The relationship between the structure of DNA
and three different biological functions
Genes are composed of DNA, which can function as hereditary material.
DNA has the ability to replicate; the capacity to carry information and to process this information within the cell.
A DNA molecule ( deoxyribonucleic acid ) has a double helix structure that consists of two polynucleotied chains: the sugar-phosphate backbone strands of DNA and the bases between the two spiralling chains bonding them together (Appendix 1).
The base pairs bonded by weak hydrogen bonds lie at the core of the molecule, they are arranged flat, rather like steps on a ladder. The sugar-phosphate backbones are on the outside of the helix, each spiralled around each other.
The functioning of DNA depends on interactions between the four different kinds of bases at the core of the molecule. Each base makes a specific pairing with only one of the other bases in what is known as base pairing. There are four bases: Adenine (A), Thymine (T), Cytosine (C) and Guanine (G). On one strand the bases can be in any order but what is important is that A only pairs with T and C only pairs with G on the other strand. This is fundamental to DNA replication and explains the double helix structure of DNA.
In the first steps of DNA replication the weak hydrogen bonds have to be broken and the two polynucleotide strands are separated. Once the strands are separated from each other, new DNA strings are synthesized.
During the process of replication the two strands unwind to expose the bases on each strand. Each of the strands act as a template or mould for the copying of DNA.
The nucleotides are added - by a special enzyme called RNA polymerase - with the rules of base pairing, and the new hydrogen bonds and new sugar-phosphates backbones form. At the end two new identical double-stranded DNA molecules are produced ( Appendix 2 ).
The process continues until all of the DNA molecule has been replicated.
DNA is copied during most cycles of cell division. DNA replication is linked to chromosome replication. Chromosomes are composed of DNA and associated with protein. A gene is part of a long DNA molecule and each DNA double helix is associated with protein. This DNA protein complex loops and coils to finally form a chromosome.
DNA replication is efficient, although sometimes errors can occur leading to mutation.
The information carried by DNA is in a simple coding within the four bases. These four letters can be arranged in many-many different ways of sequences, giving a huge store of information for the production of amino acid chains or polynucleotide chains.
This four-letter coding of DNA therefore contains potential information for many thousands of different proteins.
Information flow from DNA to polypeptide has a vital counterpart in between :
RNA or ribonucleic acid.
The structure and synthesis of RNA is somewhat different to the structure and replication of DNA. A molecule of DNA is double stranded while RNA is usually single stranded and only produced on one of the polynucleotide chains.
There are two steps in the information flow: the reading of genetic information, what is known as transcription; and the copying of information, the translation.
There are three different types of RNA molecules that help in the processing of information : mRNA, rRNA and tRNA ( messenger, ribosomal and transfer codons ).
They directly make long chains of amino acid polypeptides.
This relationship between RNA and amino acids is known as the genetic code.
In view of all the above the relatively simple structure of DNA (Appendix 1), is essential for replicating itself (Appendix 2.), for carrying vast amounts of information and for the intricate task of processing the genetic information.
The DNA within the cell undergoes a series of processes thus replicating itself so the new cells after cell division are genetically identical, and also RNA synthesis ensures information flow, and the different combinations of the four bases ensures information processing.
The fundamental process and key to the accuracy and smooth running of all of these biological functions is base pairing, which provides a solution for all of them. The important feature of DNA structure is that the genetic information it contains is copied into more DNA with the same genetic information.