Sexual Reproduction
and Meiosis
Sexual life cycle

Made up of meiosis and fertilization
Diploid cells

Haploid cells

Chapter 11
◦ Somatic cells of adults have 2 sets of chromosomes
◦ Gametes have only 1 set of chromosomes

Offspring inherit genetic material from 2
parents
2
Sexual life cycle stages



Features of Meiosis

Life cycles of sexually
reproducing organisms
involve the alternation of
haploid and diploid stages
Some life cycles include
longer diploid phases, some
include longer haploid phases
In most animals, diploid state
dominates
Meiosis includes two rounds
of division
◦ Meiosis I and meiosis II
◦ Each has prophase, metaphase,
anaphase, and telophase stages

Synapsis
◦ During early prophase I
◦ Homologous chromosomes
become closely associated
◦ Includes formation of
synaptonemal complexes
◦ Zygote first undergoes mitosis
to produce diploid cells
◦ Later in the life cycle, some of
these diploid cells undergo
meiosis to produce haploid
gametes
 Formation also called tetrad or
bivalents
3
4
The Process of Meiosis
Crossing over
Genetic recombination between nonsister
chromatids
 Allows the homologues to exchange
chromosomal material
 Alleles of genes that were formerly on
separate homologues can now be found on the
same homologue
 Chiasmata – site of crossing over



Meiotic cells have
an interphase
period that is
similar to mitosis
with G1
G1, SS, and G2
phases
After interphase,
germ-line cells
enter meiosis I

Meiosis I
◦
◦
◦
◦

Prophase I
Metaphase I
Anaphase I
Telophase I
Meiosis II
◦
◦
◦
◦
Prophase II
Metaphase II
Anaphase II
Telophase II
◦ Contact maintained until anaphase I
5
6
1
The main difference


The main point
During metaphase I, the paired homologues move to
the metaphase plate and become oriented with
homologues of each pair attached to opposite poles
of the spindle

First meiotic division is termed the “reduction
division”
◦ Results in daughter cells that contain one homologue from
each chromosome pair
◦ In mitosis
mitosis, homologues behave independently

During anaphase I, homologues are pulled to
opposite poles for each pair of chromosomes

◦ In mitosis, sister chromatids are pulled to opposite poles
No DNA replication between meiotic divisions
Second meiotic division does not further reduce the
number of chromosomes
◦ Separates the sister chromatids for each homologue
7
8
Prophase I



Metaphase I
Chromosomes coil tighter and become visible,
nuclear envelope disappears, spindle forms
Each chromosome composed of 2 sister chromatids
Synapsis
◦ Homologues become closely associated
◦ Crossing
C
over occurs between
b
nonsister chromatids
h
d
◦ Remain attached at chiasmata
 Chiasmata move to the end of the chromosome arm
before metaphase I
Terminal chiasmata hold homologues together
following crossing over
 Microtubules from opposite poles attach to
each homologue

◦ Not each sister chromatid
Homologues are aligned at the metaphase
plate side-by-side
 Orientation of each pair of homologues on the
spindle is random

9
10
11
12
2
Anaphase I

Microtubules of the spindle shorten

Homologues are separated from
each other and move to opposite
poles
Telophase I
◦ Chiasmata break
◦ Sister chromatids remain attached to
each other at their centromeres


Nuclear envelope re-forms
around each daughter
nucleus
 Sister chromatids are no
longer identical because of
crossing over (prophase I)
 Cytokinesis may or may not
occur after telophase I
 Meiosis II occurs after an
interval of variable length

Each pole has a complete haploid set
of chromosomes consisting of one
member of each homologous pair
Independent assortment of maternal
and paternal chromosomes
13
14
Meiosis II
Resembles a mitotic division
 Prophase II: nuclear envelopes dissolve and
new spindle apparatus forms
 Metaphase II: chromosomes align on
metaphase plate
 Anaphase II: sister chromatids are separated
from each other
 Telophase II: nuclear envelope re-forms around
4 sets of daughter chromosomes; cytokinesis
follows

15
16
Final result
Errors
Four cells containing haploid sets of
chromosomes
 In animals, develop directly into gametes
 In plants
plants, fungi
fungi, and many protists,
protists divide
mitotically
Nondisjunction – failure of chromosomes to
move to opposite poles during either meiotic
division
 Aneuploid
p
ggametes – ggametes with missingg or
extra chromosomes
 Most common cause of spontaneous abortion
in humans


◦ Produce greater number of gametes
◦ Adults with varying numbers of gametes
17
18
3
Meiosis vs. Mitosis
Meiosis is characterized by 4 features:
1. Synapsis and crossing over
2. Sister chromatids remain joined at their
centromeres throughout meiosis I
3. Kinetochores of sister chromatids attach to
the same pole in meiosis I
4. DNA replication is suppressed between
meiosis I and meiosis II

19
20
Please note that due to differing
operating systems, some animations
will not appear until the presentation is
viewed in Presentation Mode (Slide
Show view). You may see blank slides
in the “Normal” or “Slide Sorter” views.
All animations will appear after viewing
in Presentation Mode and playing each
animation. Most animations will require
the latest version of the Flash Player,
which is available at
http://get.adobe.com/flashplayer.
21
4