DNA STRUCTURE AND REPLICATION BASIC INFORMATION

DNA molecules are large, with RMMs up to one trillion (1012). Experimental work by Chargaff and other workers led Crick and Watson to propose that the three dimensional structure of DNA consisted of two single molecule polymer chains held together in the form of a double helix by hydrogen bonding between the same pairs of bases, namely the adenine–thymine and cytosine–guanine base pairs.

These pairs of bases, which are referred to as complementary base pairs, form the internal structure of the helix. They are hydrogen bonded in such a manner that their flat structures lie parallel to one another across the inside of the helix. The two polymer chains forming the helix are aligned in opposite directions.

In other words, at the ends of the structure one chain has a free 3’-OH group whilst the other chain has a free 5’-OH group. X-Ray diffraction studies have since confirmed this as the basic three dimensional shape of the polymer chains of the B-DNA, the natural form of DNA.

This form of DNA has about 10 bases per turn of the helix. Its outer surface has two grooves, known as the minor and major grooves respectively, which act as the binding sites for many ligands. Electron microscopy has shown that the double helical chain of DNA is folded, twisted and coiled into quite compact shapes.

A number of DNA structures are cyclic, and these compounds are also coiled and twisted into specific shapes. These shapes are referred to as supercoils, supertwists and superhelices as appropriate.

DNA molecules are able to reproduce an exact replica of themselves. The process is known as replication and occurs when cell division is imminent. It is believed to start with the unwinding of the double helix starting at either the end or more usually in a central section, the separated strands acting as templates for the formation of a new daughter strand.

New individual nucleotides bind to these separated strands by hydrogen bonding to the complementary parent nucleotides. As the nucleotides hydrogen bond to the parent strand they are linked to the adjacent nucleotide, which is already hydrogen bonded to the parent strand, by the action of enzymes known as DNA polymerases.

As the daughter strands grow the DNA helix continues to unwind. However, both daughter strands are formed at the same time in the 5’ to 3’ direction. This means that the growth of the daughter strand that starts at the 3’ end of the parent strand can continue smoothly as the DNA helix continues to unwind. This strand is known as the leading strand.

However, this smooth growth is not possible for the daughter strand that started from the 5’ of the parent strand. This strand, known as the lagging strand, is formed in a series of sections, each of which is still grows in the 5’ to 3’ direction.

These sections, which are known as Okazaki fragments after their discoverer, are joined together by the enzyme DNA ligase to form the second daughter strand. Replication, which starts at the end of a DNA helix,continues until the entire structure has been duplicated.

The same result is obtained when replication starts at the centre of a DNA helix. In this case unwinding continues in both directions until the complete molecule is duplicated. This latter situation is\more common.