She was struck by a pair of hobnailed boots

Segments of DNA can be by-passed in a way analogous to the formation of these three sentences from the "parent" sentence. The process is called gene conversion. In some species of yeast, the process generates different "mating types", and these are relevant to the organism's reproduction.

Some genes become amplified: multiple copies are made and inserted into the DNA one after the other, generating a series of more or less exact repeats. Amplification is useful or even essential to the cell when very large amounts of a gene product are needed quickly. This is the case with some components of ribosomes. (Recall that ribosomes are the protein-making machines that read the messenger RNA "photocopies". Every living cell needs very large numbers of ribosomes to cope with its protein synthesising requirements.) Other examples include the genes for the eukaryotic DNA packaging proteins, the histones. The total amount of histone required by a cell is similar to the total amount of DNA: a lot. However, histones can only be made at the moment in the cell cycle when the DNA is duplicated; excess, unbound histones would cause terminal damage to other components. Therefore, the histone genes are amplified; there are multiple copies of them. These genes are transcribed rapidly when DNA duplication occurs and switched off again immediately afterwards.

Genes are sometimes amplified when the cell has no need for multiple copies. Since the extra copies are redundant, it makes little difference to the cell if they mutate almost out of recognition. They come under the heading of "junk DNA". Nearly all eukaryotic genomes contain the mutated remnants of unwanted genes, which might be the products of amplification events in the remote evolutionary past, and these become scattered all over the genome, not necessarily adjacent to the site of the original gene. These functionless remnants are called pseudogenes. But, on a rare occasion - as a result of serial mutations - pseudogenes acquire new functions, becoming relevant to the cell and to evolution once more. Such "rare occasions" might, in fact, have happened fairly frequently. DNA has been changing for almost four thousand million years since the origin of life, so there has been plenty of opportunity for "rare events".

Occasionally a very short segment of DNA, perhaps only 3 or 4 bases, is copied over and over again, producing simple-sequence DNA. One such reiterated sequence makes up about 10% of the human genome. The difference in genome sizes among complicated organisms owes more to the quantity of simple-sequence DNA than it does to the total number of genes.

In summary, some of the more dramatic ways in which DNA can be altered are:-

  • excision, splicing and inversion;
  • gene rearrangements;
  • amplification;
  • formation of pseudogenes;
  • production of simple sequences.

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