Y chromosome: Encyclopedia II - Y chromosome - Repair of the Y chromosome

Y chromosome - Repair of the Y chromosome

Chromosomes have robust and accurate repair mechanisms. Over time random mistakes - mutations - occur throughout all chromosomes, and the existence of some high-accuracy repair mechanism is known to be necessary for the survival of the chromosome, and thus the species carrying the chromosome.

The primary repair mechanism is dependent upon the fact that all people receive two sets of each chromosome, one from their mother and one from their father. Over time damage occurs, yet at the same time, chromosome pairs swap damaged genes out and replace them with a copy of undamaged genes. Gene sequences on chromosomes are fixed by following the template on the homologous chromosome. This repair technique is called recombination, and repairs a great many errors. Errors not caught by this technique are weeded out over time through natural selection. Until recently such error-correcting mechanisms were known for all chromosomes in humans, with the exception of the Y chromosome.

While females have two X chromosomes, males only have one Y chromosome (and one X chromosome.) Without a homologous chromosome, the Y chromosome cannot utilize this repair mechanism. It is believed that when the sex chromosomes first evolved there was effectively only one type. Over time this diverged into the X and the Y chromosomes, each having roughly 1,000 protein-coding genes. As they diverged over time, the Y chromosome became significantly different from the X chromosome so that it could not swap genes. As such, without a then-extant repair mechanism, errors and deletions built up in the Y chromosome over time. Over time many of the Y chromosome's genes were damaged and then lost.

Since the Y chromosome did not have the same error-correcting machinery that all the other chromosomes have, this gave rise to widespread speculation that no error-correcting machinery existed within this chromosome at all. Without any such machinery, random errors in copying would logically and inevitably cause the destruction and disappearance of the Y chromosome in all animals. Indeed, over time it appears that the Y chromosome has indeed lost many of its original DNA material, and has become much smaller.

If this damage and loss were to continue unabated, this would lead to the disappearance of all males in any species, including humans. As a result, the only species that would survive in the long term would be those species that naturally evolved a female-only method of reproduction. However, this line of reasoning was based on the sole assumption that lack of knowledge about Y chromosome repair meant that no possible repair mechanism could exist. This assumption was shown to be in serious error in 2003.

All other chromosomes occur in pairs. They preserve genetic integrity by exchanging information with matching genes on the homologous chromosome, a process called "crossing over." But the Y chromosome lacks that option, being the only chromosome that is unpaired. What was discovered in 2003 was that the Y chromosome exchanges genes between the two copies of repeated sequences that lie near to each other as mirror images. This phenomenon is called gene conversion. It is the non-reciprocal transfer of genetic information from one DNA molecule to another. It has been previously observed on a small scale over long evolutionary timescales between repeated sequences on the same chromosome, but not at the dramatic frequency apparently employed by the Y chromosome.

A research team, led by David C. Page, M.D., a Howard Hughes Medical Institute investigator at the Whitehead Institute for Biomedical Research in Cambridge, Mass.; Richard K. Wilson, Ph.D., director of the Genome Sequencing Center at Washington University School of Medicine in St. Louis; and Robert H. Waterston, M.D., Ph.D., formerly of Washington University's sequencing center and now at the University of Washington, Seattle, discovered that many of the sequences of chemical units -- called bases or base pairs -- that carry genetic information on the Y chromosome are arranged as palindromes.

In the case of the Y chromosomes, the palindromes are not "junk" DNA; these strings of bases contain functioning genes important for male fertility. Most of the sequence pairs are greater than 99.97 percent identical. The extensive use of gene conversion appears to play a role in the ability of the Y chromosome to edit out genetic mistakes and maintain the integrity of the relatively few genes it carries.

Findings were confirmed by comparing similar regions of the Y chromosome in humans to the Y chromosomes of chimpanzees, bonobos (the pygmy chimpanzee) and gorillas. The comparison demonstrated that the same phenomenon of gene conversion appeared to be at work more than 5 million years ago, when humans and the non-human primates diverged from each other.

Adapted from the Wikipedia article "Repair of the Y chromosome", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki