Meiosis

1. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

2. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
– Interphase,
where
chromosomes
duplicate
and cell parts
are made
– The mitotic
phase, when
nuclear
division
occurs
The life cycle of a cell
Cell cycle consists of 2 major phases
Figure 8.5

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• During
interphase a
cell performs
all of its
regular
functions and
gets ready to
divide
• Metabolic
activity is very
high
Most of the life of a cell is spent in Interphase
Cell does most of its’ growth during interphase
Figure 8.5

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• Untwisting and replication of DNA
Figure 10.4B

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• Before a cell
starts dividing,
the chromosomes
are duplicated
– This process
produces sister
chromatids
– EM of human
chromosome
that has
duplicated
Centromere
Sister chromatids
Figure 8.4B

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Structure of Chromosomes
– Homologous chromosomes are identical
pairs of chromosomes.
– One inherited from mother and one from
father
– made up of sister chromatids joined at the
centromere.
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8. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• This phase spans
the time from the
completion of
DNA synthesis to
the onset of cell
division
• Following DNA
replication, the
cell spends about
2-5 hours making
proteins prior to
entering the M
phase
G2 Phase
Figure 8.5

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INTERPHASE PROPHASE
Centrosomes
(with centriole pairs)
Chromatin
Nucleolus Nuclear
envelope
Plasma
membrane
Early mitotic
spindle
Centrosome
Centrosome
Chromosome,
consisting of two
sister chromatids
Fragments
of nuclear
envelope
Kinetochore
Spindle
microtubules
Figure 8.6

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METAPHASE TELOPHASE AND CYTOKINESIS
Metaphase
plate
Spindle Daughter
chromosomes
Cleavage
furrow
Nucleolus
forming
Nuclear
envelope
forming
ANAPHASE
Figure 8.6 (continued)

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13. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

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15. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• In animals,
cytokinesis occurs by
cleavage
– This process
pinches the cell
apart
– The first sign of
cleavage is the
appearance of a
cleavage furrow
Cytokinesis differs for plant and animal cells
Figure 8.7A
Cleavage
furrow
Cleavage
furrow
Contracting ring of
microfilaments
Daughter cells

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– As the daughter
chormosomes move
to opposite poles
– The cytoplasm
constricts along the
plane of the
metaphase plate
The process of
cytokinesis divides
the cell into two
genetically identical
cells
Cytokinesis differs for plant and animal cells
Figure 8.7A
Cleavage
furrow
Cleavage
furrow
Contracting ring of
microfilaments
Daughter cells

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• When the cell
divides, the sister
chromatids
separate
– Two daughter
cells are
produced
– Each has a
complete and
identical set of
chromosomes
Centromere Sister
chromatids
Figure 8.4C
Chromosome
duplication
Chromosome
distribution
to
daughter
cells

18. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• The
human life
cycle
• Meiosis is
a special
form of
cell
division
that
produces
gametes
Figure 8.13
MEIOSIS FERTILIZATION
Haploid gametes (n = 23)
Egg cell haploid
Sperm cell haploid
Diploid
zygote
(2n = 46)
Multicellular
diploid adults
(2n = 46)
Mitosis and
development

19. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• There is a
special
mechanism to
produce
gametes
• Each gamete
has a single
set of
chromosomes
• 22 autosomes
and a single
sex
chromosomeFigure 8.13
MEIOSIS FERTILIZATION
Haploid gametes (n = 23)
Egg cell haploid
Sperm cell haploid
Diploid
zygote
(2n = 46)
Multicellular
diploid adults
(2n = 46)
Mitosis and
development

20. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• Haploid gametes keeps the chromosome
number from doubling in each succeeding
generation
• Haploid gametes are produced by a special
sort of cell division called meiosis
• Which occurs only in reproductive organs,
ovaries and testes
• Purpose of meiosis is to produce sperm and
egg
Gametes have a single set of chromosomes

