Essential Cell Biology 5th edition

Meiosis and Fertilization
655
In this section, we describe the cell biology of sexual reproduction from a
modern point of view, focusing on the elaborate dance of chromosomes
that occurs when a cell undertakes meiosis. We take a close look at how
homologous chromosomes pair, recombine, and are segregated during
meiosis, thereby shuffling the maternal and paternal genes into novel
combinations. We also discuss what happens when meiosis goes awry.
Finally, we consider briefly the process of fertilization, through which
gametes come together to form a new, genetically distinct individual.
Meiosis Involves One Round of DNA Replication
Followed by Two Rounds of Nuclear Division
Before a diploid cell divides by mitosis, it duplicates all of its chromosomes.
This duplication allows a full set of chromosomes—including a
complete maternal set plus a complete paternal set—to be transmitted to
each daughter cell (discussed in Chapter 18). Although meiosis ultimately
halves this diploid chromosome complement—producing haploid gametes
that carry only a single set of chromosomes—it, too, begins with a
round of chromosome duplication. The subsequent reduction in chromosome
number occurs because this single round of duplication is followed
by two successive rounds of nuclear division, without further DNA replication
(Figure 19−5).
It seems like it would be simpler and more direct if meiosis instead took
place by a modified form of mitotic cell division in which DNA replication
(S phase) were omitted completely; a single round of division could then,
in theory, produce two haploid cells directly. But, for reasons that are still
unclear, this is not the way meiosis works.
Meiosis begins in specialized germ-line cells that reside in the ovaries
or testes. Like somatic cells, these germ-line cells are diploid; each contains
two copies of every chromosome—a paternal homolog, inherited
from the individual’s father, and a maternal homolog, inherited from the
mother. In the first step of meiosis, all of these chromosomes are duplicated,
and the resulting copies remain closely attached to each other, as
they would during an ordinary mitosis (see Chapter 18).
The next phase of the process, however, is unique to meiosis. During this
phase, called meiotic prophase, each duplicated paternal chromosome
locates and then attaches itself along its entire length to the corresponding
duplicated maternal homolog. This process, called pairing, is of fundamental
importance in meiosis, as it allows the segregation of homologous
chromosome pairs during the first meiotic division (meiosis I).
The two duplicated chromosomes within each homolog are then separated
during the second meiotic division (meiosis II), producing four
haploid nuclei. Because chromosome segregation during meiosis I and II
is random, each haploid gamete will receive a different mixture of maternal
and paternal chromosomes.
Thus, meiosis produces four nuclei that are genetically dissimilar and
that contain exactly half as many chromosomes as the original parent
CHROMOSOME
DUPLICATION
MITOSIS 2N
diploid nucleus
MEIOSIS
2N
diploid
germ-line nucleus
CHROMOSOME
DUPLICATION
4N
4N
MITOSIS
MEIOSIS I
2N
2N
two diploid nuclei
2N
2N
MEIOSIS II
N
N
N N
four haploid nuclei
Figure 19−5 Mitosis and meiosis both
begin with a round of chromosome
duplication. In mitosis, chromosome
duplication is followed by a single round
of cell division to yield two diploid nuclei.
In meiosis, chromosome duplication in a
diploid germ-line cell is followed by two
rounds of division, without further DNA
replication, to produce four haploid nuclei.
N represents the number of chromosomes
in the haploid nucleus.