US scientists decode human chromosomes 2 and 4

A multi-institutional US research team has completed DNA analysis of human chromosomes 2 and 4, reported the journal Nature on Wednesday.

The research team, led by Washington University School of Medicine in St Louis, has detected the largest "gene deserts" known in the human genome and uncovered more evidence that human chromosome 2 arose from the fusion of two ancestral ape chromosomes, said a paper published in the April 7 issue of Nature.

The sequencing work on the chromosomes was carried out as part of the international Human Genome Project.

"This analysis is an impressive achievement that will deepen our understanding of the human genome and speed the discovery of genes related to human health and disease," said Francis Collins, director of US National Human Genome Research Institute.

"In addition, these findings provide exciting new insights into the structure and evolution of mammalian genomes," he said.

Chromosomes 2 and 4 are approximately 237 million base pairs and 186 million base pairs long respectively. Scientists confirmed the existence of a total of 1,346 protein-coding genes on chromosome 2 and 796 on chromosome 4.

Included on chromosomes 2 and 4 are genes previously linked to Huntington's disease, polycystic kidney disease, a form of muscular dystrophy, and Wolf-Hirschhorn syndrome, a condition that causes severe birth defects and mental retardation.

Scientists identified some of the human genome's largest "gene deserts" on chromosomes 2 and 4. These are large regions of DNA that contain very little in the way of protein-building instructions.

The presence of the deserts in other genomes suggests they may have important regulatory functions that scientists have yet to identify, the researchers said.

Also included on chromosome 2 is the longest protein-coding sequence yet identified, a gene called titin that spans 280,000 base pairs and produces a muscle protein that is more than 33,000 amino acids long. Protein length varies widely, but typically averages about 500 amino acids.

Scientists also identified several "hypervariable" regions, where the sequence of base pairs show significant variation among individuals.

In October 2004, the International Human Genome Sequencing Consortium published its scientific description of the finished human genome sequence in Nature. Detailed annotations and analyses have already been published for chromosomes 5, 6, 7, 9, 10, 13, 14, 16, 19, 20, 21, 22, X and Y.

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