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Types of Variation
· Variation due to genetic factors.
· In genetic variation, organisms fit into a few distinct groups e.g. Blood group, eye
colour, and is usually represented by bar graph or pie chart as the data is discrete.
· Variation due to environmental factors.
· Continuous data, so represented by a line graph, examples are height and weight.
These depend on how we are nurtured, e.g. if we eat enough protein and exercise
regularly, we will grow to our full height potential.
· Normal distribution curves on graphs are used in means and standard deviation.
The mean value is a measure at the highest point on the graph. The standard
deviation is a measure of the width of the curve and shows the spread of results.
· 1 standard deviation ­ 66% (measured either side of the mean at 33% each side)…read more

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Structure of DNA
· Nucleotide structure consists of DEOXYRIBOSE (a pentose sugar), A PHOSPHATE
GROUP and AN ORGANIC BASE.
· Single-ring bases (pyramidines) ­ cytosine and thymine.
· Double ring bases (purines) ­ adenine and guanine.
· These constituents are bonded together as the result of a condensation reaction
(where water is removed). This gives a mononucleotide. Bonding 2
mononucleotides together gives a dinucleotide. Continuous bonding makes a
polynucleotide.
· When 2 polynucleotides join together, running in anti-parallel directions and
bonded together by the bases, we get a double helix.
· Adenine always bonds to thymine by 2 hydrogen bonds.
· Guanine always bonds to cytosine by 3 hydrogen bonds.
· This is known as complimentary base pairing.…read more

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Function of DNA
· DNA is the hereditary material responsible for passing genetic information from
cell to cell. It is adapted to carry out the function in a number of ways:
· It is very stable and in most cases can be passed from generation to generation
without changing.
· The 2 strands are joined by hydrogen bonds that allow them to split during DNA
replication.
· It is very large and therefore carries a lot of information.
· By having the base pairs inside the molecule, they are to some extent, protected
by the phosphate-sugar "backbone" from outside physical and chemical forces.
· The function of the DNA depends on its base sequences.…read more

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The Triplet Code
· Genes are sections of DNA that code for the production of particular polypeptides.
The coded information is in the form of a particular sequence of bases along the
DNA strand.
· Polypeptides combine to form proteins and so genes determine the proteins of an
enzyme, therefore are vital as enzymes control the body's chemical reactions.
· Genes determine the development and nature of all organisms.
· There are 3 bases required to make an amino acid.
· There are 20 amino acids therefore there needs to be at least 3 bases to achieve
the correct number of combinations. (1 base would give 4 amino acids, 2 bases
would code for 16 amino acids, 3 bases would code for 64 amino acids)
· Amino acids can have more than one code (this is known as degenerate), and
some combinations are `start' and `stop' codes.
· Methionine is always the first amino acid in a chain (it is the start code).…read more

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DNA and Chromosomes
· In prokaryotic cells such as bacteria, DNA molecules are smaller, form a circle and
are not associated with proteins and therefore do not have chromosomes.
· In eukaryotic cells such as human, DNA molecules are larger, are linear and occur
in association with proteins to form chromosomes.
· Chromosomes are only visible as distinct structures when a cell is dividing. For the
rest of the time they are widely spread out in the cell as long DNA strands.
· DNA in double helix DNA combines with proteins DNA-Protein complex is
coiled Coils fold to form loops Loops coil and pack together to form a
chromosome.
Chromatid
Centromere…read more

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