DNA

The three dimensional structure of DNA was discovered in the 1950's by Watson and Crick. The main features of the structure are:

  -DNA is double stranded, so there are two polynucleotide strands alongside each other. The strands are anti-parallel.

  -The two strands are would around each other in a double helix.

The two strands are joined together by hydrogen bonds between the bases. The bases therefore form base pairs, which are like rungs of a ladder. The base pairs are specific (AT, CG). These are called complimentary base pairs. This means that whatever the sequence of bases along one strand, the sequence of the bases on the other strand must be complimentary to it.

The DNA molecule in a single human cell is 99cm long. In order to fit into the cell the DNA is cut into shorter lengths and each length is tightly wrapped up with histone proteins to form chromatin.

  During most of the life of a cell, the chromatin is dispersed throughout the nucleus and cannot be seen with a light microscope. A various times, parts of the chromatin will unwind so that genes on thee DNA can be transcribed. This allows the proteins that the cell needs to be made.

  Just before cell division the DA is replicated, and more histone proteins are synthesised, so there is temporarily twice the normal amount of chromatin. Following replication, the chromatin then coils up even tighter to form short fat bundles called chromosomes.

  Each chromosome is roughly X shaped. The two arms are identical as they are replicated DNA. They are called chromatids and are joined by…