Download MEIOSIS II

Meiosis
Honors Biology
Introduction to Heredity
Offspring acquire genes from parents by
inheriting chromosomes
Inheritance is possible because:
– Sperm and ova carrying each parent’s genes are
combined in the nucleus of the fertilized egg
Actual transmission of genes depends on
the behavior of chromosomes
•Chromosomes-organizational unit of hereditary
material in the nucleus of eukaryotic organisms
•Contain hundreds of thousands of genes, each of
which is a specific region of the DNA molecule, or
locus
Human Life Cycle
Each somatic cell (body cell) has 46
chromosomes or 23 matching pairs (diploid)
Karyotype:
male
Sex chromosomes:
determine gender
(XX; XY)
Autosomes: nonsex chromosomes
Human Life Cycle
Gametes (sex cells) have a single set of 22
autosomes and a single sex chromosome,
either X or Y
With 23 chromosomes, they are haploid
Haploid sperm + haploid ova
n
n
haploid number: n = 23
diploid number: 2n = 46
fertilization
zygote (2n)
2n
Meiosis
Reduces chromosome number from diploid to haploid
Increases genetic variation among offspring
Steps resemble steps in mitosis
Single replication of DNA is followed by 2
consecutive cell divisions
 Meiosis I
 Meiosis II
Produces 4 different daughter cells which have half
the number of chromosomes as the original cell
In the first division, meiosis I, homologous
chromosomes are paired
While they are paired, they cross over and exchange
genetic information
 The homologous pairs are then separated, and two
daughter cells are produced

Interphase I
Chromosomes replicate
(still as chromatin)
Duplicated
chromosomes consist of 2
identical sister chromatids
attached by centromere
Centriole pairs replicate
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 Benjamin Cummings
Metaphase
plate
Centromere
(with kinetochore)
ANAPHASE I
Sister chromatids
remain attached
Homologous
chromosomes separate
Meiosis I
This cell division separates the 2
chromosomes of each homologous pair and
reduce the chromosome number by one-half
Prophase I
Chromosomes condense
Synapsis occurs
(homologues pair)
Chromosomes seen as
distinct structures; each
chromosome has 2
chromatids, so each
synapsis forms a tetrad
Prophase I
Sister chromatids held
together by
centromeres; nonsister chromatids held
together by chiasmata
where crossing-over
occurs (exchange of
DNA)
Late Prophase I
Centriole pairs move
apart and spindle fibers
form
Nuclear envelope
disappears and nucleoli
disperse
Prophase I
Metaphase I
Homologous
chromosomes line up
along metaphase plate
Metaphase I
Anaphase I
Homologous
chromosomes separate,
independently from
others
Anaphase I
Telophase I and Cytokinesis
Each pole now has a
haploid set of chromosomes
(each with 2 sister
chromatids)
Usually, cytokinesis occurs
simultaneously with
telophase I, forming 2
haploid daughter cells
(cleavage furrow forms in
animals; cell plate forms in
plants)
Telophase I
Meiosis II is essentially the same as mitosis
The sister chromatids of each chromosome separate
 The result is four haploid daughter cells

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 Benjamin Cummings
Haploid
daughter cells
forming
Meiosis II
This cell division separates the 2 sister
chromatids of each chromosome
Prophase II
Spindle apparatus forms
and chromosomes move
toward metaphase II
plate
Prophase II
Metaphase II
Chromosomes align
singly on the
metaphase plate
Metaphase II
Anaphase II
Sister chromatids of each
pair (now individual
chromosomes) separate
and move toward
opposite poles of the cell
Anaphase II
Anaphase II
Telophase II and Cytokinesis
Nuclei form at
opposite poles of the
cell
Cytokinesis occurs
producing 4 haploid
daughter cells (each
genetically different)
Telophase II
Telophase II
Key Differences Between Mitosis
and Meiosis
Meiosis is a reduction division
 Mitotic cells produce clones (same xsome #)
 Meiosis produces haploid cells
Meiosis creates genetic variation
 Mitosis produces 2 identical daughter cells
 Meiosis produces 4 genetically different
daughter cells
Meiosis is 2 successive nuclear divisions
 Mitosis has one division
Copyright © 2001 Pearson Education, Inc. publishing Benjamin Cummings
Spermatogenesis
Process of sperm production
Results in 4 viable sperm
Oogenesis
Process of egg (ova) production
Results in 1 viable egg and 3 polar bodies
that will not survive
Polar bodies result from an uneven division
of cytoplasm
Mechanisms of Genetic Variation
Independent assortment—each pair of
homologous chromosomes separate independently

Results in gametes with different gene combinations
Crossing-over—exchange of genetic material
between non-sister chromatids

Results in genetic recombination
Random fertilization—random joining of two
gametes
Independent Assortment
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
Mechanisms of Genetic Variation
Independent assortment—each pair of
homologous chromosomes separate independently

Results in gametes with different gene combinations
Crossing-over—exchange of genetic material
between non-sister chromatids

