Essential Cell Biology 5th edition

Meiosis and Fertilization
657
(A)
MITOSIS
P1
M2
M1
(B)
MEIOSIS I
M1 P1
P2 M2
Figure 19−7 During meiosis I, duplicated
homologous chromosomes pair before
lining up on the meiotic spindle. (A) In
mitosis, the individual duplicated maternal
(M) and paternal (P) chromosomes line up
independently at the metaphase plate; each
consists of a pair of sister chromatids, which
will separate just before the cell divides.
(B) By contrast, in meiosis, duplicated
maternal and paternal homologs pair before
lining up at the metaphase plate. The
maternal and paternal homologs separate
during the first meiotic division, and the
sister chromatids separate during meiosis II.
The mitotic and meiotic spindles are shown
in green.
P2
duplicated homologous chromosomes
line up independently at the
metaphase plate
duplicated homologous chromosomes
pair before lining up at the
metaphase plate
Duplicated Homologous Chromosomes Pair During
Meiotic Prophase
Before a eukaryotic cell divides—by either meiosis or mitosis—it first
duplicates all of its chromosomes. The twin copies of each duplicated
chromosome, called sister
ECB5
chromatids,
E19.08/19.07
are tightly linked along their
length. The way these duplicated chromosomes are handled subsequently,
however, differs between meiosis and mitosis. In mitosis, as we
discuss in Chapter 18, the duplicated chromosomes line up, single file, at
the metaphase plate (Figure 19−7A). They are then segregated into the
two daughter nuclei.
In meiosis, however, the need to halve the number of chromosomes introduces
an extra demand on the cell-division machinery. The germ-line
cell must keep track of the maternal and paternal homologs—to ensure
that each of the four haploid cells produced by meiosis will receive a
single sister chromatid from each chromosome set. Meiosis therefore
begins with a complex and time-consuming process called pairing, in
which duplicated homologs are brought together during a stage called
meiotic prophase (or prophase I). It is these pairs of duplicated homologs
that line up at the metaphase plate in meiosis I (Figure 19−7B). Each
pairing forms a structure called a bivalent, in which all four sister chromatids
stick together until the cell is ready to divide (Figure 19−8).
The maternal and paternal homologs will separate during meiosis I,
and the individual sister chromatids will separate during meiosis II.
How the homologs (and the two sex chromosomes) recognize each other
during pairing is still not fully understood. In many organisms, the initial
association depends on an interaction between matching maternal
and paternal DNA sequences at numerous sites that are widely dispersed
along the homologous chromosomes. Once formed, bivalents are very
stable: they remain associated throughout meiotic prophase, a stage that
in some organisms can last for years.
duplicated
paternal
chromosome
duplicated
maternal
chromosome
centromere
sister
chromatids
Crossing-Over Occurs Between the Duplicated Maternal
and Paternal Chromosomes in Each Bivalent
The picture of meiosis we have just painted is greatly simplified, in that
it leaves out a crucial feature. In sexually reproducing organisms, the
pairing of the maternal and paternal chromosomes is accompanied by
bivalent
Figure 19−8 Duplicated maternal and
paternal chromosomes pair during
meiosis I to form bivalents. Each bivalent
ECB5 e19.09/19.08
contains four sister chromatids.