Gene cluster

A gene family is a set of homologous genes within one organism. A gene cluster is part of a gene family. A gene cluster is a group of two or more genes found within an organism's DNA that encode for similar polypeptides, or proteins, which collectively share a generalized function and are often located within a few thousand base pairs of each other. The size of gene clusters can vary significantly, from a few genes to several hundred genes. Portions of the DNA sequence of each gene within a gene cluster are found to be identical; however, the resulting protein of each gene is distinctive from the resulting protein of another gene within the cluster. Genes found in a gene cluster may be observed near one another on the same chromosome or on different, but homologous chromosomes. An example of a gene cluster is the Hox gene, which is made up of eight genes and is part of the Homeobox gene family.

Historically, four models have been proposed for the formation and persistence of gene clusters.

This model has been generally accepted since the mid-1970s. It postulates that gene clusters were formed as a result of gene duplication and divergence. These gene clusters include the Hox gene cluster, the human β-globin gene cluster, and four clustered human growth hormone (hGH)/chorionic somaomammotropin genes.

Conserved gene clusters, such as Hox and the human β-globin gene cluster, may be formed as a result of the process of gene duplication and divergence. A gene is duplicated during cell division, so that its descendants have two end-to-end copies of the gene where it had one copy, initially coding for the same protein or otherwise having the same function. In the course of subsequent evolution, they diverge, so that the products they code for have different but related functions, with the genes still being adjacent on the chromosome. Ohno theorized that the origin of new genes during evolution was dependent on gene duplication. If only a single copy of a gene existed in the genome of a species, the proteins transcribed from this gene would be essential to their survival. Because there was only a single copy of the gene, they could not undergo mutations which would potentially result in new genes; however, gene duplication allows essential genes to undergo mutations in the duplicated copy, which would ultimately give rise to new genes over the course of evolution. Mutations in the duplicated copy were tolerated because the original copy contained genetic information for the essential gene's function. Species who have gene clusters have a selective evolutionary advantage because natural selection must keep the genes together. Over a short span of time, the new genetic information exhibited by the duplicated copy of the essential gene would not serve a practical advantage; however, over a long, evolutionary time period, the genetic information in the duplicated copy may undergo additional and drastic mutations in which the proteins of the duplicated gene served a different role than those of the original essential gene. Over the long, evolutionary time period, the two similar genes would diverge so the proteins of each gene were unique in their functions. Hox gene clusters, ranging in various sizes, are found among several phyla.

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