PROCESS DIAGRAM
M eiosis • Figure 3.21
Meiosis is the orderly distribution of genetic material to newly formed haploid cells. It
includes steps very similar to those of mitosis, the main differencve being the formation of
tetrads in prophase I. Crossing over at this stage offers even more genetic variation, as the
ends of these chromosomes are close enough to exchange genetic material. Telophase I
then forms two “cells" that immediately go into prophase II, metaphase II, anaphase II, and
telophase II. These phases result in four haploid cells.
MEIOSIS I
MEIOSIS
II
PROPHASE I
PROPHASE
METAPHASE I
Meiosis includes steps
very similar to those of
mitosis, the main
difference being the
formation of tetrads in
prophase I.
Tetrads are pairs of
homologous
chromosomes that
remain close to one
another until they are
pulled apart in
anaphase I.
Crossing over offers
even more genetic
variation, as the ends
of these chromosomes
are close enough to
swap material.
ANAPHASE I
TELOPHASE I
Telophase I then forms
two cells that enclose
doubled copies of half
the chromosomes of the
original diploid cell.
METAPHASE
ANAPHASE
TELOPHASE II
The newly formed cells then immediately go
into prophase II, metaphase II, anaphase II,
and telophase II. These phases operate like
those in mitosis, resulting in four haploid cells.
Stages of meiosis
Tetrad of
Crossing-over
Genetic
sister
between
recombination
chromatids
nonsister
chromatids
Details of crossing-over during prophase
I
As mentioned previously, gametes undergo
meiosis (Figure 3.21 ). Meiosis is very similar
to mitosis. In fact, the two are so similar that
their stages are rather confusingly referred to
by the same names. One difference is that, in
the first prophase of meiosis (prophase I), pairs
homologous
(huh-
MAHL-uh-gus) Similar
in structure, function,
or sequence of ge-
netic information.
of homologous chromosomes (that is, pairs of
chromosome #1, pairs of chromosome #2, and
so on) remain close together in tight groups
called tetrads. During this phase, the chromo-
somes may exchange pieces of DNA in a process
called crossing over. Crossing over “shuffles”
70 CHAPTER 3
Cells and Tissues
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