What is Chromosome 21?
'''Chromosome 21''' is one of the 23 pairs of human [[chromosomes]]. It has 48 million [[base pairs]] (the building material of [[DNA]]). This is about 1.5 percent of the total DNA in cells. Chromosome 21 is the smallest human [[autosome]] (non-[[sex chromosome]]). There are two copies of chromosome 21 in most people. Anyone with three copies of chromosome 21 has [[Down syndrome]], also known as [[trisomy 21]]. In May 2000, researchers working on the [[Human Genome Project]] announced that the sequence of base pairs which make up this chromosome had been identified. Chromosome 21 was the second human chromosome to be completely sequenced. [[Chromosome 22]] was the first. [[Category:Chromosomes]]