HUMAN BIOLOGY

364 Chapter 18

the stages of Meiosis

Pairs of centrioles are positioned at the opposite sides of the

nucleus as the nuclear envelope breaks up (step 1). Micro-

tubules of the spindle link the chromosomes to the spindle

poles so that each chromosome is attached to one spindle

pole, and its homologous partner is attached to the other.

At metaphase I, all the chromosomes are aligned midway

between the spindle poles (step 2).

In anaphase I, the spindle microtubules separate the

homologous chromosomes and pull them toward opposite

spindle poles (step 3). During telophase I, the chromosomes

reach the spindle poles (step 4). New nuclear envelopes

form around the two clusters of chromosomes. Each of the

two haploid nuclei that form contains one set of duplicated

chromosomes. The cytoplasm divides at this point (by

cytokinesis), forming two haploid cells, but the cells’ DNA

is not copied before meiosis II begins.

n During meiosis, chromosomes of a diploid nucleus of a germ

cell are distributed into four haploid nuclei of gametes.

Before meiosis begins, DNA is replicated and a cell’s chro-

mosomes are duplicated, so each chromosome consists of

two sister chromatids. The starting nucleus is diploid—it

contains two sets of chromosomes, one from each parent.

Figure 18.11 takes us step by step through meiosis.

Meiosis i produces two haploid cells that

contain duplicated chromosomes

The first stage of meiosis I is prophase I. During this

phase, the chromosomes condense, align, and exchange

segments. (This “swap” is discussed in Section 18.8.) The

centrosome is duplicated along with its two centrioles.

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

Figure 18.11 Animated! The two consecutive divisions in meiosis reduce the parental number of chromosomes by half

(to the haploid number) for forthcoming gametes. Only two pairs of homologous chromo somes (blue and pink) are shown.

(© Cengage Learning; All photos: With thanks to the John Innes Foundation Trustees)

18.7

Copyright 2016 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).