Circadian rhythm is driven by the circadian clock. One of the core clock genes identified is Period (Per) in Drosophila and Period1 (Per1), Period2 (Per2), and Period3 (Per3) in mammals. The present study examines the presence of perfect simple sequence repeats (SSRs) in human Period genes and their orthologues in chimpanzee, gorilla, orangutan, gibbon, and mouse. Comparisons of all Per genes show a higher density of SSRs in Per1 of all organisms followed by Per3 and Per2 with exceptions. Possibly due to higher chances of mutations, CG-rich repeats are not abundant in Per genes. This analysis shows gene and species-specific distribution of repeats along with region-specific distribution of repeat types and repeat motifs in exons and introns of the Per genes. There is diverse distribution of CG-rich repeat types and motifs in the three Per and also in exons and introns of the three genes in different organisms. Long perfect SSRs are generally not common. Since perfect SSRs are prone to mutations, there is a need to study SSRs, especially found in exons of human Per genes which could serve as markers for investigators to study the role of such SSRs in Per-related disorders.
Key words: Simple sequence repeats (SSRs), Period gene, orthologues, SSR density, AT/CG richness, repeat length
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