Which human chromosome is the largest by physical length?

Human chromosome 1 is the largest chromosome by physical length. It contains roughly 249 million base pairs and represents about 8% of the DNA in a human haploid genome.

Chromosome 1 was the last human chromosome to be fully sequenced in the original Human Genome Project, with its reference sequence reported in 2005. Its size and gene content made completing the sequence a substantial technical milestone.

The chromosome contains hundreds of genes involved in many biological processes, including cell growth, metabolism, development, and DNA repair. Changes in chromosome 1 can therefore be associated with a wide range of inherited and acquired conditions.

Chromosome 2 is not the largest, although it is notable for another reason: it contains evidence of an ancient fusion that joined two ancestral ape chromosomes. Chromosome numbering is based on size, so chromosome 1 is larger than chromosome 2.