Download Meiosis and Sexual Reproduction

Chapter 10
Meiosis and Sexual Reproduction
1
10.1 Halving the Chromosome
Number
• Meiosis
 Special type of cell division
 Used only for sexual reproduction
 Halves the chromosome number prior to
fertilization
• Parents are diploid (2n)
• Meiosis produces haploid (n) gametes
• Gametes fuse in fertilization to form a diploid (2n) zygote
• The zygote becomes the next diploid (2n) generation
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Halving the Chromosome
Number
•
•
•
•
In diploid body cells, chromosomes occur in pairs
Humans have 23 different types of chromosomes
Diploid (2n) cells have two chromosomes of each type
Chromosomes of the same type are said to be
homologous chromosomes (homologues)
 They have the same length
 Their centromeres are positioned in the same place
 One came from the father (the paternal homolog) the
other from the mother (the maternal homolog)
 When stained, they show similar banding patterns
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Homologous Chromosomes
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Sister chromatids
a.
duplication
nonsister
chromatids
duplication
kinetochore
centromere
chromosome
homologous pair
paternal chromosome
b.
chromosome
maternal chromosome
a: © L. Willatt/Photo Researchers, Inc.
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Halving the Chromosome
Number
• Homologous chromosomes have genes controlling the
same trait at the same position
 Each gene occurs in duplicate
• Many genes exist in several variant forms in a large
population
• Homologous copies of a gene may encode identical or
different genetic information
• The variants that exist for a gene are called alleles
• An individual may have:
 Identical alleles for a specific gene on both homologs
(homozygous for the trait), or
 A maternal allele that differs from the corresponding paternal
allele (heterozygous for the trait)
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Halving the Chromosome Number
• Overview of Meiosis
 Meiosis I
• Chromosomes are replicated prior to meiosis I
– Each chromosome consists of two identical sister chromatids
• Homologous chromosomes pair up – synapsis
• Homologous pairs align themselves against each other side by side at the
metaphase plate
• The two members of a homologous pair separate
• Each daughter cell receives one duplicated chromosome from each pair
 Meiosis II
• DNA is not replicated between meiosis I and meiosis II
• Sister chromatids separate and move to opposite poles
• The four daughter cells contain one daughter chromosome from each pair
• Each daughter chromosome consists of a single chromatid
• The daughter cells are haploid
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Overview of Meiosis
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
First division
centrioles
nucleolus
centromere
Second division
Four haploid
daughter cells
sister chromatids
synapsis
chromosome
duplication
2n = 4
2n = 4
MEIOSISI
Homologous pairs
synapse and then separate.
n=2
n=2
MEIOSISII
Sister chromatids separate,
becoming daughter chromosomes.
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10.2 Genetic Variation
• Meiosis brings about genetic variation in two key
ways:
 Crossing-over between homologous chromosomes, and
 Independent assortment of homologous chromosomes
• Crossing Over:
 Exchange of genetic material between non-sister
chromatids during meiosis I
 At synapsis, a nucleoprotein lattice (called the
synaptonemal complex) appears between homologues
 Then homologues separate and are distributed to different
daughter cells
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Crossing Over Occurs During
Meiosis I
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nucleoprotein lattice
sister chromatids
of its homologue
sister chromatids
of a chromosome
A
A
a
a
B
B
b
b
chiasmata of
nonsister
chromatids c
1 and 3
C
C
c
D d
d
A a
B
b
C
c
D
D
d
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Bivalent
forms
a.
b.
1 2 3 4
Crossing-over
has occurred
c.
a: Courtesy Dr. D. Von Wettstein
1
2 3 4
Daughter
chromosomes
d.
