Deoxyribonucleic acid (DNA) is made up of nucleotides (phosphate esters of nucleosides), consisting of a 5-carbon sugar, attached to a phosphate at the 5′ carbon, and a base at the 1′ carbon. The bases include the pyrimidines cytosine, and thymine, and the purines guanine, and adenine. The nucleotides join together through phosphodiester bonds, to form a chain where the hydroxyl end is always free (3′-OH), and the strand starts from the 5’PO4 end. The DNA structure is usually in the form of two anti-parallel strands which spiral around each other base pairing using weak hydrogen bonds. Adenine pairs with thymine with a double hydrogen bond, and Guanine pairs with cytosine with a triple hydrogen bond.
The DNA has a phosphate backbone, which is negatively charged, and so can form weak interactions. It has major, and minor grooves present due to its spiral nature, proteins specifically interact with these regions of the DNA, mainly the major grooves. B-DNA is the usual type of DNA, right handed, bases 3.4A apart, 10 bases each turn, with anti-parallel strands. There is also A-DNA, which is usually found at low humidity, which is also right handed, but has 11 bases per turn, and is tilted on its axis with a central hole. Z-DNA is very messy, with bases sometimes sticking out on the outside of the DNA, it is also a left handed spiral, and looks like a stretched out version of the typical B-DNA.
DNA can be separated, for example when it is heated, however it also re-anneals again after wards, this leads to hybridization as strands can re-anneal to different strands than they started with. Sometimes due to inverted repeat sequences, the same piece of DNA can anneal to itself, this can cause hair pin, and cruciform structures. It can also form tetraplexes (unusual DNA which plays a role in metabolism), which can be parallel, or anti-parallel, and are four stranded structures, for example the guanosine tetraplex.
DNA is usually visually seen by scientists packaged into chromosomes, this tight packaging involves two coils of DNA around histone protein complexes (nucleosomes). These nucleosomes are packaged into a 30nm fiber, which forms a rosette of 6 loops, 6 loops forms one coil, and each chromatid has 10 coils. Not only are coils important to package DNA, but they can also be introduced to the DNA itself, for example into bacterial circular DNA the linking number of the DNA (number of times the DNA revolves around itself) can be changed by increasing the stress on the DNA, this super-coiling gives extra energy to the DNA, which can be used in processes such as replication.
