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Sylv
ia S
. Ma
der
Copyright © The McGraw Hill Companies Inc. Permission required for reproduction or display
PowerPoint® Lecture Slides are prepared by Dr. Isaac Barjis, Biology Instructor
BIOLOGY 10th Edition
Meiosis & Sexual Reproduction
Chapter 10: pp. 169 - 188
1
egg
FERTILIZATION MEIOSIS
MITOSIS
MITOSIS
sperm
n
n
2n
2n
2n
2n
zygote
haploid (n)
n = 23
2n = 46
diploid (2n)
SPERMATOGENESIS
OOGENESIS
Metamorphosis
and maturation
Primary
spermatocyte
Primary
oocyte
zygote
egg
Secondary
oocyte
Meiosis II is completed
after entry of sperm
spermatids
sperm
Secondary
spermatocytes
First
polar body
Second
polar body
fusion of sperm
nucleus and
egg nucleus
sperm nucleus
2n
2n
2n
n
n
n
n
n
n
n
n
Meiosis I
Meiosis II
Meiosis I
Fertilization
cont'd
Meiosis II
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
2
Outline
Reduction in Chromosome Number Homologous Pairs
Meiosis Overview
Genetic Variation Crossing-Over
Independent Assortment
Fertilization
Phases of Meiosis Meiosis I
Meiosis II
Meiosis Compared to Mitosis
Human Life Cycle
Changes in Chromosome Number and Structure
3
Meiosis: Halves the Chromosome Number
Special type of cell division
Used only for sexual reproduction
Halves the chromosome number prior to
fertilization
Parents diploid
Meiosis produces haploid gametes
Gametes fuse in fertilization to form diploid zygote
Becomes the next diploid generation
4
Homologous Pairs of Chromosomes
In diploid body cells chromosomes occur in pairs
Humans have 23 different types of chromosomes
Diploid cells have two of each type
Chromosomes of the same type are said to be homologous
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
A location on one homologue contains gene for the same trait that occurs at this locus on the other homologue
Although the genes may code for different variations of that trait
Alternate forms of a gene are called alleles
5
Homologous Chromosomes
a. sister chromatids
homologous pair chromosome chromosome
Nonsister
chromatids duplication duplication
centromere
kinetochore
paternal chromosome maternal chromosome
b.
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7
Homologous Pairs of Chromosomes
Homologous chromosomes have genes controlling the same trait at the same position Each gene occurs in duplicate A maternal copy from the mother A paternal copy from the father
Many genes exist in several variant forms in a large population
Homologous copies of a gene may encode identical or differing 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)
8
Overview of Meiosis
n = 2 n = 2
2n = 4 2n = 4
MEIOSIS I
Homologous pairs
synapse and then separate.
centrioles sister chromatids synapsis
nucleolus
centromere
chromosome
duplication
MEIOSIS II
Sister chromatids separate,
becoming daughter chromosomes.
Four haploid
daughter cells
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10
Phases of Meiosis I: Prophase I &
Metaphase I
Meiosis I (reductional division):
Prophase I
Each chromosome internally duplicated (consists of two identical sister chromatids)
Homologous chromosomes pair up – synapsis
Physically align themselves against each other end to end
End view would show four chromatids – Tetrad
Metaphase I
Homologous pairs arranged onto the metaphase plate
11
Phases of Meiosis I: Anaphase I &
Telophase I
Anaphase I Synapsis breaks up
Homologous chromosomes separate from one another
Homologues move towards opposite poles
Each is still an internally duplicate chromosome with two chromatids
Telophase I Daughter cells have one internally duplicate
chromosome from each homologous pair
One (internally duplicate) chromosome of each type (1n, haploid)
12
Phases of Meiosis I: Cytokinesis I &
Interkinesis
Cytokinesis I Two daughter cells
Both with one internally duplicate chromosome of each type
Haploid
Meiosis I is reductional (halves chromosome number)
Interkinesis
Similar to mitotic interphase
Usually shorter
No replication of DNA
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14
Genetic Variation: Crossing Over
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 nonsister chromatids
during meiosis I At synapsis, a nucleoprotein lattice (called the
synaptonemal complex) appears between homologues Holds homologues together Aligns DNA of nonsister chromatids Allows crossing-over to occur
Then homologues separate and are distributed to different daughter cells
15
Crossing Over
A
B
b B
C
c C
D
D D
A A a
b
B b
c
C c
d
d d
a a
nucleoprotein lattice sister chromatids
of a chromosome
sister chromatids
of its homologue
chiasmata of
nonsister
nhromatids
1 and 3
Bivalent
forms
b.
1 2 3 4 1 2 3 4 1 2 3 4
a. c. d.