Results in genetic recombination
Random fertilization—random joining of two
gametes
Tetrad
Chaisma
Centromere
Figure 8.18A
Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings
Coat-color genes
Eye-color genes
Brown
Black
C
E
c
e
White
Pink
Tetrad in parent cell
(homologous pair of
duplicated chromosomes)
Figure 8.17A, B
Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings
C
E
C
E
c
e
c
e
Chromosomes of
the four gametes
Coat-color
genes
Eye-color
genes
Tetrad
(homologous pair of
chromosomes in synapsis)
How crossing over
leads to genetic
recombination
1
Breakage of homologous chromatids
2
Joining of homologous chromatids
Chiasma
3
Separation of homologous
chromosomes at anaphase I
4
Separation of chromatids at
anaphase II and completion of meiosis
Parental type of chromosome
Recombinant chromosome
Recombinant chromosome
Parental type of chromosome
Figure 8.18B
Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings
Gametes of four genetic types
Crossing Over
In Prophase I of Meiosis I, synapsis occurs
allowing the crossing over of genetic
material between non-sister chromatids
Creates new
combinations of
genes not seen
in either parent
Mechanisms of Genetic Variation
Independent assortment—each pair of
homologous chromosomes separate independently

Results in gametes with different gene combinations
Crossing-over—exchange of genetic material
between non-sister chromatids

Results in genetic recombination
Random fertilization—random joining of two
gametes
Random Fertilization
Random as to which gametes join and form
a gamete
Importance of Genetic Variation
Essential to evolution (change over time)
Variation can cause changes that leads to
different traits
Some favorable
 Some unfavorable

Errors and Exceptions in
Chromosomal Inheritance
Alterations in chromosome number or
structure causes some genetic disorders
Physical and chemical disturbances
 Errors during meiosis

ALTERATIONS OF CHROMOSOME
NUMBER AND STRUCTURE
To study human chromosomes microscopically,
researchers stain and display them as a
karyotype

A karyotype usually shows 22 pairs of autosomes
and one pair of sex chromosomes
Preparation of a karyotype
Blood
culture
Packed red
And white
blood cells
Hypotonic solution
Stain
White
Blood
cells
Centrifuge
3
2
1
Fixative
Fluid
Centromere
Sister
chromatids
Pair of homologous
chromosomes
4
Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings
5
Figure 8.19
Human female bands
Figure 8.19x1
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Human female karyotype
Figure 8.19x2
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Human male bands
Figure 8.19x3
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Human male karyotype
Figure 8.19x4
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Alterations of Chromosome
Numbers
Nondisjunction—pair of homologues do not
move apart during Meiosis I, or sister
chromatids do not separate during Meiosis II

Results in uneven distribution of chromosomes to
daughter cells
Alterations of Chromosome
Numbers
Aneuploidy: abnormal chromosome number
Trisomy: three copies of chromosomes
 Monosomy: one copy of a chromosome
 Trisomy and monosomy are usually lethal

Accidents during meiosis can
alter chromosome number
Abnormal
chromosome count
is a result of
nondisjunction

Either
homologous
pairs fail to
separate
during
meiosis I
Copyright © 2003Pearson Education, Inc. publishing Benjamin Cummings
Nondisjunction
in meiosis I
Normal
meiosis II
Gametes
n+1
n+1
n–1
n–1
Number of chromosomes
Figure 8.21A

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
Copyright © 2003Pearson Education, Inc. publishing Benjamin Cummings
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 © 2003Pearson Education, Inc. publishing Benjamin Cummings
Trisomy 21 (Down Syndrome)
*Short stature, characteristic facial features,
and heart defects (varying severity)
*Most common serious birth defect (1 out of
700 births)
*Mothers 35+ years of age have higher
chance of having a Down baby
The chance of having a Down syndrome
child goes up with maternal age
Figure 8.20C
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Down syndrome karyotype
Figure 8.20Ax
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Nondisjunction with Sex Chromosomes
Table 8.22
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Klinefelter’s karyotype
Figure 8.22Ax
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XYY karyotype
Figure 8.22x
Alterations of Chromosome
Structure
Breakage of a chromosome can lead to four
types of changes in chromosome structure
Deletion: chromosomal fragment is lost during
cell division
 Duplication: fragment may join to the
homologous chromosome
 Inversion: fragment may reattach to the original
chromosome but in the reverse orientation
 Translocation: fragment joins a nonhomologous
chromosome

Chromosome Mutation: Deletion
Deleted region
Before
Deletion
After
Deletion
Cri du Chat Syndrome: Partial deletion 5p
Chromosome Mutation: Inversion
Inverted region
Before
inversion
After
inversion
Chromosome Mutation: Translocation
Before Translocation
Region
being
moved
After Translocation
Chromosome 20
Chromosome 20
Chromosome 4
Chromosome 4
Chromosomal changes in a somatic cell can
cause cancer

A chromosomal translocation in the bone marrow is
associated with chronic myelogenous leukemia
Chromosome 9
Chromosome 22
Reciprocal
translocation
“Philadelphia chromosome”
Activated cancer-causing gene
Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings
Figure 8.23C
Philadelphia
Chromosome
t(9,22)
Translocation
Figure 8.23Bx
Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings
Acknowledgements
Unless otherwise noted, illustrations are credited to Pearson Education
which have been borrowed from BIOLOGY: CONCEPTS AND
CONNECTIONS 4th Edition, by Campbell, Reece, Mitchell, and
Taylor, ©2003. These images have been produced from the originals
by permission of the publisher. These illustrations may not be
reproduced in any format for any purpose without express written
permission from the publisher.
BIOLOGY: CONCEPTS AND CONNECTIONS 4th Edition, by
Campbell, Reece, Mitchell, and Taylor, ©2001. These images have
been produced from the originals by permission of the publisher. These
illustrations may not be reproduced in any format for any purpose
without express written permission from the publisher.