Chimeric genes (literally, made of parts from different sources) form through the combination of portions of two or more coding sequences to produce new genes. These

mutation In biology, a mutation is an alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA. Viral genomes contain either DNA or RNA. Mutations result from errors during DNA or viral replication, m ...

s are distinct from

fusion genes A fusion gene is a hybrid gene formed from two previously independent genes. It can occur as a result of translocation, interstitial deletion, or chromosomal inversion. Fusion genes have been found to be prevalent in all main types of human neopla ...

which merge whole gene sequences into a single reading frame and often retain their original functions.

Formation

Chimeric genes can form through several different means. Many chimeric genes form through errors in

DNA replication In molecular biology, DNA replication is the biological process of producing two identical replicas of DNA from one original DNA molecule. DNA replication occurs in all living organisms acting as the most essential part for biological inherita ...

DNA repair DNA repair is a collection of processes by which a cell identifies and corrects damage to the DNA molecules that encode its genome. In human cells, both normal metabolic activities and environmental factors such as radiation can cause DNA da ...

so that pieces of two different genes are inadvertently combined.Rogers, RL, Bedford, T and Hartl DL. "Formation and Longevity of Chimeric and Duplicat Genes in ''Drosphila''". ''Genetics''. 181: 313-322. Chimeric genes can also form through

retrotransposition A transposable element (TE, transposon, or jumping gene) is a nucleic acid sequence in DNA that can change its position within a genome, sometimes creating or reversing mutations and altering the cell's genetic identity and genome size. Trans ...

where a retrotransposon accidentally copies the transcript of a gene and inserts it into the genome in a new location. Depending on where the new retrogene appears, it can recruit new

exon An exon is any part of a gene that will form a part of the final mature RNA produced by that gene after introns have been removed by RNA splicing. The term ''exon'' refers to both the DNA sequence within a gene and to the corresponding sequenc ...

s to produce a chimeric gene. Finally,

ectopic recombination Ectopic recombination is an atypical form of recombination in which crossing over occurs at non-homologous, rather than along homologous, loci. //This needs to be edited, as it is 1)incorrect and 2)contradicts what's written below, namely the nee ...

, when there is an exchange between portions of the genome that are not actually related, can also produce chimeric genes. This process occurs often in human genomes, and abnormal chimeras formed by this process are known to cause

color blindness Color blindness or color vision deficiency (CVD) is the decreased ability to see color or differences in color. It can impair tasks such as selecting ripe fruit, choosing clothing, and reading traffic lights. Color blindness may make some aca ...

Evolutionary Importance of Fusion Proteins

Chimeric genes are important players in the evolution of genetic novelty. Much like

gene duplication Gene duplication (or chromosomal duplication or gene amplification) is a major mechanism through which new genetic material is generated during molecular evolution. It can be defined as any duplication of a region of DNA that contains a gene ...

s, they provide a source of new genes, which can allow organisms to develop new

phenotype In genetics, the phenotype () is the set of observable characteristics or traits of an organism. The term covers the organism's morphology or physical form and structure, its developmental processes, its biochemical and physiological prop ...

s and adapt to their environment. Unlike duplicate genes, chimeric proteins are immediately distinct from their parental genes, and therefore are more likely to produce entirely new functions. Chimeric fusion proteins form often in genomes, and many of these are likely to be dysfunctional and eliminated by natural selection. However, in some cases, these new peptides can form fully functional gene products that are selectively favored and spread through populations quickly.

Functions

One of the most well known chimeric genes was identified in

Drosophila ''Drosophila'' () is a genus of flies, belonging to the family Drosophilidae, whose members are often called "small fruit flies" or (less frequently) pomace flies, vinegar flies, or wine flies, a reference to the characteristic of many s ...

and has been named Jingwei.Long, M., C. H. Langley 1993. "Natural selection and the origin of ''jingwei'', a chimeric processed functional gene in ''Drosophila''." Science 260: 91-95. This gene is formed from a retrotransposed copy of

Alcohol dehydrogenase Alcohol dehydrogenases (ADH) () are a group of dehydrogenase enzymes that occur in many organisms and facilitate the interconversion between alcohols and aldehydes or ketones with the reduction of nicotinamide adenine dinucleotide (NAD+) to N ...

that united with the yellow emperor gene to produce a new protein. The new amino acid residues that it recruited from yellow emperor allow the new protein to act on long chain alcohols and diols, including growth hormones and pheremones.Zhang J, Dean AM, Brunet F, Long M. 2004. "Evolving protein functional diversity in new genes of ''Drosophila''." ''Proc Natl Acad Sci U S A'' 101: 16246-50. These changes affect fly development. In this case, the combination of different

protein domains In molecular biology, a protein domain is a region of a protein's polypeptide chain that is self-stabilizing and that folds independently from the rest. Each domain forms a compact folded three-dimensional structure. Many proteins consist of se ...

resulted in a gene that was fully functional and favored by selection. The functions of many chimeric genes are not yet known. In some cases these gene products are not beneficial and they may even cause diseases such as cancer.

References

{{DEFAULTSORT:Chimeric Gene Mutation Genes Molecular evolution Evolutionary biology