Gene families
Gene families are groups of related genes that arise from the duplication of an original gene, allowing for structural and functional similarities among their members. These families can contain identical copies of a gene, which are essential for producing proteins required in abundance, such as ribosomal RNA (rRNA) in eukaryotes. Alternatively, gene families may feature nonidentical members that serve different but related functions, as seen in the human beta-globin gene family, which includes genes expressed at various developmental stages.
The evolution of gene families contributes to the complexity of genomes and organisms, as duplication can lead to mutations that generate new functions. Various mechanisms create these duplicate genes, including chromosome duplication and unequal crossing over during meiosis. While some duplicated genes may become nonfunctional, or pseudogenes, others can evolve independently, altering their function or protein output. Additionally, gene family members may undergo concerted evolution, evolving together as they retain similarities.
Studying gene families provides insights into genomic evolution, suggesting that the formation of these gene clusters can facilitate the emergence of new genes. Recent research indicates that this evolutionary process may be somewhat predictable, with implications for fields such as medicine and synthetic biology.
Gene families
SIGNIFICANCE: Gene families contain multiple copies of structurally and functionally related genes, derived from duplications of an original gene. Some gene families represent multiple identical copies of an important gene, while others contain different versions of a gene with related functions. Evolution of gene families can lead some genes to take on completely new functions, allowing greater complexity of the genome and perhaps the organism.
Evolutionary Origin of Gene Families
Gene families are a class of low or moderately repetitive DNA, consisting of structurally and functionally related genes resulting from gene duplication events. Usually, members of are clustered together on a chromosome, but members of a family can be located on more than one chromosome. Several mechanisms can generate tandem copies of genes: chromosome duplication, unequal crossing over, and slippage. Duplication of chromosomal segments is often a result of in heterozygotes and creates tandem repeated segments. Unequal crossing over occurs when segments do not line up correctly during meiosis and one of the crossover products has a duplicated segment. Replication slippage occurs when the “slips” during DNA replication and copies part of the strand again. Once there are two copies of a gene in tandem, the latter two mechanisms are more likely to generate additional copies.
A member of a gene family may be functional or functionless. If the gene was not copied completely or further mutations render it nonfunctional, it is called a pseudogene. Further sequence changes in a functional copy may result in a gene with an altered function, such as producing a similar but different form of a protein that can serve some biochemical need or a protein that has a much different function than the original.
Identical Gene Families
Identical gene families contain functional member genes that produce proteins that are identical or very nearly so. These gene families usually contain genes for protein products that need to be found in abundance in the cell because of a crucial function. Multiple copies of the genes allow greater transcription and protein production.
For example, in eukaryotes, ribosomal RNA (rRNA) genes are repeated in tandem several hundred times. In contrast, there are only seven copies of genes in the Escherichia coli, and they are dispersed throughout its single chromosome. The rRNA products of these genes make up part of the structure of the ribosome, the organelle responsible for the important process of protein synthesis.
The genes for eukaryotic histone proteins, which are important in maintaining the structure of DNA in chromosomes and in regulating the rate of transcription of many genes, are another example of clustered repeats of the same set of genes. In this case, there are five histone genes, separated by short, unrelated noncoding sequences, repeated several hundred times. The repeats are found in tandem in many invertebrate animal genomes but are dispersed in mammalian genomes.
Nonidentical Gene Families
The human beta-globin gene family is an example of a nonidentical gene family, which has functional member genes that serve different, but usually related, functions. In this case, the different protein products are alternate forms of the same type of protein, perhaps expressed at different times in the organism’s development. There are five functional genes and one pseudogene clustered together on chromosome 11. One gene is expressed in the human embryo stage, two in the fetus, and two in the adult. The related alpha-globin gene family, with three genes and four pseudogenes, is a cluster on chromosome 16.
Evolutionary Role of Gene Families
Gene families serve as an example of how genes may be accidentally duplicated by several possible processes, and then by mutation and further duplication the various copies can diverge in function. It is known that long-term genomic evolution (with the exceptions of symbiotic and parasitic genomes) usually involves increases in the number of genes. Although there are a number of mechanisms for this, including polyploidization, it is believed that the formation of gene families can be a first step toward the evolution of “new” genes. Mutations in different members of the gene family cause them to diverge independently, and some may evolve to produce completely different proteins. The presence of gene copies still coding for the original protein allows redundant copies to evolve freely without detrimental changes to cellular physiology.
Although gene family members can evolve to be more different, they may also undergo concerted evolution, in which the various copies evolve together. Unequal crossing over not only changes the number of copies of members of a gene family but also does so by actual duplication, so that some copies are identical. Repeated events of this type can result in all of the genes in the family being identical. In fact, natural selection will sometimes favor this process if it is to the organism’s advantage to have multiple identical copies, as with the rRNA and histone identical gene families. A 2023 study (Beavan, et al.) using machine learning suggests evolution is predictable to some degree; some genes regularly emerge with or without specific others. The researchers suggested this discovery could have applications in environmental science, medicine, and synthetic biology.
Key Terms
- concerted evolutiona process in which the members of a gene family evolve together
- pseudogenesnonfunctional segments of DNA that resemble functional genes
- repetitive DNAa DNA sequence that is repeated two or more times in a DNA molecule or genome
Bibliography
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