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1. Meiosis and chromosome number
2. Steps in meiosis
3. Source of genetic variation
a. Independent alignment of homologues
b. Recombination
• Somatic cells are diploid.
• Gametes are haploid, with only one set of
chromosomes
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Meiosis reduces the number of genomes
from diploid to haploid
Haploid gametes (n = 23)
• human life
cycle
• Meiosis
creates
gametes
Egg cell
Sperm cell
MEIOSIS
• Mitosis of
the zygote
produces
adult bodies
Figure 8.13
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FERTILIZATION
Diploid
zygote
(2n = 46)
Multicellular
diploid adults
(2n = 46)
Mitosis and
development
Steps in meiosis I
MEIOSIS I: Homologous chromosomes separate
INTERPHASE
Centrosomes
(with
centriole
pairs)
Nuclear
envelope
PROPHASE I
METAPHASE I
Microtubules
attached to
Spindle kinetochore
Sites of crossing over
Chromatin
Sister
chromatids
Tetrad
Figure 8.14, part 1
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Metaphase
plate
Centromere
(with kinetochore)
ANAPHASE I
Sister chromatids
remain attached
Homologous
chromosomes separate
• In meiosis I, homologous chromosomes are
paired
– While paired, they cross over and exchange
genetic information (DNA)
– homologous pairs are then separated, and two
daughter cells are produced
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
MEIOSIS II: Sister chromatids separate
TELOPHASE I
AND CYTOKINESIS
PROPHASE II
METAPHASE II
ANAPHASE II
TELOPHASE II
AND CYTOKINESIS
Cleavage
furrow
Sister
chromatids
separate
Figure 8.14, part 2
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Haploid
daughter cells
forming
• Meiosis II is essentially the same as mitosis
– sister chromatids of each chromosome separate
– result is four haploid daughter cells
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Diploid
MITOSIS
MEIOSIS
1
somatic
cell
gamete
precursor
2n
Diploid
2n
duplication
2
2n
2n
3
2n
2n
2n
4
2n
division
diploid
2n
2n
haploid
5
1n
1n
6
division
7
1n
1n
1n
1n
Homologous chromosomes carry different
versions of genes (alleles) at corresponding loci
• Each chromosome of a homologous pair comes
from a different parent
– Each chromosome thus differs at many points
from the other member of the pair
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Independent alignment of homologous chromosomes
POSSIBILITY 1
POSSIBILITY 2
Two equally probable
arrangements of
chromosomes at
metaphase I
Metaphase II
Gametes
Combination 1
Combination 2
Figure 8.16
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Combination 3
Combination 4
Crossing over further increases genetic variability
• the exchange of corresponding segments
between two homologous chromosomes
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Tetrad
Chaisma
Centromere
Figure 8.18A
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
MEIOSIS I
END OF
INTERPHASE
PROPHASE I
METAPHASE I
ANAPHASE I
Genetic recombination results from crossing
over during prophase I of meiosis
MEIOSIS
TELOPHASE I
PROPHASE II
METAPHASE II
ANAPHASE II
TELOPHASE II
INDEPENDENT ASSORTMENT
TELOPHASE II
METAPHASE II
METAPHASE I
METAPHASE I
a
SPERMATOGENESIS
b
OOGENESIS
spermatogonium
oogonium
primary
spermatocyte
primary
oocyte
meiosis l
secondary
spermatocyte
secondary
oocyte
meiosis ll
polar
body
spermatids
polar bodies
(will be degraded)
egg
Accidents during meiosis can alter
chromosome number
• Abnormal
chromosome count
is a result of
nondisjunction
– Either
homologous
pairs fail to
separate
during
meiosis I
Nondisjunction
in meiosis I
Normal
meiosis II
Gametes
n+1
n+1
n–1
n–1
Number of chromosomes
Figure 8.21A
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
– Or sister chromatids fail to separate during
meiosis II
Normal
meiosis I
Nondisjunction
in meiosis II
Gametes
n–1
n+1
n
Number of chromosomes
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n
Figure 8.21B
• Fertilization after nondisjunction in the mother
results in a zygote with an extra chromosome
Egg
cell
n+1
Zygote
2n + 1
Sperm
cell
n (normal)
Figure 8.21C
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Connection: An extra copy of chromosome 21
causes Down syndrome
• This karyotype shows three number 21
chromosomes
• An extra copy of chromosome 21 causes Down
syndrome
Figure 8.20A, B
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• The chance of having a Down syndrome child
goes up with maternal age
Figure 8.20C
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Connection: Abnormal numbers of sex
chromosomes do not usually affect survival
• Nondisjunction can also produce gametes with
extra or missing sex chromosomes
– Unusual numbers of sex chromosomes upset the
genetic balance less than an unusual number of
autosomes
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Table 8.22
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings