chr2

Chromosome 2, often abbreviated chr2, is the second-largest autosome in the human genome and one of the 23 pairs of chromosomes. It spans about 243 million base pairs and contains roughly 2,000 genes, along with extensive regulatory sequences and noncoding RNA genes. As with other autosomes, chr2 is present in two copies in diploid cells.

Evolutionary origin: In humans, chromosome 2 is the result of an ancient fusion of two ancestral chromosomes (often referred to as 2A and 2B) that existed in a common ancestor with other African great apes. Evidence for this event includes the presence of a vestigial telomere sequence near the fusion site and remnants of a second centromere. The fusion is estimated to have occurred around 6 to 8 million years ago, after the split from the lineage leading to chimpanzees, gorillas, and orangutans.

Genomic content: chr2 contains genes across many functional classes, including those involved in development, cell signaling, metabolism, and neural function. It also houses numerous regulatory elements, noncoding RNAs, and segmental duplications. The gene density is uneven, with regions of high density interspersed with gene deserts. Like other chromosomes, chr2 is organized into regions of active and inactive chromatin that influence gene expression.

Clinical significance: Variation in chr2 contributes to a range of human diseases and traits through copy-number changes, single-nucleotide variants, and structural rearrangements. Specific loci on chr2 have been implicated in developmental disorders, metabolic traits, and cancer susceptibility, though disease associations are often complex and involve multiple genes and regulatory contexts.

Model organisms: In nonhuman primates, chr2 is not the result of a fusion; their chromosome numbering differs. The human chromosome 2 corresponds largely to two separate chromosomes in other great apes (designated 2A and 2B).