DNA stands for DeoxyriboNucleic Acid. DNA is a polymer; polymers are composed of many individual units called monomers, linked together in a chain.
Nucleotides
The monomer of DNA is the nucleotide. The nucleotide itself is a complex molecule, consisting of three seperate parts:
i. The sugar component: in DNA the sugar component of the nucleotide is a pentose (contains five carbon atoms) sugar called 2’-deoxyribose.
ii. The nitrogenous bases: these are attached to the sugar in the nucleotide. In DNA, any one of four nitrogenous bases can be attached to the sugar. These are adenine (A) and guanine (G), which are purines and thymine (T) and cytosine (C), which are pyrimidines.
iii. The phosphoric acid component: also attached to the sugar.
Polynucleotides
The nucleotides are linked together by their phosphate groups to form a polymer, the polynucleotide. The phosphate group of one nucleotide joins to the carbon ring of the sugar in the next nucleotide. The linkage between the nucleotides is a phosphodiester bond.
An important feature of polynucleotides is that they have chemically distinct ends. This chemical distinction between the two ends means that polynucleotides have a direction. The direction of the polynucleotide is very important in molecular genetics.
Polynucleotides can be any length and have any sequence. For example within the human nucleus, the smallest chromosome contains 50 x 10 6 nucleotide pairs (stretched full-length this molecule would extend 1.7 cm) and there are up to 250 x 10 6 nucleotide pairs in the largest chromosome (which would extend 8.5 cm). In addition, there are no restrictions on the order in which the nucleotides can join together in a single strand. At any point in the polynucleotide theoretically the nucleotide could be A, T, C or G. If we consider a polynucleotide just ten nucleotides in length, it could have any one of 4 10 = 1 048 576 different sequences. Imagine the number of different sequences possible for a polynucleotide a thousand nucleotides in length or one million.
The double helix
Once the structure of the polynucleotide was known, the next step was to discover the exact structure DNA took up within the cell.
Research by Chargaff, discovered that the number of adenines equalled the number of thymines and that the number of guanines equalled the number of cytosines.
The second piece of research was the X-ray diffraction analysis performed by Franklin; this indicated that DNA was a helical molecule.
The famous Watson and Crick took both these pieces of research and decided that the only structure that could fit the data they had on DNA was the double helix.
There are seven important features of the double helix:
i. The double helix consists of two polynucleotides.
ii. The nitrogenous bases are on the inside of the helix with the sugar phosphate backbone on the outside.
iii. The bases of the two complementary polynucleotides are bonded by hydrogen bonding.
iv. The double helix turns every ten base pairs.
v. The two polynucleotide strands are antiparallel, that is one strand runs one direction and the other strand runs the opposite direction.
vi. The double helix contains two different grooves, a major and a minor groove.
vii. The double helix is right handed, this means that if the double helix was a spiral staircase, if you climbed up it, the banister (the sugar phosphate backbone) would be on your right hand side.
Sources:
Genetics – a molecular approach, TA Brown pub. Chapman and Hall.
