AQA AS Biology Unit 2 Revision

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AS Biology
Unit 2 Revision
By Lisa :D

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Interspecific variation: variation between species
Intraspecific variation: variation within species.
Making measurements
Biologists cannot take measurements from every single living organism about
the things they want to know, so they take a sample. E.g. the number of red
blood cells in 1mm3 of blood varies a lot, so many measurements have to be
taken to determine the mean number.
This takes measurements of individuals from the target population. This should
be representative of the whole population.…read more

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Use means, and standard variation to see how much chance has influenced the
Causes of variation
Genetic differences:
-Mutations: changes in genes and chromosomes may be passed on.
-Meiosis: cross-over and independent assortment causes variation as it
increases the different combinations of genes.
-Fusion of gametes: this is completely random from each parent, and increases
variety of offsprings.
The alleles that individuals have in the species means they have different
genotypes (combination of genes), and so will have different phenotypes
(appearances displayed).…read more

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Variation due to environmental influences
The variation is due to environmental factors. E.g. Height and weight. This is
not controlled by a single gene, but many genes (polygenes). The data from
this type of variation shows a normal distribution.
Mean: the maximum height of the curve.
Standard deviation: how spread out the data is.…read more

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A smaller standard deviation (blue, thinner curve) means there is less variation,
a larger standard deviation (red, wider curve) means there is more variation.…read more

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Structure of DNA
DNA is made of individual nucleotides which are
made of:
-Nitrogenous base
Two nucleotides may be linked together by condensation, forming a
dinucleotide. Many of these nucleotides together forms a polynucleotide.
DNA is made of two long strands of polynucleotides. The two strands are joined
by hydrogen bonds.…read more

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Function of DNA
It's stable, and can be passed from generation to generation without
The two strands are only joined by hydrogen bonds, so it can separate
during DNA replication and protein synthesis.
It carries a lot of information
The base pairs are protected by the Deoxyribose-phosphate backbone.
8.2 The Triplet Code
Genes are sections of DNA that contain the coded information for making
polypeptides.…read more

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Features of the triplet code
Non-overlapping: each base in the code is read only once, so 123456 is
read as 123, 456. Not 123, 234, 345, 456.
Degenerate: most amino acids are coded by more than one combination
of triplet codes because there are 64 combinations for 20 amino acids.
Universal: it makes the same amino acid wherever it is. E.g. in plants, or
Three triplet codes do not code for any amino acids, they just mark the
ends of a polypeptide.…read more

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Diploid and Haploid
Diploid: A cell/organism which contains two sets of chromosomes. It's written
as 2n. E.g. in humans, 2n=46.
Haploid: A cell/organism which contains a single set of chromosomes. Shown
as n. E.g. in humans, n=23.
Homologous chromosomes
Chromosomes occur in pairs because the sexually produced organisms need a
set of chromosomes from both the mother and father. So you get a pair of
chromosomes from both parents. Each homologous pair of chromosomes
contains the same genes, with different alleles. E.g.…read more

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Meiosis and genetic variation
First division: The homologous chromosomes pair up, and their chromatids
wrap around each other. Crossing over occurs, where portions of the
chromatids swap places.
Second Meiotic division: chromatids move apart.
Genetic Variation is caused by:
Independent assortment of homologous
Crossing over
Key words
Gene: a section of DNA that codes for a
Locus: position of a gene on a chromosome
Allele: one of the different forms of a particular
gene.…read more


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