated’, it can develop into different cells, tissues and
organs; stem cells are an example of an ‘undifferentiated’ cell.
Stem cells can be obtained from embryos and could be potentially used to treat
many medical conditions including Parkinson’s disease and paralysis.
Many people have a problem with the use of stem cells because they think that it’s
wrong that embryos are destroyed. However, some people think that this is an
acceptable issue because it can be used to treat life-threatening diseases.
Stem cells are also found in adults but these do not offer the same range types that
embryonic cells do; also adult stem cells are not as easy to find.
Plant and Animal Growth:
Plant and animal growth are very different:
Feature
Pattern of growth
Plants
Often can grow continuously
How growth
happens
Where cell division
happens
Mainly by cell enlargement
(increase in cell size)
Mainly at meristems – found
at the tips of shoots and roots
Animals
Tend to grow to a maximum
size
Increasing the number of
cells
In most tissues
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Cell differentiation
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Many cells can differentiate
Most cells lose the ability to
differentiate at an early
stage
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Cell Structure:
Liver and muscle cells have large numbers of mitochondria due to large amounts of
respiration taking place. Some structures in cells, such as ribosomes, are too small to
be seen with the light microscope. Ribosomes are in the cytoplasm and are the site
of protein synthesis.
DNA Structure and Genetic Code:
Describe the structure of DNA as two strands coiled to form a double helix, each
strand containing chemicals called bases, of which there are four different types,
with cross links between the strands formed by pairs of bases.
-
A chromosome is a long, coiled molecule of DNA, divided up into regions
called genes.
Each gene contains a different sequence of bases and codes for a particular
protein.
Proteins are made in the cytoplasm; a copy is made from the nucleus
because the gene itself cannot leave the nucleus.
The four bases of DNA are A, T, C and G. Complementary base pairings: A – T
and G – C.
The base pairs
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Protein structure is determined by the DNA base code, to include: the base
sequence determines amino acid sequence and each amino acid is coded
for by a sequence of 3 bases
Ribosomes are the site of protein synthesis. They are found in the cytoplasm but DNA
is found in the nucleus. The genetic code needed to make a particular protein is
carried from the DNA to the ribosomes by a molecule called mRNA. Making:
-
mRNA from DNA is called transcription
Proteins from mRNA is called translation
Ribosomes are smaller than mitochondria and are found in the cytoplasm. They are
too small to be seen with a light microscope.
The code needed to produce a protein is carried from the DNA to the ribosomes by
a molecule called mRNA.
DNA controls cell function by controlling the production of proteins, some of which
are enzymes.
Discovery of DNA Structure:
Watson and Crick used data from other scientists to build a model of DNA, using Xray data showing that there were two chains wound in a helix and data indicating
that the bases occurred in pairs.
New discoveries, such as Watson and Crick’s, are not accepted or rewarded
immediately. Here are reasons why:
Cell Division – Revision Pack (B3)
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Other scientists repeating or testing the work gives public confidence if their
studies back up the original studies
Other theories were more widely accepted
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Selective Breeding:
There are many problems with selective breeding; sometimes SB can lead to the
inbreeding, where two closely related individuals mate. This can cause health
problems for the species.
Inbreeding can lead to a reduction in the variety of alleles in the population (this is
also known as the gene pool). This can lead to:
-
An increased risk of harmful recessive characteristics showing up in offspring
A reduction in the ability to change easily; lack of variation
Genetic Engineering:
Key examples of genetically engineered organisms include:
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Beta-carotene being put in rice to increase vitamin A content for countries
where there is little vitamin A and a reliance on rice
Bacteria that have been made to contain human insulin
Crop plants have been engineered so that they are frost or damage resistant
There are advantages and disadvantages to genetic engineering:
Advantage
Organisms with the desired
characteristics can be created quickly
and efficiently
Disadvantage
There is a risk that the inserted genes
may have unprecedented and harmful
side effects
There are also quite a lot of ethical issues surrounding genetic engineering:
-
Some people think it is just morally wrong
Others think that there may be long-term side effects like a damage to
ecosystems by GE animals / plants
There are four steps to genetic engineering:
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STEP 1: Identify the desired characteristic
STEP 2: The desired genes are identified and removed from an organism (isolation)
STEP 3: These genes are then inserted into another organism
STEP 4: These organisms then reproduce and replicate
(This is shown on the following page in a diagram as an example)
Gene therapy:
The use of genetic engineering to change a person’s genes and cure certain
disorders is called gene therapy.
Gene therapy could involve gametes or body cells. The changing of gametes is the
most controversial because it could lead to ‘designer babies’.
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Types of Proteins:
Proteins are made of long chains of amino acids. Each protein has its own number
and sequence and number of amino acids, which results in differently shaped
molecules, which have different functions.
The function of proteins includes:
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Structural – used to build cells and tissues (limited to collagen)
Hormones – carry messages to control a reaction (limited to insulin)
Carrier Molecules – self explanatory (limited to haemoglobin which carries
oxygen)
Enzymes
Enzymes:
Describe enzymes as:
-
Biological catalysts (speed up reactions in the body)
Catalysing chemical reactions occurring in living cells: respiration,
photosynthesis, protein synthesis
Having a high specificity for their substrate.
