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Reading the instructions


By now most people will be familiar with the double spiral of the DNA molecule, from the number of times it has appeared in the media. It is often portrayed as a twisted ladder and, very briefly, the way it works is as follows:

The sides of the ladder are molecules of a sugar, deoxyribose, joined together in long chains. The rungs are pairs of nucleotide molecules, with the property of bonding very firmly to the deoxyribose molecules, and less firmly to each other.

The nucleotides are of four kinds: guanine, cytosine, thymine and adenine, referred to as bases. However, they are usually known as G, C, T and A for short. At any point on one side of the ladder, the base can be G, C, T or A, but the bases on the opposite sides that link together must be C with a G (or vice versa) or A with a T (or vice versa). Thus the two together make a base pair. It is the order of these bases along the strand of DNA that forms the genetic code.

These bases are read in groups of three: CAC, CGT, ATC and so on, referred to as codons, and sequences of codons are the genes. At each end of each gene, which may be from a few hundred to tens of thousands of base pairs long, are triplets that mark the beginning and end.

The process is in two stages, transcription whereby a copy of the code is prepared in the form a length of RNA, and translation whereby the RNA is the template for preparing a protein.



Under the influence of an enzyme, RNA polymerase, the double strand of DNA parts and free nucleotide molecules bind to the codons on one side of the DNA, beginning at the relevant start point, and proceeding as far as the stop point. RNA polymerase binds these molecules together, producing a molecule which is a copy of the codons on the DNA for the gene which is being read. The RNA detaches, leaving the DNA to join together again.

The end product is messenger RNA, which leaves the nucleus and enters the surrounding cell, the cytoplasm. However there are actually two stages in the transcription process.

Introns and Exons.

Most genes contain within them stretches of redundant code. The initial copy of the transcription unit within the gene is made to what is known as the primary transcript in the form of pre-messenger RNA.

The second step in the transcription process, therefore, is to remove the redundant code to form the functional messenger RNA.



At this point, ribosomes, which have two components, a protein and another form of RNA, ribosomal RNA, attach themselves to the messenger RNA.

The structure of the codons has then to be matched with their relevant amino acids. This is achieved by yet another RNA, transfer RNA. Specific tRNA molecules attach to specific amino acids. A tRNA molecule attaches itself to an amino acid molecule, then moves towards the mRNA, attaching itself to the relevant codon.

The ribosome attaches the amino acid to itself, freeing the tRNA to repeat the process as necessary. It then moves to the next codon on the mRNA, and hence the next tRNA molecule, repeats the process, and so on. Each time the amino acid molecule is bound to the previous amino acid molecule on the ribosome.

At the end of the process, there is, attached to the ribosome, a chain of amino acids, in the correct order to form the required protein.

So it is, that the end result of any particular gene code of several hundred triplets along the DNA, is a complex protein molecule.

Enzymes and Proteins.

It is not the intention here to give an exhaustive account of the gene, but one aspect does not become clear in many accounts. It is not the gene that is important so much as the protein it codes for.

The biochemical activity of these, often enormous, molecules depends to a great extent on their three dimensional shape, influenced by the electrical charges within them and around them.

The cell is an immensely complex mixture of processes going on together, in close proximity, in an environment containing many free ions. A given reaction then may not work in exactly the same way as one might predict from experimental evidence derived on the laboratory bench.

NEXT Copying genes - Mitosis

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Citation: Bland, J., (2003) Reading the Instructions
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Derby TV/TS Group. Text copyright Jed Bland. 01.01.99 amended 07.06.02, 12.08.03, 23.11.03