Chromosomes 2 and 4 completed

US researchers have unveiled the 'gold-standard' versions of the DNA sequence of human chromosomes two and four. Together, these two bundles of genetic material make up five per cent of the entire human genome. The analysis of chromosome four has revealed the largest 'gene deserts' identified so far - vast stretches of DNA that do not contain any protein-coding genes. The international team of scientists, based at the Washington University School of Medicine in St Louis, published its results in the journal Nature.

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 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 (numbers 1-22 plus the X and Y sex chromosomes), 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. Detailed analyses of chromosomes 5, 6, 7, 9, 10, 13, 14, 16, 19, 20, 21, 22, X and Y have already been published, with the remaining eight due shortly.

Chromosome four, which is home to the genes involved in Huntington's disease and polycystic kidney disease, contains a total of 796 genes. But the chromosome also contains large stretches of DNA that do not contain any genes. The researchers say these gene deserts probably still have some important biological function, since they have been conserved throughout evolution since humans shared a common ancestor with birds. Chromosome two, the second largest human chromosome, contains 1,346 genes. The new study confirms that it arose when two chromosomes in an ape ancestor fused together. Interestingly, the researchers also identified a gene on chromosome two that makes a protein that is possibly unique to humans and chimpanzees.

Francis Collins, director of the National Human Genome Research Institute (NHGRI), called the analysis 'an impressive achievement', which will 'deepen our understanding of the human genome and speed the discovery of genes related to human health and disease'. He also said the findings would provide 'exciting new insights into the structure and evolution of mammalian genomes'.