The substrate molecule fits into the active site like a key into a lock:
-
This is why enzymes are described as working in a ‘lock and key mechanism’
It also explains why each enzyme can only work with a certain substrate. This
is called specificity and happens because the substrate must be the right
shape to fit into the active site
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Enzymes work best at a particular temperature and pH. This is called optimum and
any change away from either optimum will slow the reaction down.
When explaining how enzyme activity is affected by pH and temperature, include:
-
lower collision rates at low temperatures
denaturing at extremes of pH and high temperatures
denaturing as an irreversible change inhibiting enzyme function
denaturing changing the shape of the active site
It’s possible to work out how temperature affects the rate of reaction by calculating
the temperature coefficient, called Q10. This is done for a 10oC change in
temperature using:
Q10 = rate at higher temperature
Rate at lower temperature
Mutations:
Gene mutations may lead to the production of different proteins. Mutation may
occur spontaneously but can be made to occur more often by radiation or
chemicals. Mutations are often harmful but may be beneficial or have no effect.
Only some of the full set of genes is used in any one cell; some genes are switched
off. The genes switched on determine the functions of a cell.
Changes to genes alter, or prevent the production of the protein which is normally
made, this is because they change to base code of DNA, and so change the order
of amino acids in the protein.
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Aerobic Respiration:
The symbol equation for aerobic respiration:
C6H12O6 + 6O2→ 6CO2 + 6H2O
Glucose + oxygen → carbon dioxide + water
Measuring Respiration Rate:
Two different experiments can be used to measure rate of respiration. These two
ways involve:
-
Measuring how much oxygen is used up (the faster it’s consumed, the faster
the respiration rate
The rate at which carbon dioxide is made
The rate of oxygen consumption can be used as an estimate of metabolic rate
because aerobic respiration requires oxygen using the respiratory quotient (RQ)
formula:
RQ = carbon dioxide produced / oxygen used
The rate of respiration is influenced by changes in temperature and pH. This is
because enzymes are involved in respiration, and their activity varies with
temperature and pH.
Anaerobic Respiration:
Muscles often don’t receive sufficient oxygen during exercise. They start to use
anaerobic respiration as well as aerobic respiration. The word equation is:
Glucose  Lactic Acid (+ energy)
Anaerobic respiration makes lactic acid which builds up in muscles. This can cause
extreme pain and fatigue (tiredness).
It also released much less energy per glucose molecule than aerobic respiration.
Oxygen debt is when the incomplete breakdown of glucose results in the build up of
lactic acid.
Sometimes during recovery the heart rate and breathing rate stay high; this is
because:
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Rapid blood flow allows the lactic acid to be carried away to the liver
Extra oxygen can be supplied, enabling the liver to break down the lactic
acid
ATP:
ATP is a substance that is used as the energy source for many processes in cells. ATP
is produced as a result of respiration. For example, one glucose molecule can
release enough energy during respiration for the production of:
-
38 ATP molecules by aerobic respiration
-
2 ATP molecules by anaerobic respiration
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Blood:
Red Blood Cells:
Feature
Reason
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Small size
Flattened disc shape
Contains
haemoglobin
Does not contain a
nucleus
Lets red blood cells pass through narrow capillaries
Provides a large surface area, allowing rapid diffusion of
oxygen
Haemoglobin absorbs oxygen in the lungs and releases
oxygen in the rest of the body
Increases amount of space inside the cell for hemoglobin
Plasma:
Plasma transports dissolved substances around the body, including:




Hormones
Antibodies
Nutrients, such as water, glucose, amino acids, minerals and vitamins
Waste substances, such as carbon dioxide and urea
Haemoglobin:
Haemoglobin in red blood cells reacts with oxygen in the lungs to form oxyhaemoglobin. When this oxy-haemoglobin reaches tissues it releases the oxygen.
The flattened disc shape of red blood cells (biconcave shape) provides larger
surface area to volume ratio to exchange oxygen more quickly; it is the
haemoglobin in red blood cells that combines with oxygen.
Blood Vessels:
Arteries:
-
transport blood away from the heart
have thick muscular and elastic walls to resist the high pressure
Veins:
-
transport blood to the heart
have large lumen and valves to try and keep the blood moving back to the
heart because the pressure is low
Capillaries:
-
exchange materials with tissues; link arteries to veins
have thin, permeable walls that allow substances to be transferred between the
blood and the tissues
The Heart:
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The right side:
-
Vena Cava (vein) brings deoxygenated blood from the body
This passes through the tricuspid valve into the right ventricle
The right ventricle then pumps blood up through the pulmonary artery
The pulmonary artery then takes deoxygenated blood to the lungs where it gets
oxygenated
The left side:
-
The pulmonary vein brings oxygenated blood from the lungs
This is passed through the bicuspid valve
The left ventricle then pumps this blood up through the aorta
The aorta then takes this oxygenated blood all around the body
Additional Notes:
The bicuspid and tricuspid are there to prevent any back-flow.
This is known as a ‘double-circulatory system’. This means that the blood is at a much
higher pressure and gets to the tissues at a faster rate.
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