Describe the structure of a DNA molecule and explain how this relates to its function.

DNA is made up of two nucleic acid strands joined by hydrogen bonding. Nucleic acids are polymers made up of many nucleotide monomers linked together by phosphodiester bonds. This type of bond is a covalent link between the phosphate at the 5' carbon of the pentose sugar of one nucleotide, and the hydroxyl group at the 3' postion of another nucleotide. This forms a very strong sugar-phosphate backbone which contributes to the stability of DNA. Each polynucleotide strand has a 5' end and a 3' end, and the two run antiparallel (in opposite directions) within DNA. As well as a pentose monosaccharide and negatively charged phosphate group, each nucleotide wihtin the chain has one of the four nitrogenous bases within its structure. These bases can be divided into the pyrimidines, cytosine and thymine and the purines, adenine and guanine. DNA winds into a double helix, with hydrogen bonding between the bases of each strand like the rungs of a ladder. This structure enables DNA to coil so that the long molecule is compact and lots of information can be stored in a small space. The bases within DNA undergo complimentary base pairing with cytosine forming three hydrogen bonds to guanine, and adenine forming two hydrogen bonds to thymine. Complimentary base pairing means that a larger purine always binds to a smaller pyramidine, keeping a constant distance between the two DNA backbones and resulting in parallel polynucleotide chains. The two strands are thus opposite and complimentary to each other, with sequence of the bases allowing DNA to carry coded information used for protein synthesis. Complimentary base pairing also functions to enable accurate DNA replication, as once the two strands are spearated, complimentary base pairing will reproduce an identical copy of the DNA moelcule. The bases are held together by hydrogen bonds, numerous enough to have a strong cumulative effect on the stability of the DNA molecule as a whole, but weak individually so that they can be broken and DNA can be 'unzipped' easily for replication. Finally, the DNA double helix has the strong sugar-phosphate backbone on the outside with the bases on the inside, making the molecule stable and protecting the vital coding bases from damage.