Chromosome 21 is one of the 23 pairs of chromosomes found in human cells. It is the smallest human chromosome, containing about 48 million base pairs and approximately 200 genes. These genes play important roles in various biological processes, including development and metabolism. An extra copy of chromosome 21 is associated with Down syndrome, a genetic condition that can lead to intellectual disabilities and various health issues. This condition occurs due to a process called nondisjunction, where chromosomes fail to separate properly during cell division, resulting in three copies of chromosome 21 instead of the usual two.