Biology AQA AS Unit 2 Notes

For AQA AS Unit 2 - doesn't include the basics of Classification.

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Biology Unit 2 Notes
Variation
The differences that exist between individuals, there are two types:
o Interspecific ­ variation that exists between different species
o Intraspecific ­ differences that occur within a species, caused by genetic and
environmental factors.
Individuals of the same species may seem similar but no two are exactly alike.
Genetic Factors:
All the members of a species have the same genes which makes them come from the same
species, but individuals within a species can have different alleles (different versions of
those genes)
The alleles an organism has make up its genotype - different genotypes result in variation in
phenotype (the characteristics displayed by an organism)
Examples of variation in humans caused by genetic factors include eye colour and blood type
Genes are inherited from parents thus genetic variation is inherited.
Environmental Factors:
Phenotype is also affected by the environment
Plant growth is affected by the amount of minerals, such as nitrate and phosphate, available
in the soil
Fur colour of the Himalayan rabbit is affected by temperature ­ most of its fur is white
except the ear, feet and tail which are black, the black only develops in temperatures below
25 deg. C.
Identical twins are genetically identical ­ same alleles thus any differences are due to the
environment.
Variation is often a combination of genetic and environmental factors. An individual may have the
genetic information for a particular characteristic, but environmental factors may affect the
expression of this characteristic.
In any group of individuals, there is a lot of variation however it's not always clear if this variation is
caused by the genes, the environment, or both.
Overeating ­ thought to be caused only by environmental factors, later discovered that food
consumption increases brain dopamine levels in animals. Once enough dopamine was
released, people would stop eating. Researchers discovered that people with one particular
allele had 30% fewer dopamine receptors. They found that people with this particular allele
were more likely to overeat. Therefore based on this evidence, scientists now think that
overeating has both genetic and environmental causes.
Antioxidants ­ many foods contain antioxidants- compounds that are thought to play a role in
preventing chronic diseases, e.g. berries. Scientists thought that the berries produced by
different species of plant contained different levels of antioxidants because of genetic

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Experiments were carried out to see if environmental conditions affected
antioxidant levels found that the environmental conditions caused a great deal of variation.
Scientists now believe that antioxidant levels in berries are due to both genetic and
environmental factors.
Genetics
DNA is a polynucleotide ­ made up of lots of
nucleotides joined together. Each nucleotide is
made from a pentose sugar, a phosphate
group and a nitrogenous base. The sugar in
DNA nucleotides is a deoxyribose sugar. Each
nucleotide has the same sugar and phosphate.…read more

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Contain linear DNA molecules ­ chromosomes ­ thread like structures each
made up of one long molecule of DNA, so long it has to be wound up around
proteins (histones) to fit into the nucleus.
Histones also help to support the DNA.
DNA + protein are then coiled up v. tightly to make a compact chromosome.
o Prokaryotic cells:
Also carry DNA as chromosomes but it is shorter and circular.
Isn't wound around proteins ­ condenses to fit in the cell by supercoiling.…read more

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Mutations are changes in the base sequence of an organism's DNA. Thus mutations can produce
new alleles of genes. A gene codes for a particular protein so if the sequence of bases in a gene
changes, a non-functional or different protein could be produced. All enzymes are proteins, if
there's a mutation in a gene that codes for an enzyme, then that enzyme may not fold up properly,
producing an active site that isn't complementary (a non-functional enzyme).…read more

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Type of cell division where the cells are diploid to start with, but from meiosis haploid cells
are formed ­ the chromosome number has halved.
The DNA unravels + replicates so there are 2 copies of each chromosome called chromatids.
The DNA condenses to form double-armed chromosomes made from 2 sister chromatids.
Meiosis I (the 1st division) ­ chromosomes arrange themselves into homologous pairs which
then separate, halving the chromosome number.…read more

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The survivors reproduce + a larger population
is created from a few individuals.
E.g. Northern Elephant Seals ­ hunted by humans in the late 1800s, original population was
reduced to around 50 seals ­ reproduced to give a new population of around 100'000 ­
very little genetic diversity compared to the southern elephant seals who haven't ever
suffered such a reduction in numbers.…read more

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Variation in Biochemistry + Cell Structure
Haemoglobin
RBCs contain haemoglobin (Hb) ­ large protein with a quaternary structure ­ 4 polypeptide
chains which each have a haem group that contains iron.
Hb has a high affinity for oxygen ­ each molecule can carry four oxygen molecules, in the
lungs, oxygen bonds to Hb in RBCs to form oxyhaemoglobin ­ reversible reaction ­ when
oxygen dissociates from the oxyhaemoglobin near the body cells, it turns back into
haemoglobin.…read more

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When the curve is steep, a small change in pO2 causes a big change in the amount of oxygen
carried by the Hb.
Hb also gives up its oxygen more
readily at high pCO2 to get more
oxygen to cells during activity.
When cells respire, they produce
Carbon dioxide which increases
the pCO2 increasing the rate of
oxygen unloading, the ODC shifts
down. The saturation of blood
with oxygen is lower for a given
pO2 meaning more oxygen is
being released... BOHR EFFECT.…read more

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Main energy storage material in plants
Cells get energy form glucose, the plants store the excess glucose as starch and when the
plant needs more energy it breaks down the starch to release the glucose.
Mixture of two polysaccharides of alpha-glucose ­ amylose and amylopectin
o Amylose ­ long, unbranched chain of alpha glucose. The angles of the glycosidic
bonds give it a coiled structure, almost like a cylinder thus compact so good for
storage because you can fit many into a small space.…read more

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Some parts of photosynthesis happen in the grana and
others happen in the stroma (a thick fluid found in the chloroplasts).
Cell Cycle + DNA Replication
The cell cycle is the process that all body cells from
multicellular organisms use to grow and divide:
It starts when a cell has been produced by cell
division and ends with the cell dividing to
produce two identical cells.…read more

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