DNA:The code of life
In 1953, two scientists Englishman Frances Crick and American James Watson worked out the shape of Deoxyribonucleic Acid, DNA, the code of life. The Double Helix is two strands linked together like rungs on a ladder and twisted into the helix shape.
DNA has a rich language, build on only the four letters of the two sets of base pairs, A,C,G,T. Adenine, Cytosine, Guanine and Thymine. A and C pair with each other and G and T pair with each other making the rungs of the ladder. These bases are written onto long chains of sugars and phosphates.
Cells contain chromosomes – in humans each parent contributes 23 chromosomes to the cell, making 46. Each chromosome is made of a long pair of DNA molecules. Another molecule, messenger RNA, Ribonucleic Acid, transcribes the language written in the DNA and makes a copy. RNA is only slightly different to DNA in that it has Uracil instead of Thymine to bind with Cytosine. It is thought an RNA world existed before DNA evolved.
Ribosomes translate the copy into three letter groups known as codons that form the basis of another alphabet that makes amino acids – humans need 20 amino acids to make proteins – every protein has a different job to perform, whether for a human or a bacterium or any other form of life on Earth. Every protein represents a gene translated from the chromosome.
Mutation happens when a mistake is made in the transcription process. It is at this molecular level that evolution happens, and can be seen to be happening.
The genome – the total of all the genes – an organism has depends on its complexity. It can vary from less than a few thousand in a bacterium to hundreds of thousands in a human.
The organization and complexity of DNA is puzzling. Prof Paul Davies at the Australian Centre for Astrobiology at Macquarie University, Sydney, has likened it to putting a bomb under a pile of bricks, setting it off and expecting a house to be built. The question of when life got going on Earth is one of strong debate among scientists. The Earth formed around 4.6 billion years ago. The planet was under heavy bombardment as it came together over 500 million years. If the stromatolites in the Pilbara indicate life was flourishing 3.5 billion years ago, it means there were less than 500 million years for DNA, and subsequently, single celled organisms to have evolved.
The possibility of the swiftness of life appearing indicates that life may get going very easily and – geologically speaking – very quickly once conditions are right. This is a key question in the search for past life on Mars. If it was wet and warmer in its past, how long was that for and was it long enough for life to have got going – perhaps before the right conditions appeared on Earth.
More information about RNA and DNA.
Ridley, M. (2000) Genome, Fourth Estate Ltd
Jones, S. (2000) The Language of Genes, Flamingo, London.