Chromosome 22

Size: 49 Mb (million base pairs)
Approx. no. of genes: 600

Originally defined as chromosome 22 because earlier cytogenetic analyzes suggested that it was the smallest chromosome in the human genome. Chromosome 21 is now known to be smaller than chromosome 22, by only a few Mb. Although chromosomes 21 and 22 are similar in size and structure (both are small and acrocentric and both contain a rRNA gene cluster and satellite formation), chromosome 22 has a much higher gene density. Indeed, chromosome 22 is the second chromosome with the highest gene density (17 genes per Mb, average 9-10 genes). It also has twice the density of CpG islets from the genomic average (up to 20 per Mb), high content of repetitive sequences (44% versus 40% average), and high GC pairs content (48% compared to the genomic average of 41%) . Two well-known syndromes are associated with cytogenetic abnormalities of chromosome 22. These are DiGeorge syndrome (sometimes referred to as catch-22 syndrome) caused by deletion within the 22q11 region and cat-eye syndrome, which involves partial 22q11 duplication (sometimes occurring as extra chromosome).