Variation and similarities between members can be due to:

• genetic factors
• environmental factors
• both

Genetic or inherited variation is due to the fact that in sexual reproduction the offspring inherit half their genetic information from their mother and half from their father.

Environmental variation can be down to a number of factors including:

• diet
• lifestyle
• accidents
• access to energy sources e.g. food or light

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DNA

DNA’s structure

Deoxyribonucleic acid or DNA is a polymer. Each of its monomers is called a nucleotide and its specific polymer name is polynucleotide.

Nucleotides are composed of:

• a phosphate group
• a pentose sugar called deoxyribose
• an organic base of either adenine, guanine, thymine or cytosine

Adenine and guanine are known collectively as purines and are composed of nine atoms. They have a double ring structure and are the larger of the bases.

Cytosine and thymine are called pyrimidines. They’re composed of six atoms with a single ring structure.

DNA consists of a sugar-phosphate backbone which keeps the molecule stable. When a sugar and a phosphate join this causes a condensation reaction in which water is eliminated. The bond between sugar and phosphate is known as a phosphodiester bond.

DNA is formed of two polynucleotide chains which are bonded together to form a double helix. Each of the chains runs in opposite directions or antiparallel. The backbones of each chain wind around the helix of the axis with the bases on the inside, stacked on top of each other similar to a staircase.

At the end of one chain is a spare 5′ sugar atom while at the other end is a spare 3′ atom. On the other chain the location of the spare sugar atoms is reversed. The bases are reversed all the way up the chains and bond together with weak hydrogen bonds. The bases pair in a specific way:

• adenine with thymine (A-T)
• guanine with cytosine (G-C)

This amounts to one purine with one pyrimidine. This pairing ensures that the distance between the chains is kept constant.

The bond angle of the base pairs causes the spiralling effect and a complete turn is completed at every tenth base pair.