The third of 23 human chromosomes has been sequenced, and scientists have found major discrepancies between the new sequence and the highly publicized human genome map published in February.
Researchers say the differences are a reality check that points to the need to continue to refine the human genome map and correct inaccuracies.
A publicly funded group that goes by the collective name the Human Genome Project, as well as a for-profit company, Celera, each published a genome map in scientific journals in February.
In June 2000, the rivals made nice and declared the race to their maps a tie. But in 1998, when Celera scientists announced they would map the entire human genome in less than a year and for a fraction of the cost of the 10-year, $2 billion Human Genome Project, critics questioned the relevance of the publicly funded project.
Some say that discrepancies found in chromosome 20 are proof that the public project is still vital.
"While it is every intent of the public effort to continue to use the mapping approach until all the chromosomes are finished, Celera Genomics, as far as I know, does not intend to ... try and resolve such discrepancies," said Todd D. Taylor, a research scientist at the Genomic Sciences Center of the RIKEN Yokohama Institute in Japan.
It's important to get the genetic sequences correct because the order of the genes is key to understanding some diseases, researchers say. Chromosome 20, for example, is the home of many diseases.
The genes that cause Creutzfeldt–Jakob disease, severe combined immunodeficiency, type 2 diabetes, obesity, cataract, and eczema have been linked to chromosome 20.
Knowing the genetic sequence of the diseases will lead to better diagnosis and possibly treatment.
Researchers at the Wellcome Trust Sanger Institute in the United Kingdom have sequenced 99.5 percent of chromosome 20's genes. It contains 60 million bases, represented by the letters A, C, T and G, which stand for the nucleotides adenine, cytosine, thymine and guanine.
Sanger Institute scientists started working on chromosome 20 in 1996. Considering Celera mapped a rough draft of the whole human genome in about a year, that seems like a long time for one chromosome. But the scientists say they had to build their sequencing technology from scratch.
"It may sound a long time ago but at the time we had to put in place all the necessary mapping resources ourselves. Chromosome projects that started later benefited much more from the concerted efforts to generate whole-genome resources," said Panos Deloukas, a human genetics senior group leader at the Sanger Institute who led the study.
The project is detailed in the December 20/27 issue of the scientific journal Nature.
The researchers not only looked at chromosome 20's sequence, but they also compared it to sequences in other species like the mouse and pufferfish.
By comparing the genomes of different creatures, researchers can pinpoint which genes have survived hundreds of millions of years of evolution.
"The identification of these conserved segments is important for understanding the entire structure and function of the human and other genomes," Taylor said.
Chromosome 20 ranks about fifth compared to other chromosomes in gene density, although it ranks about 20th as far as number of genes.
