Paralogue

In molecular biology and genetics, the term "paralog" refers to genes that have evolved within the same genome through a gene duplication process. Paralogs and orthologs are two fundamentally different types of homologous genes that have evolved by vertical descent from a single ancestral gene or by duplication. Molecules that occur in different variations in one and the same organism as a result of gene duplications, e.g. alpha- and beta-hemoglobin, are also referred to as paralogs. The opposite of paralog is ortholog.

Gene duplication: Paralogously produced genes are created by the duplication of an original gene. After a gene has been duplicated, each of the duplicates develops independently and can take on different functions in the course of evolution.

Structural similarity: Since parallel genes (paralogs) have evolved from a common genetic template, they often have similar sequences and structural features, even if they diverge over time.

Functional consequences:

• Functional divergence: Despite their similarity, paralogs can fulfill different functions. While they were often initially redundant, subsequent mutation and selection often allows the evolution of new or specialized functions.
• Contribution to genetic diversity: Gene duplication and the resulting emergence of paralogs are important mechanisms of molecular evolution and contribute to the complexity and adaptability of organisms.

Differentiation from related terms:

• Orthologous genes: In contrast to paralogs, which arise within the same species through duplication, orthologs are genes that have arisen in different species from a common ancestor. Orthologous genes often fulfill similar functions in different organisms.
• Homologous genes: Both paralogous and orthologous genes belong to the larger category of homologous genes, as they descend from a common ancestor, but they differ in their evolutionary origin.

Orthology and paralogy are key concepts in evolutionary genomics. A clear distinction between orthologs and paralogs is crucial for establishing a robust evolutionary classification of genes and a reliable functional annotation of newly sequenced genomes. Genome comparisons show that orthologous relationships to genes of taxonomically distant species can be established for most genes of each sequenced genome (Koonin EV 2005).

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2. Koonin EV (2005) Orthologs, paralogs, and evolutionary genomics. Annu Rev Genet 39:309-338.
3. Studer RA, Robinson-Rechavi M. How confident can we be that orthologs are similar, but paralogs differ? Trends Genet 25:210-216.