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4.4 Meiosis: Cell Divisions That Halve Chromosome Number 103

spindle, kinetochores, or centrosomes, are one source of im- Figure 4.14 An overview of meiosis: The chromosomes
proper segregation. Other problems occur in cells where the replicate once, while the nuclei divide twice. In this figure, all
normal restraints on cell division, such as checkpoints, have four chromatids of each chromosome pair are shown in the same
broken down. Such cells may divide uncontrollably, leading shade of the same color. Note that the chromosomes duplicate before
to a tumor. We present the details of cell-cycle regulation, meiosis I, but they do not duplicate between meiosis I and meiosis II.
checkpoint controls, and cancer formation in Chapter 20.

essential concepts 2n
Chromosomes
• Through mitosis, diploid cells produce identical diploid duplicate
progeny cells.
• At metaphase, the sister chromatids are being pulled at
their kinetochores toward opposite spindle poles; these
poleward forces are balanced because the chromatids Meiosis I
are connected at their centromeres. No duplication
• At the beginning of anaphase, the connections between Meiosis II
sister centromeres are severed so sister chromatids No duplication
separate and move to opposite spindle poles.
• Cell cycle checkpoints help ensure correct duplication
and separation of chromosomes.
n n n n
The union of haploid gametes at fertilization yields dip-
loid offspring that carry the combined genetic heritage of
4.4 Meiosis: Cell Divisions That two parents. Sexual reproduction therefore requires the alter-
Halve Chromosome Number nation of haploid and diploid generations of cells. If gametes
were diploid rather than haploid, the number of chromo-
somes would double in each successive generation. In hu-
learning objectives mans, for example, the children would have 92 chromosomes
per cell, the grandchildren 184, and so on. Meiosis prevents
1. Describe the key chromosome behaviors during this lethal, exponential accumulation of chromosomes.
meiosis that lead to haploid gametes.
2. Compare chromosome behaviors during mitosis In Meiosis, the Chromosomes Replicate
and meiosis.
3. Explain how the independent alignment of homologs, Once but the Nucleus Divides Twice
and also crossing-over during the first meiotic division, Unlike mitosis, meiosis consists of two successive nuclear di-
each contribute to the genetic diversity of gametes. visions, logically named division I of meiosis and division II of
meiosis, or simply meiosis I and meiosis II. With each round,
the cell passes through a prophase, metaphase, anaphase, and
During the many rounds of cell division within an embryo, telophase. In meiosis I, the parent nucleus divides to form two
most cells either grow and divide via the mitotic cell cycle daughter nuclei; in meiosis II, each of the two daughter nuclei
just described, or they stop growing and become arrested in divides, resulting in four nuclei (Fig. 4.14). These four
G 0 . These mitotically dividing and G 0 -arrested cells are the nuclei—the final products of meiosis—become partitioned in
so-called somatic cells whose descendants continue to make four separate daughter cells because cytokinesis occurs after
up the vast majority of each organism’s tissues throughout both rounds of division. The chromosomes duplicate at the
the lifetime of the individual. Early in the embryonic devel- start of meiosis I, but they do not duplicate in meiosis II, which
opment of animals, however, a group of cells is set aside for explains why the gametes contain half the number of chromo-
a different fate. These are the germ cells: cells destined for a somes found in somatic cells. A close look at each round of
specialized role in the production of gametes. Germ cells meiotic division reveals the mechanisms by which each gam-
arise later in plants, during floral development instead of ete comes to receive one full haploid set of chromosomes.
during embryogenesis. The germ cells become incorporated
in the reproductive organs—ovaries and testes in animals, During Meiosis I, Homologs Pair,
ovaries and anthers in flowering plants—where they ulti- Exchange Parts, and Then Segregate
mately undergo meiosis, the special two-part cell division
that produces gametes (eggs and sperm) containing half the The events of meiosis I are unique among nuclear divisions
number of chromosomes other body cells have. (Fig. 4.15, meiosis I). The process begins with the replication