Gene cluster

A gene cluster is a group of two or more genes found within an organism's DNA that encode similar polypeptides or proteins which collectively share a generalized function and are often located within a few thousand base pairs of each other.

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

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.

[1][6] 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.

Hox gene clusters, ranging in various sizes, are found among several phyla.

[7] It is unknown the exact number of genes contained in the duplicated Protohox cluster; however, models exist suggesting that the duplicated Protohox cluster originally contained four, three, or two genes.

[3] The Fisher Model was considered unlikely and later dismissed as an explanation for gene cluster formation.

[9] thus the formation of clustered genes generates a high local concentration of cytoplasmic protein products.

Spatial segregation of protein products has been observed in bacteria; however, the Molarity Model does not consider co-transcription or distribution of genes found within an operon.

Gene clusters change over a long evolutionary time period, which does not result in genetic complexity.

[12] Ectopic gene conversion occurs when one homologous DNA sequence is replaced by another.

In the eukaryotic genome, tandemly arrayed genes make up ribosomal RNA.

In this situation, tandem repeats of the gene allow a sufficient amount of RNA to be provided.

For example, human embryonic cells contain 5-10 million ribosomes and double in number within 24 hours.

Hox genes have been observed among various phylum. Eight genes make up the Hox gene Drosophila. The number of Hox genes may vary among organisms, but the Hox genes collectively make up the Homeobox family.
Intrachromosomal duplication is the duplication of genes within the same chromosome over the course of evolution (a-1). Mutations may occur in the duplicated copy, such as observed with the substitution of guanine with adenine (a-2). Alignment of DNA sequences exhibits homology between the two chromosomes (a-3). All segments were duplicated from the same ancestral DNA sequence as observed by the comparisons in b(i-iii).
Tandem duplication is the process in which one gene is duplicated and the resulting copy is found adjacent to the original gene. Tandemly arrayed genes are formed as a result of tandem duplications.