糖心视频

(糖心视频Org.com) -- David Page, director of the Whitehead Institute and professor of biology at Massachusetts Institute of Technology, says research indicates the much-maligned Y chromosome plays a more critical role in genetics than previously believed.

In comparison to the X chromosome, says David Page, the Y chromosome is a 鈥渄emure, rather shy little fellow鈥� traditionally believed by scientists to be decaying or stagnating to the point where some researchers have predicted its eventual extinction.

鈥淚 have spent the better part of the last 25 years defending the honor of this small, downtrodden chromosome in the face of numerous insults to its character,鈥� said Page, director of the Whitehead Institute and a professor of biology at Massachusetts Institute of Technology, during a lively lecture Thursday (April 15) titled 鈥淭he Evolutionary and Genetic Basis of Human Reproduction,鈥� the final talk this semester in the 鈥淓volution Matters鈥� series sponsored by the Harvard Museum of Natural History.

Talking as if he were teaching a class, Page dispensed with the traditional format of holding a question-and-answer session at the end of his lecture, and instead invited audience participation. He began with a 鈥渃rash course鈥� he called 鈥淗uman Genome 101,鈥� asking questions such as 鈥淗ow many cells do you have in your body?鈥� (10 trillion); 鈥淗ow many genomes per cell?鈥� (two, except for in gametes, which have one); and the trickier 鈥淗ow old is sex?鈥� (that depends whether you鈥檙e talking about bacteria, yeast, turtles, or humans), before tackling gene recombination.

During recombination, he explained, genes usually work in pairs, swapping material to lead to DNA repair and more robust genetic diversity. Every cell in the human body contains 23 pairs of chromosomes, and 22 of those pairs are matched. The 23rd in about 50 percent of people (that is, men) are not a matched pair but an XY pair. The Y chromosome is passed from father to son and contains the genes necessary for forming testicles, and therefore making sperm. Until Page鈥檚 laboratory learned differently, scientists believed that the Y chromosome, which has about 80 genes compared with the X鈥檚 1,000 or so, did not pair-swap genetic material, and therefore was a weakened player.

But Page and his colleagues discovered that the Y chromosome does swap genetic material. The twist is, it swaps with itself. The Y, Page鈥檚 lab learned, stores DNA as a palindrome that reads the same in either direction 鈥� like the name Otto, for example. 鈥淭he palindromes on Y are spectacular,鈥� Page said. 鈥淚t has almost perfect left-arm-to-right-arm symmetry,鈥� with only .06 percent divergence.

One thing scientists knew was true was genes on the Y did not come in pairs, which would mean that Y chromosomes are very young, evolutionarily speaking, only about half a million years old. 鈥淣ow all of a sudden we realized genes on Y come in pairs, just not from Mom and Dad, but on the arms of the palindrome.鈥� The arms of the palindrome engage in 鈥渘onreciprocal recombination,鈥� folding over on themselves to 鈥渙verwrite鈥� faulty genetic material.

鈥淭his implies that the palindrome existed in the chimp/human ancestor 6 million years ago,鈥� said Page, whose lab also sequenced the chimp Y chromosome and discovered that the Y has continued to evolve in the 6 million years since chimps and humans emerged from a common ancestor.

Page and his colleagues also discovered that the Y chromosome may be linked to Turner syndrome in women, which is characterized by the lack of one sex chromosome, and can cause short stature, heart defects, and infertility due to ovarian malfunction. The syndrome may be the result of Y chromosome recombination gone awry, Page speculates, when the chromosome inadvertently becomes a palindrome with no gap in the center.

Known as the centromere, the middle space between the two arms of the Y chromosome is key to its health. If two centromeres are inadvertently created, as they were on 18 of 60 patients studied who had low sperm production, there are anomalies of the Y chromosome, or discordance between chromosomal constitution and anatomy 鈥� that is, feminization. 鈥淚t turns out these centromeres play a critical role in passing out one copy of each chromosome to each daughter cell,鈥� said Page. 鈥淚ronically, the more Y you have, the more likely you鈥檙e a female.鈥