The total human genome contains 3.2 Gbp of DNA, and 2.95 Gbp of this represents euchromatin (q.v.). There are about 31,000 genes that transcribe mRNAs and at least 750 genes that encode other RNAs. For example, there are about 500 different tRNA genes. The genes are distributed over 22 pairs of chromosomes that range in size from autosome 1 (263 Mbp) to autosome 21 (50 Mbp). The X and Y chromosomes are 164 and 59 Mbp, respectively. The most gene-rich chromosome is autosome 19 (with 23 genes/Mbp), while the least gene-rich chromosomes are autosome 13 and the Y (each with 5 genes/Mbp). Genes (or at least their coding regions) make up only 1–2% of the genome. Just over 40% of the genes that encode proteins have orthologs in Drosophila and Caenorhabditis. Hundreds of human genes have resulted by horizontal transfer from bacteria at some point during the evolution of vertebrates. More than half of the genome is composed of repetitious DNA (q.v.). About 45% of the genome is derived from transposable elements (q.v.). Although the human genome has only about twice as many protein-coding genes as Drosophila or Caenorhabditis, human genes are more complex and often undergo alternative splicing (q.v.) to produce an array of different transcripts. More than 1,000 genes that cause specific diseases when they mutate have been mapped and are listed in OMIM (q.v.). The human mitochondrial chromosome is sometimes referred to as chromosome 25 or M. See Chronology, 2001, Collins and Venter et al.; 2003, International Human Genome Sequencing Consortium; Down syndrome, gene, genome annotation, horizontal transmission, human mitochondrial DNA, repetitious DNA.
Subjects: Genetics and Genomics.