Pseudogene

Pseudogenes are DNA segments that are indeed built like a gene, but no longer serve as a template for a functional protein.

Emergence of pseudogenes

For a small number of pseudogenes, it is believed that they were functioning genes once and were set by mutations overridden. However, most pseudogenes are non-functional duplicates of functional genes. The lack of function of a pseudo gene could have arisen in two different ways:

Besides the formation of pseudogenes by gene duplication we also discussed the possibility of retrotransposition. By reverse transcription of mRNA functioning genes produced double-stranded DNA which is then inserted somewhere in the genome. Pseudogenes, which are created in this way have no introns, as the underlying mRNA also contained no introns ( RNA splicing ). This intron -less genes are also referred to as pseudo- processed pseudogenes.

Importance of pseudogenes

Both methods described above - duplication and retrotransposition - can enlarge the number of genes in an organism. The duplicated gene is not subject to the same selection pressure as the original gene, as a non- functioning of the duplicate is not more tragic if the original gene is still working. In the duplicate itself can therefore relatively easily accumulate mutations. This can quickly duplicate again become nonfunctional (and thus turn into a pseudogene ), without this having a negative effect on the phenotype. On the other hand, allow mutations in the duplicated genes but not until the emergence of new advantageous features. For example, the green opsin genes of mammals have arisen by gene duplication from the red opsin genes.

Pseudogenes of ribosomal proteins

Some types of genes are especially likely to be pseudogenes. Only 1/4 of all the known gene families has produced pseudogenes. Especially extreme as this is in the genes for ribosomal proteins. There are about 80 genes encoding ribosomal proteins. These 80 genes have produced over 2,000 pseudogenes, ie 1/10 of all known pseudogenes. An extreme case, the gene RPL21, of which there are more than 140 pseudogenes.

Pseudogenes and selection pressure

If pseudogenes are really useless, they are not subject to more selection pressure. In the course of time, so a lot of mutations should accumulate indiscriminately. Now, however, been shown that there are pseudogenes, which are quite conserved, have thus virtually unchanged. This strongly suggests that this is a selection pressure was effective, as we know it with important " real" genes. This raises the suspicion that - at least some - but pseudogenes have a function. In this case, but must be transcribed pseudogenes. Indeed, it has been found that about 10% of the pseudo- genes are transcribed.

A new study discusses the function of many pseudogenes as a regulator of miRNAs. It was shown that the 3 'UTR of a tumor suppressor pseudogene ( PTENP1 ) has binding sites of miRNAs that bind well to its cognate protein-coding gene ( PTEN ). The sequestered in the pseudogene miRNAs can no longer compromise, which implies a certain down-regulation of miRNA action is the encoding gene.

Possible functions of the pseudogene mRNA

The mRNA of pseudogenes can influence the regulatory RNA transcription ( or translation) of real genes.

• Example 1: In the neurons of a water screw, both the gene for the nitric oxide synthase (NOS ) and a corresponding transcribed pseudogene. The pseudogene RNA inhibits the translation of the correct NOS RNA.

• Example 2: developmental disorders of mice could be attributed to a regulatory gene called Makorin1. Somehow, this gene was disrupted. However, no mutations found in the base sequence of this gene. But they found a mutation in a pseudogene. If you destroyed this pseudogene experimentally, could not function without the genuine gene.

Nowadays there are more than 20 such pseudogenes, which are regulatory action. However, it seems questionable whether these pseudogenes had such a regulatory function from the beginning.

Reactivation of pseudogenes

It seems also cases in evolution to have given, in which useless pseudogenes were back to functioning genes. Also, individual differences are possible. In certain individuals, an olfactory receptor pseudogene has no function, other people it is transcribed and even translated. Corn is seems especially common for the reactivation of pseudogenes occur when the plants are exposed to stressful environmental conditions. Seen pseudogenes can be viewed not only as dead genes, but also as a source of new genes.