HUMAN BIOLOGY

CeLL reprOduCtIOn 365

What are the key events of meiosis?

• In meiosis the nucleus of a diploid germ cell divides twice.
  Ultimately four haploid nuclei form, each with 23 chromosomes—
  one of each type.


• Meiosis takes place in two steps, called meiosis I and meiosis II.
  Similar to mitosis, each step occurs in stages called prophase,
  metaphase, anaphase, and telophase.


• Cytokinesis occurs at the close of both meiosis I and meiosis II.
  The result is four haploid cells.

taKe-hoMe Message

Meiosis ii produces four haploid cells, each

with a set of unduplicated chromosomes

In prophase II and metaphase II, the same events occur in

each new nucleus as in prophase I and metaphase I (steps 5

and 6). In anaphase II, the spindle microtubules pull the

sister chromatids apart (step 7). Each chromosome now con-

sists of one DNA molecule. During telophase II, the chro-

mosomes (which are no longer duplicated) cluster around

each spindle pole. There are four clusters in all. Each one

is a set of unduplicated chromosomes. New nuclear enve-

lopes form around the four clusters, forming four haploid

nuclei—two in each cell (step 8). Cytokinesis then divides

the cytoplasm of each cell, producing four haploid cells.

No DNA

replication

nuclear envelope centrosome

breaking up

plasma

membrane spindle

one pair of

homologous chromosomes

Prophase I. Homologous
chromosomes condense, pair
up, and swap segments. Spindle
microtubules attach to them as
the nuclear envelope breaks up.

Metaphase I. The homologous

chromosome pairs are aligned

midway between spindle poles.

Anaphase I. The homologous

chromosomes separate and begin

heading toward the spindle poles.

Te lophase I. Two clusters of

chromosomes reach the spindle

poles. A new nuclear envelope

forms around each cluster, so

two haploid nuclei form.

Prophase II. The chromosomes

condense. Spindle microtubules

attach to each sister chromatid as

the nuclear envelope breaks up.

Metaphase II. The (still

duplicated) chromosomes are

aligned midway between poles

of the spindle.

Anaphase II. All sister chro-

matids separate. The now undu-

plicated chromosomes head to

the spindle poles.

Te lophase II. A cluster of

chromosomes reaches each

spindle pole. A new nuclear

envelope encloses each cluster,

so four haploid nuclei form.

1 2 3 4 5 6 7 8

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