L15-Zimmer 2009

L15-Zimmer 2009 - Sexual Reproduction it so widespread. The...

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For Darwin, sex was a big question mark. “We do not even in the least know the final cause of sexuality; why new beings should be pro- duced by the union of the two sexual ele- ments,” he wrote in 1862. “The whole subject is as yet hidden in darkness.” Today, biologists understand the molecular nuts and bolts of sex fairly well. Each new human being (or bird or bee) needs a set of chromosomes from each parent. But that’s the how . The why of sex is still fairly mysterious. Bacteria don’t have to search for a mate; they just grow and divide in two. An aspen tree can simply send out shoots that grow into new trees. No muss, no fuss with finding a partner, fertilizing an egg, and joining two genomes. Why should so many species take such a labyrinthine path to reproduction, when straightforward routes are available? Biologists first began to give the question “Why sex?” serious attention about 40 years ago, and today they’re using genomics and other 21st century tools to search for the answer. They are finding hidden signs of sex in the DNA of supposedly asexual organisms and are tracking the evolutionary impact of sex among living populations of animals and plants. Some use sophisticated mathematical models to assess the conditions under which sex can arise. These efforts are providing new hints about how sex first emerged some 2 billion years ago and about the forces that have made it so widespread. The studies bolster a handful of hypotheses: Sex may speed up evolution, for example, or it may provide a better defense against parasites. In the past, scientists have focused on just one of these hypotheses at a time, but today many argue that several forces may be at work at once. Mating of molecules Sex gives nature much of its spice. Fireflies flash through the night to find a mate; a flower’s perfume lures insects to carry pollen to distant partners; male bullfrogs croak to impress females. But despite this dizzying diversity, all sexually reproducing organisms take the same key steps to make new offspring: They shuffle their own DNA and then combine some of it with the DNA of another member of their species to produce a new genome. The key to this novelty is a process called meiosis. As with those of other vertebrates, almost all human cells are diploid: Each one contains two copies of very similar, or homologous, chromosomes. As precursor sex cells divide, they give rise to haploid sex cells of sperm and eggs, each with only one chromosome from each pair. Only when one sex cell fuses with another does it become part of a new diploid genome. Meiosis creates new variations in two ways. There’s a 50-50 chance that a parent will pass down either chromosome of a given pair to his or her offspring. And during the development of sex cells, homo- logous chromosomes undergo recombination: They line up with each other and swap segments of their DNA. So even if two sib- lings get the same chromosome from their mother, their chromo- somes aren’t identical. In 1971, the late British evolu-
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This note was uploaded on 08/27/2011 for the course BIONB 2210 taught by Professor Seeley during the Fall '10 term at Cornell.

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L15-Zimmer 2009 - Sexual Reproduction it so widespread. The...

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