Origin of new genes is a driving power for evolution in all living things. The processes that produce new genes, includes a diversity of molecular events, all of which must take place in the germ line to be inherited by the subsequent generation. Following the germ-line mutational event, the new gene will be polymorphic (population that has more than one relatively common allele present at a given locus) in the population. Consequently, the two most possible outcomes for the new gene are fixation or extinction.
Over the years, researchers have suggested various mechanisms by which new genes are created.
Gene duplication - it was the initial mechanism of gene generation to be indicated and this mechanism does certainly emerge to be the most general way of generating new genes. Duplications are normally classified according to the size of the segment of the genome that is duplicated. Thus duplication may be depicted as, including total genome, big portions of a genome, individual genes, individual exons, or even precise parts of exons .The processes that create duplicate genes are diverse, and more details about these processes are constantly being discovered. These mechanisms include:
- Entire genome duplications originating through nondisjunction (inability of homologous chromosomes or sister chromatids to detach in meiosis or mitosis).
- Tandem duplications aroused due to unequal crossover.
- Retroposition (the integration of a sequence derived from RNA into a DNA genome. mRNA is reverse-transcribed and reintegrated into the genome) originating through the retrotranscription of an RNA intermediate.
- Transpositions relating the transposable elements.
- Duplications resulting after rearrangements and successive repair of staggered breaks.
Such duplications include not only protein-coding genes, but also noncoding RNA genes. Much of the present anticipation about gene duplication stems from the information that with the number of sequenced genomes now accessible scientists have more precise estimates of how often genes duplicate, and these rates seems to be very high. Further enthusiasm comes from the understanding that duplications occur so often that individuals of the same species differ very much in DNA content and gene number. It is assessed that, on average, two humans will differ by approximately 5 megabases of information.
Transposable Element(TE) protein domestication - transposable elements known as "selfish" segments of DNA codes for the proteins that allow these segments to copy or move themselves inside a genome. There are 2 classes of transposable elements, DNA transposons and retrotransposons. DNA transposons are capable of excising themselves out of the genome and be inserted elsewhere, while retrotransposons copy themselves through an RNA intermediate. Analogous to viral insertions in the genome, TE insertions can lead to mutations and furnish to increased genome size, but they generally do not code for cellular proteins. Fascinatingly, one way for a genome to acquire novel genes is by recruiting transposable element proteins and utilizing them as cellular proteins. Such circumstances are termed as domestications of transposable element proteins. Numerous instances of domestication have been depicted in Drosophila, vertebrates and in plants.
Lateral Gene Transfer - it is referred to the situation in which a gene does not have a vertical origin (that is direct inheritance from parent to offspring) but comes from an unrelated genome. It is recognized that this type of transfer occurs between bacteria, and that it also has taken place between the genomes of the cell organelles like mitochondria and chloroplasts and the nuclear genomes.
Gene Fusion and Fission - persisting genes can also fuse (that is 2 or more genes can form part of the same transcript) or undergo fission (that is a single transcript can break into 2 or more separate transcripts), thus forming new genes.
De Novo Gene Origination - New genes can additionally generated de novo from noncoding regions of DNA. In fact, several new genes derived from noncoding DNA, in recent times have been described in Drosophila. To the newly formed Drosophila genes with protein-coding capabilities, no homologues in other species have been found. But the de novo genes found in various species till now consists of both protein-coding as well as noncoding genes.
As the case of any mutation, when novel genes becomes established in a genome and they add to the dissimilarities among species and acts as the means for evolution.
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