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• Meiosis involves 2 cell divisions
• Meiosis produces 4 cells from 1 parental cell
• Each of the 4 daughter cells has 23 individual
chromosomes rather than 23 pairs of
chromosomes
• Meiosis reduces the chromosome number from
diploid to haploid
• Meiosis, like mitosis, is preceded by chromosome
duplication
– However, in meiosis the cell divides twice to
form four daughter cells
MEIOSIS

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Figure 8.15
MITOSIS MEIOSIS
PARENT CELL
(before chromosome replication)
Site of
crossing over
MEIOSIS I
PROPHASE I
Tetrad formed
by synapsis of
homologous
chromosomes
PROPHASE
Duplicated
chromosome
(two sister chromatids)
METAPHASE
Chromosome
replication
Chromosome
replication
2n = 4
ANAPHASE
TELOPHASE
Chromosomes
align at the
metaphase plate
Tetrads
align at the
Metaphase plate
METAPHASE I
ANAPHASE I
TELOPHASE I
Sister chromatids
separate during
anaphase
Homologous
chromosomes
separate
during
anaphase I;
sister
chromatids
remain together
No further
chromosomal
replication; sister
chromatids
separate during
anaphase II
2n 2n
Daughter cells
of mitosis
Daughter cells of meiosis II
MEIOSIS II
Daughter
cells of
meiosis I
Haploid
n = 2
n n n n

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Figure 8.14, part 1
MEIOSIS I: Homologous chromosomes separate
INTERPHASE PROPHASE I METAPHASE I ANAPHASE I
Centrosomes
(with
centriole
pairs)
Nuclear
envelope
Chromatin
Sites of crossing over
Spindle
Sister
chromatids
Tetrad
Microtubules
attached to
kinetochore
Metaphase
plate
Centromere
(with kinetochore)
Sister chromatids
remain attached
Homologous
chromosomes separate

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Figure 8.14, part 2
MEIOSIS II: Sister chromatids separate
TELOPHASE I
AND CYTOKINESIS
PROPHASE II METAPHASE II ANAPHASE II
Cleavage
furrow
Sister
chromatids
separate
TELOPHASE II
AND CYTOKINESIS
Haploid
daughter cells
forming

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27. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 8.16
POSSIBILITY 1 POSSIBILITY 2
Two equally probable
arrangements of
chromosomes at
metaphase I
Metaphase II
Gametes
Combination 1 Combination 2 Combination 3 Combination 4

28. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• Each synapsis is
made up of 2 pairs
of sister
chromatids
• This matched set
of 4 chromatids is
called a tetrad
MEIOSIS AND CROSSING OVER
Chromosomes are matched in homologous pairs
Chromosomes
Centromere
Sister chromatids Figure 8.12

29. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

30. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• Crossing over is the exchange of
corresponding segments between two
non-sister chromatids of homologous
chromosomes
• Genetic recombination results from
crossing
over during prophase I of meiosis
– This increases variation further
Crossing over further increases genetic variability

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• How crossing
over leads to
genetic
recombination
• Nonsister
chromatids
break in two at
the same spot
• The 2 broken
chromatids join
together in a new
way Figure 8.18B
Tetrad
(homologous pair of
chromosomes in synapsis)
Breakage of homologous chromatids
Joining of homologous chromatids
Chiasma
Separation of homologous
chromosomes at anaphase I
Separation of chromatids at
anaphase II and completion of meiosis
Parental type of chromosome
Recombinant chromosome
Recombinant chromosome
Parental type of chromosome
Gametes of four genetic types
1
2
3
4
Coat-color
genes
Eye-color
genes

32. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• A segment of one
chromatid has
changed places
with the equivalent
segment of its
nonsister
homologue
• If there were no
crossing over
meiosis could only
produce 2 types of
gametes
Figure 8.18B
Tetrad
(homologous pair of
chromosomes in synapsis)
Breakage of homologous chromatids
Joining of homologous chromatids
Chiasma
Separation of homologous
chromosomes at anaphase I
Separation of chromatids at
anaphase II and completion of meiosis
Parental type of chromosome
Recombinant chromosome
Recombinant chromosome
Parental type of chromosome
Gametes of four genetic types
1
2
3
4
Coat-color
genes
Eye-color
genes

33. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings