satDNA

Satellite DNA, abbreviated satDNA, refers to a class of tandemly repeated, non-coding DNA sequences that are present in long arrays in the genomes of many eukaryotes. The repeats are arranged head-to-tail in large blocks, often in pericentromeric and centromeric regions, and to a lesser extent in other heterochromatic or euchromatic regions. Individual repeat units range from about a few base pairs to several hundred base pairs; the most well-studied examples in humans are alphoid (alpha-satellite) sequences with roughly 171 base pairs that organize into higher-order repeat structures at centromeres. Satellite DNA can constitute a substantial fraction of the genome, particularly in plants and animals, where the amount and composition of satDNA can differ greatly between species and even among populations.

SatDNA is typically non-coding and not transcribed into functional proteins, although some satellites can be transcribed into noncoding RNAs under certain conditions.

Evolution: satDNA evolves rapidly; a common mechanism is concerted evolution via unequal crossing over and gene conversion, leading to homogenization of repeats within a genome and divergence between species. The copy number of repeat units can vary widely.

Functions and uses: despite often being considered “junk DNA,” satellite DNA is implicated in centromere and kinetochore formation, heterochromatin organization, and chromosome pairing; its repetitive nature makes it useful as a cytogenetic marker, especially via fluorescent in situ hybridization (FISH) to distinguish chromosomes and species.