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Genetic Variation
• Independent assortment
 When homologous chromosome pairs
align at the metaphase plate:
• They separate in a random manner
• The maternal or paternal homologue may
be oriented toward either pole of mother
cell
 Causes random mixing of blocks of
alleles into gametes
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Independent Assortment
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Combination 1
Combination 2
Combination
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Combination 5
Combination 3
Combination 4
Combination 6
Combination 8
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Genetic Variation
• Fertilization – union of male and female
gametes
 Chromosomes donated by the parents are
combined
 In humans, (223)2 = 70,368,744,000,000
chromosomally different zygotes are possible
• If crossing-over occurs only once
 (423)2, or 4,951,760,200,000,000,000,000,000,000
genetically different zygotes are possible
• Crossing-over may occur several times in
each chromosome
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Genetic Variation
• Significance of genetic variation:
 Asexual reproduction produces genetically identical
clones
 Sexual reproduction causes genetic recombinations
among members of a population
 Asexual reproduction is advantageous when the
environment is stable
 However, if the environment changes, genetic
variability introduced by sexual reproduction may be
advantageous
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10.3 The Phases of Meiosis
• Meiosis I:
 Prophase I
•
•
•
•
A spindle forms
The nuclear envelope fragments
The nucleolus disappears
Each chromosome is duplicated (consists of two identical sister
chromatids)
• Homologous chromosomes pair up and physically align themselves
against each other side by side (synapsis)
• Synapsed homologs are referred to as a bivalent (two homologues) or
a tetrad (four chromatids)
 Metaphase I
• Homologous pairs are arranged at the metaphase plate
• Bivalents are aligned independently of one another
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The Phases of Meiosis
• Meiosis I
 Anaphase I
• Homologous chromosomes of each bivalent
separate from one another
• Homologues move towards opposite poles
• Sister chromatids do not separate
• Each is still a duplicated chromosome with two
chromatids
 Telophase I
• Daughter cells have one duplicated chromosome
(n) from each homologous pair
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The Phases of Meiosis
• Interkinesis
 Two haploid (n) daughter cells, each with one
duplicated chromosome of each type
 Interkinesis is similar to mitotic interphase
except
• It is usually shorter
• DNA replication does not occur
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The Phases of Meiosis
• Meiosis II (mitosis of two haploid cells)
 Prophase II – Chromosomes condense
 Metaphase II – Chromosomes align at
metaphase plate
 Anaphase II
• Centromere dissolves
• Sister chromatids separate and become daughter
chromosomes
 Telophase II and cytokinesis
• Four haploid (n) cells all genetically unique
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Meiosis I in Plant and Animal
Cells
Meiosis I in Plant and Animal
Cells
Meiosis II in Plant and Animal
Cells
Meiosis II in Plant and Animal
Cells
10.4 Meiosis Compared to
Mitosis
• Meiosis
 Requires two nuclear
divisions
 Chromosomes synapse and
cross over
 Centromeres survive
Anaphase I
 Halves chromosome
number
 Produces four daughter
nuclei
 Produces daughter cells
genetically different from
parent and each other
 Used only for sexual
reproduction
• Mitosis
 Requires one nuclear
division
 Chromosomes do not
synapse nor cross over
 Centromeres dissolve in
mitotic anaphase
 Preserves chromosome
number
 Produces two daughter
nuclei
 Produces daughter cells
genetically identical to
parent and to each other
 Used for asexual
reproduction and growth
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Meiosis Compared to Mitosis
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Meiosis Compared to Mitosis
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Meiosis Compared to Mitosis
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
2n = 4
Prophase I
Synapsis and
crossing-over occur.
MetaphaseI
Homologous pairs align
independently at the metaphase plate.
Anaphase I
Homologous chromosomes
separate and move toward the poles.
MEIOSISI
2n = 4
Prophase
MITOSIS
Metaphase
Chromosomes align
at the metaphase plate.
Anaphase
Sister chromatids separate and
become daughter chromosomes.
Meiosis Compared to Mitosis
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Telophase I
Daughter cells are forming
and will go on to divide again.
n=2
Daughter cells
Sister chromatids
separate and become
daughter chromosomes.
n=2
MEIOSIS I cont'd
Four haploid daughter cells.
Their nuclei are genetically
different from the parent cell.
MEIOSIS II
Daughter cells
Telophase
Daughter cells
are forming.
MITOSIS cont'd
Two diploid daughter cells.
Their nuclei are genetically
identical to the parent cell.
