Re: Fwd: Y Chromosome Depends on Itself to Survive

From: Chris Taylor (
Date: Thu 19 Jun 2003 - 13:18:32 GMT

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    Cheers Wade that is fantastic :)

    Wade T. Smith wrote:
    > June 19, 2003
    > Y Chromosome Depends on Itself to Survive
    > 19GENE.html?pagewanted=print&position=
    > Biologists have made a fundamental discovery about how the human Y
    > chromosome, a genetic package inherited by men, protects itself against
    > evolutionary decay.
    > As part of the work, the scientists have tallied the exact number of
    > genes on the Y chromosome, finding more than they had expected. That
    > and other research has led the researchers to assess the genetic
    > differences between men and women as being considerably greater than
    > thought.
    > Although most men are unaware of the peril, the Y chromosome has been
    > shedding genes furiously over the course of evolutionary time, and it
    > is now a fraction the size of its partner, the X chromosome. Sex in
    > humans is determined by the fact that men have an X and a Y chromosome
    > in each of their body's cells. Women have a pair of X's.
    > The decay of the Y stems from the fact that it is forbidden to enjoy
    > the principal advantage of sex, which is, of course, for each member of
    > a pair of chromosomes to swap matching pieces of DNA with its partner.
    > The swapping procedure, known to biologists as recombination, occurs
    > between the chromosomes inherited from the mother's and the father's
    > side as a first step to produce the eggs or sperm. Not only does that
    > swapping create novel combinations of genes, making each individual
    > different, but it also enables bad genes — those damaged by mutation or
    > DNA changes — to be replaced by their good counterparts on the other
    > chromosome.
    > Nature has barred the Y chromosome from recombining with the X, except
    > at its very tips, because otherwise the male-determining gene, carried
    > on the Y chromosome, would sneak into the X, making everyone male.
    > The cost of this abstinence, however, is that most of the Y's genes
    > have been rendered useless by mutation and physically shed. The X and
    > the Y chromosomes were once as similar as the 22 other pairs of human
    > chromosomes, and each carried about 1,000 genes. Now the Y carries
    > fewer than 100. What prevents it losing even those?
    > A team of researchers led by Dr. David C. Page, a biologist at the
    > Whitehead Institute in Cambridge, Mass., has made a startling
    > discovery. Denied the benefits of recombining with the X, the Y
    > recombines with itself.
    > The Y chromosome is made of a single DNA molecule that is 51 million
    > units of DNA in length. Within the chromosome, Dr. Page and his
    > colleagues report in Nature today, lie eight vast palindromes, regions
    > that carry identical sequences of DNA units that run in opposite
    > directions like the letters in the sentence "Madam, I'm Adam."
    > By making a hairpin bend in the middle of a palindrome, the two arms
    > can be brought together, aligning two long stretches of almost
    > identical DNA sequence. That is the same step that precedes
    > recombination between the maternal and paternal members of each
    > ordinary chromosome pair, which also have almost identical sequences.
    > In the case of the Y, the alignment of the palindromic sequences leads
    > to gene conversion. A mutated gene on one arm of the palindrome can be
    > converted to the undamaged sequence preserved on the other arm.
    > This narcissistic process of salvation by palindrome seems to be what
    > has saved men from extinction so far. It serves at least to
    > counterbalance the decay caused by the lack of recombination. But Dr.
    > Page and others say it is too soon to say which force is now uppermost.
    > "This is a pretty striking result," said Dr. William Rice, an expert on
    > the evolution of the sex chromosomes at the University of California at
    > Santa Barbara.
    > The mechanism, Dr. Rice said, is novel in human biology. It will take
    > more study, he added, to see whether it can reverse Muller's Ratchet,
    > the name that geneticists give to the grim process of irreversible
    > genetic decay that affects asexual organisms and nonrecombining genome
    > parts like the Y chromosome.
    > "This changes our view of the Y as being an X chromosome wannabe," said
    > Dr. Evan Eichler, an expert on chromosome structure at Case Western
    > Reserve.
    > The X chromosome, too, is denied the benefits of recombination when
    > paired with the Y. But an X chromosome spends two-thirds of its time in
    > a woman, where it can recombine with another X, dodging the Muller's
    > Ratchet that has so eroded the Y.
    > The palindromes that make gene conversion possible sometimes foster
    > another result, large deletions of DNA, including the genes that they
    > carry. Those losses are a major cause of male infertility, Dr. Page has
    > found.
