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AS Biology
Unit 2 Revision

By Lisa :D

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7.1 Variation

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…

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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…

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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…

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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.

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8.1 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…

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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.


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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.…

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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…

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8.4 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

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