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Round two for Chromosome 22
  
By
Edward R. Winstead


Scientists have revised the gene map of human chromosome 22, eliminating errors, adding newly discovered genes and fusing misplaced DNA fragments. The new version has virtually the same number of genes as the original sequence, published in 1999, but the roster of genes has changed.

"We were able to show that some genes we originally thought were there probably are not there," says Ian Dunham of the Wellcome Trust Sanger Institute in Cambridge, U.K., who led the study. "And we also were able to join together fragments of the same gene."


Deletion of genes in DiGeorge syndrome can be visualized by a fluorescent signal on only one of the two copies of chromosome 22.

The accuracy of the DNA sequence is critical to scientists who study diseases linked to chromosome 22, such as the DiGeorge syndrome. Infants born with this rare condition are missing a large portion of chromosome 22. As a result, they develop recurrent infections and heart problems.

"When you're working on a chromosomal region where a gene for a disease has been mapped, it's important to know exactly which genes are in that region," says Dunham, who also worked on the original sequence. The new analysis, he believes, makes it fairly certain that scientists now know all of the genes on chromosome 22 that code for proteins. The findings appear in Genome Research.

When the new analysis of chromosome 22 was completed, the total number of known genes increased only by one, to 546. But the number of 'pseudogenes' more than doubled (to 234). Pseudogenes are gene-like stretches of DNA that apparently are not real genes; it is not clear what these pseudogenes do or why they are there.

The researchers found many duplicated fragments of genes—places where part of a gene has been duplicated and has moved to another part of the same chromosome. One of the challenges facing the researchers now is to determine which of these fragments have biological functions.

Another challenge is to figure out what the vast majority of human DNA, which lies outside of genes, does. Dunham and his team have begun to analyze regions outside genes that produce the chemical RNA but unlike real genes, do not produce a protein. These are known as non-coding RNAs.

"There is a lot of interest right now in non-coding RNAs and whether they have functions in the cell," says Dunham. "We need to look now at regions of the chromosome outside those that we and others have identified as genes."

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»RNA study finds activity in ‘silent’ regions of human genome

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Collins, J.E. et al. Reevaluating human gene annotation: a second-generation analysis of chromosome 22. Genome Res 13, 27-36 (January 2003).
 

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