    > Dr. Page's discovery is a fruit of a collaboration with the genome
    > sequencing center at the Washington University School of Medicine in
    > St. Louis. Under its previous director, Dr. Robert H. Waterston, and
    > his successor, Dr. Richard K. Wilson, the center decoded the precise
    > DNA sequence in the Y chromosome, a two-year effort.
    > Dr. Huntington Willard, a genome expert at Duke, said the sequencing
    > effort was "nearly heroic."
    > "Most people," Dr. Willard said, "would have thrown their hands in the
    > air and said this is too much like heavy lifting."
    > Although most of the human genome was decoded using DNA from several
    > people, the Y had to be decoded from one man, because the natural
    > variation between two men would have swamped the very small differences
    > in the arms of the Y's palindromic DNA.
    > The donor of this Y chromosome is anonymous and designated by a sample
    > number. But it is known that he was recruited locally by the Roswell
    > Park Cancer Institute in Buffalo. So it can only be said that the
    > person who revealed the secret of male survival is a Buffalo man known
    > to science as Mr. RPCI-11.
    > In the course of a long study of the Y chromosome, Dr. Page's team has
    > now tallied that it contains 78 genes, some concerned with male
    > fertility and sperm production and others with general biological
    > functions. The fertility genes are almost all sited in the palindromic
    > regions of DNA. Dr. Page theorizes that the other genes are on their
    > way out or that the damage from failure to recombine may drop off after
    > just a handful of genes is left.
    > The finding of 78 active genes on the Y contradicts an earlier
    > impression of the chromosome as being a genetic wasteland apart from
    > its male-determining gene. But if the Y is not a wasteland, important
    > consequences ensue for the differences between men and women.
    > As often noted, the genomes of humans and chimpanzees are 98.5 percent
    > identical, when each of their three billion DNA units are compared. But
    > what of men and women, who have different chromosomes?
    > Until now, biologists have said that makes no difference, because there
    > are almost no genes on the Y, and in women one of the two X chromosomes
    > is inactivated, so that both men and women have one working X chromosome.
    > But researchers have recently found that several hundred genes on the X
    > escape inactivation. Taking those genes into account along with the new
    > tally of Y genes gives this result: Men and women differ by 1 to 2
    > percent of their genomes, Dr. Page said, which is the same as the
    > difference between a man and a male chimpanzee or between a woman and a
    > female chimpanzee.
    > Almost all male-female differences, whether in cognition, behavior,
    > anatomy or susceptibility to disease, have usually been attributed to
    > the sex hormones. But given the genomic differences that are now
    > apparent, that premise has to be re-examined, in Dr. Page's view.
    > "We all recite the mantra that we are 99 percent identical and take
    > political comfort in it," Dr. Page said. "But the reality is that the
    > genetic difference between males and females absolutely dwarfs all
    > other differences in the human genome."
    > Dr. Rice commented that he would have to think through this argument,
    > noting that many genes - up to 15 percent in some animals - are more
    > active in one sex than the other. These differences in gene activity
    > might dwarf the genomic differences described by Dr. Page, he said.
    > Another difference that has emerged between men and women concerns
    > their ribosomes, the numerous small engines in the cell that build its
    > working parts from the instructions in the genes. A general purpose
    > gene on the Y makes a ribosome component. Its counterpart gene on the X
    > makes a slightly different protein.
    > That means that every ribosome in a man's body is slightly different
    > from those in a woman's. Though the difference is pervasive, Dr. Page
    > said, it was not known what significance it may have, if any.
    > One thing his study had made him sure of was the complexity with which
    > nature accomplishes its ends.
    > "It's a great irony that though the Y has been called a sex
    > chromosome," Dr. Page said, "the bulk of it is asexual. Nothing is as
    > it appears."
    > Copyright 2003 The New York Times Company
    > ===============================================================
    > This was distributed via the memetics list associated with the
    > Journal of Memetics - Evolutionary Models of Information Transmission
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    > see:

      Chris Taylor ( »people»chris
    This was distributed via the memetics list associated with the
    Journal of Memetics - Evolutionary Models of Information Transmission
    For information about the journal and the list (e.g. unsubscribing)

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