2.1.16 Biology- DNA

DNA is a stable polynucleotide:

DNA is a long-chain polymer of nucleotide monomers. This polymer is called a polynucleotide. A DNA molecule forms when two polynucleotide strands come together. They form what looks like a ladder – the sugar phosphate backbones of the two chains from the uprights, and the bases project towards each other to form the rungs.

Hydrogen bonds between the bases in opposite uprights strengthen the rungs of the ladder. This makes DNA a very stable structure, which is vital as it carries the instructions to make an organism, if it were unstable, the instructions could go wrong too easily.


Hydrogen bonding and base pairing – getting it right:

The two DNA strands run parallel to each other because the space between them is taken up by the nitrogenous bases projecting inwards. The term ‘antiparallel is used because the strands run in opposite directions to each other (so they can be joined together).

The chains are always the same distance apart because the bases pair up in a specific way. Where a pyrimidine appears on one side, a purine appears on the other. Even more important is that where the purine is adenine (A) the pyrimidine is always thymine (T). Where the purine is guanine (G), the pyrimidine is always cytosine (C). As the strands come together, hydrogen bonds form between the bases. The different structure of the bases means that the base-pairing rules always apply. The base pairing is described as complimentary. So A is complimentary to T and


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