Science News Review

Genes and Not-Genes: Human Genome Shrinks Again

Jan 16 at 5:05pm by Aileen

Human Genome loses nearly 5,000 genes

Back before the human genome had been sequenced biologists estimated that we might have up to 150,000 genes to work with. Genes are those stretches of DNA code sequences between “start” and “stop” codons that are transcribed and used to create functional proteins from amino acids. High initial estimates of how many genes it takes to be a human were the natural result of a “gene-centric” point of view in biology, which assumed that all the particular traits of any organism would be determined by specific genes for those traits. We now know that things are more complicated than that.

When the first draft of the human genome was published in 2001 the approximate number of genes had been pared down - somewhat surprisingly to many - to a mere ~35,000 genes. That number has been whittled down even further over the next few years, and now has been reduced yet again by nearly 5,000. Current estimate: a mere 20,500 genes in the entire human genome.

Researchers from the Broad Institute of MIT and Harvard in Cambridge identified these nearly 5,000 “mistaken genes” by comparing their sequences with mouse and genomes and finding that there were no counterparts. Evolution tends to preserve gene sequences with relatively little substitution because the proteins encoded are so important to biological functions. The researchers considered that perhaps these genes had no analogs in the mouse or dog genomes because they were unique to primates. So they compared them to the genomes of chimpanzees and macaques. The suspect sequences were also absent from these more closely related genomes, thus they lost the title of ‘gene’.

Those sequences were included as genes in the online genome databases and will now have to be removed. The researchers are not convinced that they are actually “Junk DNA” as they may prove to have some function, but they are not protein coding genes. The research was published in the November 27 issue of PNAS.

Links:

Human gene count tumbles again

Human Genome Analysis Hints at New Proteins Involved in Gene Expression [2001]

Largest Human-Mouse Genome Comparison Spotlights Parental Competition

Estimates of Human Gene Number Too Low [!!! 2001]