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10.1 An Overview of Meiosis
T
alk about stopping the presses. In the spring of 2004, presses that were printing biology textbooks may literally have been stopped when their authors heard the
news you can see in the story above. For decades, every biology textbook confi-
dently told its readers that all the eggs a female mammal makes are formed within her prior
to her birth—while she herself is an embryo, in other words. This initial store of eggs then
slowly dwindles over time, the textbooks said, and this is the primary reason that women
can't get pregnant past early middle age: Their supply of eggs runs out. By contrast, men remain fertile throughout life because their sperm are produced each day by “stem cells,”
which is to say cells that never lose their ability to produce more sperm.
Then came the news that, at least in mice, female mammals do possess stem cells that are ca-
pable of giving rise to eggs and that these stem cells remain active throughout life. Now, imagine the implications if such cells could be found or coaxed
into development in human females: Fertility might be prolonged for older women or enhanced for women of any
age. This news was big, and it was good.
Buried within it, however, was something that would be
easy to miss. Note the implication that eggs and sperm
develop from cells. Indeed, eggs and sperm are cells themselves. In Chapter 9, you went over the process by which
one cell becomes two, and you might think that this kind
A human egg, surrounded by accessory cells, moves into
a woman’s Fallopian tube during the process of ovulation.
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CHAPTER 10 Preparing for Sexual Reproduction: Meiosis
of cell division would be at work in producing eggs and sperm, but that's not the
case. Eggs and sperm can't develop the way regular cells do, because eggs and
sperm are destined to come together—sperm fertilizing egg—to create offspring.
Why should this affect the way they develop? Well, a little story from regular life
can show you the problem that would ensue if sperm and egg were produced
through normal cell division.
A real-life couple—let’s call them Jack Fennington and Jill Kent—combine their
last names when they get married and thus become Jack and Jill Fennington-Kent.
Now, what would happen if the Fennington-Kent's children were to continue in
this tradition? Their daughter Susie might marry, say, Ralph Reeson-Dodd, which
would make her Susie Fennington-Kent-Reeson-Dodd. If her daughter, Alicia, were
to keep this up, she might be fated to become Alicia Fennington-Kent-ReesonDodd-Garcia-Lee-Minderbinder-Green, and so on.
Now consider our cells. Remember that regular human cells have 23 pairs of
chromosomes or 46 chromosomes in all. If an egg and sperm each brought 46 chromosomes to their union—to their merger in conception—the result would be an
embryo with 92 chromosomes. The next generation down would have 184 chromosomes, the next after that 368, and so on. This would be as functional as having
Fennington-Kent-Reeson-Dodd-Garcia-Lee-Minderbinder-Green as a last name.
Thus, eggs and sperm have to be produced in a special way. What way is this?
In sexual reproduction, chromosome union is preceded by chromosome reduction.
The reduction occurs in the cells that give rise to sperm and egg. When these cells
divide, the result is sperm or egg cells that have only half the usual number of
chromosomes. Human sperm or eggs, in other words, have only 23 chromosomes
in them. Each 23-chromosome sperm can then unite with a 23-chromosome egg to
produce a 46-chromosome fertilized egg that develops into a new human being. In
each generation, then, there is first a halving of chromosome number (when egg
and sperm cells are produced), followed by a coming together of these two halves
(when sperm and egg unite). In this chapter, you'll learn about this process and its
relation to the fantastic variety we see in the living world.
Some Helpful Terms
The kind of cell division that results in the halving of chromosome number is
called meiosis. This stands in contrast to mitosis, which you looked at in Chapter
9. The cells that reproduce through mitosis are known as somatic cells. And which
cells are these? In animals, all the cells in the organism, except for the reproductive
cells—the eggs and sperm—which are known as gametes.
Egg and sperm are said to be in the haploid state, the term haploid meaning
“single number.” When egg and sperm unite, however, it marks a return to the
diploid, or “double number” state of cellular existence. By definition, haploid cells
possess a single set of chromosomes while diploid cells possess two sets of chromosomes. In human beings, haploid cells have 23 chromosomes, while diploid
cells have 46 chromosomes. Meiosis can be defined as a process in which a single
diploid cell divides to produce haploid reproductive cells. Diploid cells are also
sometimes referred to as 2n cells (the “2” here standing for a doubled number of
chromosomes), while haploid cells are said to be 1n. In what follows, we will be
looking at meiosis as it occurs in human beings.
10.2 The Steps in Meiosis
How does the chromosomal halving take place? Let’s go over the process of meiosis and see. FIGURE 10.1 shows meiosis, in a stripped-down form, as compared to
the mitosis described in Chapter 9. Two essential differences between the two
processes can be seen in the figure. First, you may remember that the formula for
mitosis was: duplicate once, divide once. Duplicate the chromosomes once, then
divide the original cell once. With meiosis, on the other hand, the formula is: duplicate once, divide twice. Meiosis includes one chromosome duplication followed