問2 下の文章中にある下線が引かれた部分(A)~(E)を日本語訳しなさい。
The End of Men? by Joe Palca
As Y Chromosome Shrinks, End of Men Pondered
Each of our cells contains 23 pairs of chromosomes. Twenty-two of those pairs are matched pairs, shared by men and women. The 23rd is different.
In women, the 23rd pair is made up of two X chromosomes. In men, it's made up of an X chromosome and a Y chromosome. That Y chromosome determines maleness in humans -- it holds genes necessary for forming testes and making sperm.
The fact that it doesn't have a matching pair poses a bit of a problem for the Y chromosome.(A)
All the other chromosomes come in two copies. Every time a cell divides, mistakes in genes can creep in. In paired chromosomes, that means that if there is a mistake on one chromosome, a cell can always get the correct gene sequence from the other chromosome.
Over time, mistakes have crept into the Y chromosome, too. But every time a gene on the Y chromosome went bad, it basically disappeared. Scientists theorize that the X and Y chromosome started out with about the same amount of genes -- about 1,000. Today, the Y chromosome has less than 80 genes.
Some geneticists think the Y chromosome is now little more than a genetic wasteland that will eventually just disappear. If that were to happen, it would certainly spell the end of sexual reproduction.(B)
But David Page of MIT's Whitehead Institute vigorously disagrees. "At the same time that it is continuing to lose genes, it's found some new ways of replenishing itself," Page says.
Last year, Page and his colleagues reported a finding that brightened the outlook for the future of men: The Y chromosome has been secretly creating backup copies of its most important genes. These are stored in the DNA as mirror images, or palindromes -- which read the same way forwards and backwards. ("Madam, I'm Adam" is a famous example.)
In Y chromosome palindromes, the first half contains the gene and the second half contains the same information, just in reverse.(C)
That means that many of the genes on the Y chromosome do occur as pairs. Page says members of these pairs appear to be swapping out or recombining with each other -- allowing the genes to repair themselves when they get damaged.
Page says this helps explain why these genes have been able to persist despite millions of years of assault from random mutations. And, he says, it means the Y chromosome won't simply keep shrinking away until it disappears altogether.
Making Sperm in the Lab
The secret of producing sperm without men appears to lie in embryonic stem cells. Under the right conditions, these cells can replenish themselves indefinitely in the laboratory, and they can, in theory, turn into any type of cell in the body.
But for a long time, many questioned whether embryonic stem cells could turn into sperm -- a highly specialized cell that's only formed after puberty. The smallest cells in the male body, sperm are programmed to find and fertilize an egg.
Last year, scientists proved that they could not only create sperm in a petri dish, but also use that sperm to fertilize a mouse egg.(D)
Led by George Daley of Children's Hospital in Boston, the researchers began their process by culturing mouse embryonic stem cells to form globular clusters called embryoid bodies.
Cells in these embryoid bodies differentiated into primitive germ cells (the precursors of eggs and sperm). The germ cells were tagged with a fluorescent chemical that allowed the scientists to isolate and track the germ cells as the body developed.
Those embryoid bodies that were allowed to grow contained cells that became mature male sex cells similar to sperm, minus the tails. Those tail-less sperm were injected directly into mouse egg cells, essentially fertilizing them. The fertilized eggs then developed into early embryos called blastocysts.
Although embryonic stem cells can make primitive spermatids, it's not known yet whether these resulting tail-less sperm will be able to do all the things that regular sperm are supposed to do.
Daley's team has not yet proved that eggs fertilized with sperm derived from embryonic stem cells will grow into a mouse pup when implanted in a mouse mom. But he's confident he'll be able to do that.
Daley also believes that what works with mouse embryonic stem cells will also work with human embryonic stem cells. But he's not trying to put men out of business -- he simply wants to study sperm cell development and infertility.
"The implications for being able to make sperm in a dish allows you to ask questions about normal sperm development, and abnormal sperm development," Daley says.(E)
Besides, Daley notes, there's still one thing that makes men indispensable: "It's clear to make a sperm cell, you do need the Y chromosome. So insofar as men are the only ones who harbor the Y chromosome, you still need a male cell."
Of course, once embryonic stem cells start producing sperm, they can keep doing so forever.
