DNA is a stable polynucleotide
DNA is a long-chain polymer of nucleotide monomers. This polymer is called polynucleotide. A DNA molecule forms when two polynucleotide strands come together form 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. The sugars are pointing in opposite directions.
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 adenine (purine) always pairs up with thymine (pyrimidine), and guanine (purine) always pairs up with cytosine (pyrimidine). 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 complementary.
In a complete DNA molecule, the antiparallel chains twist, like twisting a rope ladder, to form the final structure. This is known as a double helix.

Making copies
When a cell divides, each new cell must receive a full set of instructions. Each cell must have a full copy of all the DNA for that organism. This means that there has to be a way of copying DNA strands precisely. This DNA replication takes place in interphase of the cell cycle and is the process that creates identical sister chromatids.
In order to make a new copy of a DNA molecule:

the double helix is untwisted
hydrogen bonds between the bases are broken apart to 'unzip' the DNA - this exposes the bases
free DNA nucleotides are hydrogen-bonded onto the exposed bases according to the base pairing rules
covalent bonds are formed between the phosphate of one nucleotide and the sugar of the next to seal the backbone

This continues all the way along the molecule until two new DNA molecules are formed, each with an exact replica of the original DNA molecule because of the base-pairing rules.
This process of DNA replication is described as semi-conservative replication. Each new DNA molecule consists of one conserved strand plus one newly built strand. Pioneering work by the scientists Meselsohn and Stahl showed how semi-conservative DNA replication was responsible for the production of new DNA molecules.

Structure to function in DNA

the sequence of bases is an example of information storage. The information is in the form of codes to build proteins.
The molecules are long, so a large amount of information can be stored.
The base-pairing rules mean that complementary strands of information can be replicated.
The double helix structure gives the molecule stability.
Hydrogen bonds allow easy unzipping for copying and reading information.

	Created by siobhan.quirk over 11 years ago

Undergraduate Biology (Biological Molecules) Note on DNA - Information Storage, created by siobhan.quirk on 29/05/2013.