Scientists have published the complete DNA sequences of chromosomes 19 and 13, bringing the total number of fully decoded human chromosomes to nine. A team lead by researchers at the Stanford Human Genome Center in Paolo Alto, US, carried out the chromosome 19 project, while scientists at the Wellcome Trust Sanger Centre in Hinxton, UK, lead the chromosome 13 work. Both groups published their findings in the journal Nature. The results show that chromosome 19, packed with nearly 1,500 genes, is the most gene-dense chromosome analysed in detail so far. In contrast, chromosome 13 appears to have one of the lowest gene densities, with an estimated 633 genes along its length.
An international consortium unveiled the final version of the entire human genome on 14 April 2003. They found that it is made up of 2.9 billion base-pairs (chemical 'letters') of DNA, and contains an estimated 25-30,000 different genes. Researchers have since been looking at each of its 24 different chromosomes in detail, to identify the 'coding' stretches of DNA that make up these genes. They have also been filling gaps - stretches of DNA sequence that could not easily be determined - and double-checking for any errors. As well as chromosomes 19 and 13, scientists have also published complete, fully-analysed sequences for chromosomes 22, 21, 20, 14, 7, 6 and the Y chromosome.
Chromosome 13, which harbours the breast cancer gene BRCA2, as well as genes involved in retinoblastoma (inherited eye cancer) and schizophrenia, is made up of around 96 million DNA base-pairs. Of its 633 genes, 231 had already been identified. 'Having this sort of information out there is going to make life so much easier for a lot of people', said Andrew Dunham, who lead the chromosome 13 project. Dunham also said that some of the 'barren' gene-poor areas of chromosome 13 contain DNA sequences important for gene control.
Chromosome 19, known to carry genes involved in diabetes, hereditary high cholesterol levels and myotonic muscular dystrophy, is made up of around 56 million DNA base-pairs. It contains an estimated 1461 genes, including 141 that were previously unknown. Although nearly 100 chromosome 19 genes have already been linked to inherited traits or diseases, the full chromosome sequence should speed up the search for at least another 20 genes involved in inherited disorders.