n=2
The Cycle of Life
• In animals:
 “Individuals” are diploid and produce haploid
gametes
 The only haploid part of the life cycle is the gametes
 The products of meiosis are always gametes
 Meiosis occurs only during gametogenesis
• Production of sperm
– Spermatogenesis
– All four cells become sperm
• Production of eggs
– Oogenesis
– One of the four nuclei receives the majority of
the cytoplasm
» Becomes the egg or ovum
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» Others wither away as polar bodies
The Cycle of Life
• Human Life Cycle:
 Sperm and egg are produced by meiosis
 A sperm and egg fuse at fertilization
 Results in a zygote
• The one-celled stage of an individual of the next generation
• Undergoes mitosis
 Results in a multicellular embryo that gradually takes
on features determined when the zygote was formed
 All growth occurs as mitotic division
 As a result of mitosis, each somatic cell in the body
• Has same number of chromosomes as zygote
• Has the same genetic makeup, which was determined when
the zygote was formed
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Life Cycle of Humans
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
MITOSIS
2n
2n
2n
MITOSIS
2n
zygote
2n = 46
diploid (2n)
FERTILIZATION
MEIOSIS
haploid (n)
n = 23
n
n
egg
sperm
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Spermatogenesis and
Oogenesis in Mammals
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
SPERMATOGENESIS
primary
spermatocyte
OOGENESIS
primary
oocyte
2n
2n
Meiosis I
Meiosis I
secondary
spermatocytes
first
polarbody
n
secondary
oocyte
n
Meiosis II
n
spermatids
n
Meiosis II
second
polarbody
Meiosis II is completed
after entry of sperm
(fertilization)
n
Metamorphosis
and maturation
egg
Fertilization
sperm
n
Sperm nucleus
n
n
fusion of sperm
nucleus and
agg nucleus
zygote
2n
10.6 Changes in Chromosome
Number and Structure
• Euploidy is the correct number of
chromosomes in a species.
• Aneuploidy is a change in the
chromosome number
 Results from nondisjunction
• Monosomy - only one of a particular type of
chromosome,
• Trisomy - three of a particular type of
chromosome
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Nondisjunction
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
pair of
homologous
chromosomes
nondisjunction
pair of
homologous
chromosomes
Meiosis I
normal
Meiosis II
normal
nondisjunction
Fertilization
Zygote
2n + 1
a.
2n + 1
2n - 1
2n - 1
2n
2n
2n + 1
2n - 1
b.
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Changes in Chromosome
Number and Structure
• Trisomy occurs when an individual has
three of a particular type of chromosome
• The most common autosomal trisomy
seen among humans is Trisomy 21
 Also called Down syndrome
 Recognized by these characteristics:
•
•
•
•
•
short stature
eyelid fold
flat face
stubby fingers
wide gap between first and second toes
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Trisomy 21
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
extra chromosome 21
a.
b.
a: © Jose Carrilo/PhotoEdit; b: © CNRI/SPL/Photo Researchers
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Changes in Chromosome
Number and Structure
• Changes in sex chromosome
number:
 Results from inheriting too many or too few X
or Y chromosomes
 Nondisjunction during oogenesis or
spermatogenesis
 Turner syndrome (XO)
• Female with a single X chromosome
• Short, with broad chest and widely spaced nipples
• Can be of normal intelligence and function with
hormone therapy
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Changes in Chromosome
Number and Structure
• Changes in sex chromosome
number:
 Klinefelter syndrome (XXY)
• Male with underdeveloped testes and prostate;
some breast overdevelopment
• Long arms and legs; large hands
• Near normal intelligence unless XXXY, XXXXY,
etc.
• No matter how many X chromosomes are present,
the presence of a chromosome Y renders the
individual male
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Changes in Sex Chromosome
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
a. Turner syndrome
missing
chromosome X
b. Klinefelter syndrome
a(top): Courtesy UNC Medical Illustration and Photograph; b(top): Courtesy Stefan D. Schwarz,
http://klinefeltersyndrome.org; a, b(bottom): © CNRI/SPL/Photo Researchers, Inc
extra
chromosome X
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Changes in Chromosome
Number and Structure
• Changes in chromosome structure include:
 Deletion
• One or both ends of a chromosome breaks off
• Two simultaneous breaks lead to loss of an internal segment
 Duplication
• Presence of a chromosomal segment more than once in the
same chromosome
 Translocation
• A segment from one chromosome moves to a nonhomologous chromosome
• Follows breakage of two non-homologous chromosomes and
improper re-assembly
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Changes in Chromosome
Number and Structure
 Changes in chromosome structure include:
• Inversion
– Occurs as a result of two breaks in a
chromosome
– The internal segment is reversed before reinsertion
– Genes occur in reverse order in the inverted
segment
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Types of Chromosomal Mutation
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
a
b
c
b
c
a
b
c
a
b
c
d
e
d
e
f
+ a
d
e
d
e
d
e
f
f
f
g
g
g
g
a. Deletion
b. Duplication
a
b
c
a
b
d
d
e
c
f
f
g
g
c. Inversion
e
a
b
c
d
e
l
m
n
a
b
c
d
l
m
n
e
f
o
p
f
o
p
g
q
q
g
h
r
r
h
d. Translocation
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