(Dec. 2004, http://www.npr.org)
The End of Men? by Joe Palca
As Y Chromosome Shrinks, End of Men Pondered
Each of our cells contains 23 pairs of chromosomes. Twenty-two of those pairs are matched pairs, shared by men and women. The 23rd is different.
In women, the 23rd pair is made up of two X chromosomes. In men, it's made up of an X chromosome and a Y chromosome. That Y chromosome determines maleness in humans -- it holds genes necessary for forming testes and making sperm.
The fact that it doesn't have a matching pair poses a bit of a problem for the Y chromosome.(A)
All the other chromosomes come in two copies. Every time a cell divides, mistakes in genes can creep in. In paired chromosomes, that means that if there is a mistake on one chromosome, a cell can always get the correct gene sequence from the other chromosome.
Over time, mistakes have crept into the Y chromosome, too. But every time a gene on the Y chromosome went bad, it basically disappeared. Scientists theorize that the X and Y chromosome started out with about the same amount of genes -- about 1,000. Today, the Y chromosome has less than 80 genes.
Some geneticists think the Y chromosome is now little more than a genetic wasteland that will eventually just disappear. If that were to happen, it would certainly spell the end of sexual reproduction.(B)
But David Page of MIT's Whitehead Institute vigorously disagrees. "At the same time that it is continuing to lose genes, it's found some new ways of replenishing itself," Page says.
Last year, Page and his colleagues reported a finding that brightened the outlook for the future of men: The Y chromosome has been secretly creating backup copies of its most important genes. These are stored in the DNA as mirror images, or palindromes -- which read the same way forwards and backwards. ("Madam, I'm Adam" is a famous example.)
In Y chromosome palindromes, the first half contains the gene and the second half contains the same information, just in reverse.(C)
That means that many of the genes on the Y chromosome do occur as pairs. Page says members of these pairs appear to be swapping out or recombining with each other -- allowing the genes to repair themselves when they get damaged.
Page says this helps explain why these genes have been able to persist despite millions of years of assault from random mutations. And, he says, it means the Y chromosome won't simply keep shrinking away until it disappears altogether.
Making Sperm in the Lab
The secret of producing sperm without men appears to lie in embryonic stem cells. Under the right conditions, these cells can replenish themselves indefinitely in the laboratory, and they can, in theory, turn into any type of cell in the body.
But for a long time, many questioned whether embryonic stem cells could turn into sperm -- a highly specialized cell that's only formed after puberty. The smallest cells in the male body, sperm are programmed to find and fertilize an egg.
Last year, scientists proved that they could not only create sperm in a petri dish, but also use that sperm to fertilize a mouse egg.(D)
Led by George Daley of Children's Hospital in Boston, the researchers began their process by culturing mouse embryonic stem cells to form globular clusters called embryoid bodies.
Cells in these embryoid bodies differentiated into primitive germ cells (the precursors of eggs and sperm). The germ cells were tagged with a fluorescent chemical that allowed the scientists to isolate and track the germ cells as the body developed.
Those embryoid bodies that were allowed to grow contained cells that became mature male sex cells similar to sperm, minus the tails. Those tail-less sperm were injected directly into mouse egg cells, essentially fertilizing them. The fertilized eggs then developed into early embryos called blastocysts.
Although embryonic stem cells can make primitive spermatids, it's not known yet whether these resulting tail-less sperm will be able to do all the things that regular sperm are supposed to do.
Daley's team has not yet proved that eggs fertilized with sperm derived from embryonic stem cells will grow into a mouse pup when implanted in a mouse mom. But he's confident he'll be able to do that.
Daley also believes that what works with mouse embryonic stem cells will also work with human embryonic stem cells. But he's not trying to put men out of business -- he simply wants to study sperm cell development and infertility.
"The implications for being able to make sperm in a dish allows you to ask questions about normal sperm development, and abnormal sperm development," Daley says.(E)
Besides, Daley notes, there's still one thing that makes men indispensable: "It's clear to make a sperm cell, you do need the Y chromosome. So insofar as men are the only ones who harbor the Y chromosome, you still need a male cell."
Of course, once embryonic stem cells start producing sperm, they can keep doing so forever.
(Dec. 2004, http://www.npr.org)