27 August 2007

Is Sex Necessary? Part 1

Motile Plant Gametes

motile plant gametes drawing from http://biodidac.bio.uottawa.ca/

"Mommy, Where Do Gametes Come From?"

"I'm glad you asked that, Honey. We usually don't see gametes, but just because they are small doesn't mean they're not important.

"You see, gametes are special haploid cells produced by a kind of cell division called 'meiosis'. Two gametes can unite to form a diploid cell again. That's called 'fertilization' or 'syngamy'. Diploid cells have a set of pairs of chromosomes, but haploid cells have just one copy of each chromosome. In people, most cells have 23 pairs of chromosomes. Chickens have 39 pairs of chromosomes in their diploid cells. Mosquitoes have 4, isn't that cute?

Day-Old Chick
day-old chick photo from http://en.wikipedia.org/wiki/Chicken
"There are two reasons gametes are important, Snookums. First, although human gametes can't survive on their own, they can live long enough for a swimming gamete from a daddy to get to a big round nonmotile gamete in a mommy. When they join they bring genes from the daddy and genes from the mommy together. So a baby has its own set of genes, some from its daddy and some from its mommy. With our 23 pairs of chromosomes there are lots of ways the daddy's genes, half of which he got from his daddy and half from his mommy, can be shuffled and distributed in his gametes. In fact there are about 8 million possible results from shuffling 23 chromosomes, so you can see the possibility of two gametes from the same individual having the same assortment of grandpa and grandma's chromosomes is really, really tiny.

"And there's a second thing even more wonderful about meiosis, Dear. When the pairs of sister chromatids are in the pachytene stage of prophase I, non-sister chromatids can exchange some of their DNA by 'crossing over'. So the DNA from
A Boy's Chromosomes
karyotype of male from http://www.mathemagic.org/MOBM/DynamicDNA.html
the grandpa and grandma can be even more mixed up! Here, maybe this will be clearer after you watch this little movie.

"All this shuffling and exchanging of DNA means that the baby that grows from the zygote formed by the union of those two gametes probably has a unique set of genes never born on Earth before! Of course if the zygote splits into twins, they will each have the same genetic makeup, but you get the idea.

"This way of making babies is called 'sexual reproduction'. Lots of different life forms do it, but not always in the same way. Bacteria and archaea don't do it at all, since they can't do meiosis. Still, they seem to get along OK. In fact, they are the dominant life forms on our planet!

A Girl's Chromosomes
karyotype of female from http://www.biologyreference.com/Ce-Co/Chromosome-Eukaryotic.html
"If such successful and important organisms as bacteria and archaea can get along without it, why do so many other kinds of protists, fungi, plants and animals go to all the trouble to use sexual reproduction? I think the answer, Sweetie, is that by creating so many combinations of genes, and by mixing them up generation after generation, sexually reproducing organisms can try out new combinations of mutations faster. So there is more variation for natural selection to work on, you see. And if the environment is changing novel combinations of genes might be better able to handle the changing selective pressures, so a baby with an adaptive combination of genes might be more likely to grow up to make gametes of its own.

"Lots and lots of eukaryotes have meiosis and fertilization as part their life cycle. Did you know that we are eukaryotes? We (or our ancestor eukaryotes) evolved meiosis and most of us have never given it up. It is strongly selected by natural selection. One result of the more flexible evolution that results from using sexual reproduction is the origin over the ages of so many new species with so many different ways of living -- like us, Pumpkin! That's why people have sex! Isn't that interesting?"

"Yes, I guess so. . . . Mommy, what are 'cells'?"

Two good articles:

Wikipedia on Evolution of Sex

Good review article

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