Crossing-over
has occurred
Daughter
chromosomes
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Courtesy Dr. D. Von Wettstein
Animation
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17
Genetic Variation: Independent Assortment
Independent assortment:
When homologues 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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19
Independent Assortment
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
20
Recombination
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
© American Images, Inc/Getty Images
21
Genetic Variation: Fertilization
When gametes fuse at fertilization:
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
22
Genetic Variation: Significance
Asexual reproduction produces genetically identical clones
Sexual reproduction cause novel genetic recombinations
Asexual reproduction is advantageous when environment is stable
However, if environment changes, genetic variability introduced by sexual reproduction may be advantageous
Offspring adapt to that environment
23
Phases of Meiosis II: Similar to Mitosis
Metaphase II Overview
Unremarkable
Virtually indistinguishable from 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 II Four haploid cells
All genetically unique
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25
Meiosis I & II in Plant Cells (Cont.)
Prophase I
Chromosomes have duplicated.
Homologous chromosomes pair during
synapsis and crossing-over occurs.
Metaphase I
Homologous pairs align
Independently at the metaphase plate.
Anaphase I
Homologous chromosomes separate
and move toward the poles.
Prophase II
Cells have one chromosome
from each homologous pair.
Metaphase II
Chromosomes align
at the metaphase plate.
Anaphase II
Sister chromatids separate and
become daughter chromosomes.
kinetochore
MEIOSIS I
MEIOSIS II
Animal Cell
at Interphase
centrosome has
centrioles
Plant Cell
at Interphase
2n = 4
n = 2
n = 2
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
© Ed Reschke
26
Meiosis I & II in Plant Cells
Telophase I
Daughter cells have one chromosome
from each homologous pair.
Interkinesis
Chromosomes still
consist of two chromatids.
Telophase II
Spindle disappears, nuclei form,
and cytokinesis takes place.
Daughter cells
Meiosis results in four
haploid daughter cells.
MEIOSIS I cont'd
MEIOSIS II cont'd
n = 2
n = 2
n = 2
n = 2
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
© Ed Reschke
Animation
27
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the latest version of the Flash Player,
which is available at
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28
Meiosis vs. 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
29
Meiosis Compared to Mitosis
Prophase I
Synapsis and
crossing-over occur.
Metaphase I
Homologous pairs align
independently at the metaphase plate.
Anaphase I
Homologous chromosomes
separate and move towards the poles.
Prophase Metaphase
Chromosomes align
at the metaphase plate.
Anaphase
Sister chromatids separate and
become daughter chromosomes.
MEIOSIS I
MITOSIS
2n = 4
2n = 4
Telophase I
Daughter cells are forming
and will go on to divide again.
Telophase
Daughter cells
are forming.
Sister chromatids
separate and become
daughter chromosomes.
Two diploid daughter cells.
Their nuclei are genetically
identical to the parent cell.
Four haploid daughter cells.
Their nuclei are genetically
different from the parent cell.
MEIOSIS I cont'd MEIOSIS II
MITOSIS cont'd
Daughter cells
Daughter cells
n = 2
n = 2
n = 2
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Animation
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33
Life Cycle Basics: Plants
Haploid multicellular “individuals” alternate with diploid multicellular “individuals”
The haploid individual: Known as the gametophyte
May be larger or smaller than the diploid individual
The diploid individual: Known as the sporophyte
May be larger or smaller than the haploid individual
Mosses are haploid most of their life cycle
Ferns & higher plants have mostly diploid life cycles
In fungi and most algae, only the zygote is diploid
In plants gametes are produced by haploid individuals Plants have both haploid and diploid phases in their life cycle
34
Life Cycle Basics: Animals
In familiar animals: “Individuals” are diploid; produce haploid
gametes Only haploid part of 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 Only one of four nuclei get cytoplasm
Becomes the egg or ovum Others wither away as polar bodies
35
The 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 multicellular embryo that gradually takes on features determined when zygote was formed
All growth occurs as mitotic division
As a result of mitosis, each somatic cell in body Has same number of chromosomes as zygote
Has genetic makeup determined when zygote was formed
36
The Human Life Cycle
egg
FERTILIZATION MEIOSIS
MITOSIS
MITOSIS
sperm
n
n
2n
2n
2n
2n
zygote
haploid (n)
n = 23
2n = 46
diploid (2n)
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
SPERMATOGENESIS
OOGENESIS
Metamorphosis
and maturation
Primary
spermatocyte
Primary
oocyte
zygote
egg
Secondary
oocyte
Meiosis II is completed
after entry of sperm
spermatids
sperm
Secondary
spermatocytes
First
polar body
Second
polar body
fusion of sperm
nucleus and
egg nucleus
sperm nucleus
2n
2n
2n
n
n
n
n
n
n
n
n
Meiosis I
Meiosis II
Meiosis I
Fertilization
cont'd
Meiosis II
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
37
Gametogenesis in Mammals
38
Changes in Chromosome Number
Euploid is the correct number of
chromosomes in a species.
Aneuploid is 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
39
Changes in Chromosome
a. b.
pair of
homologous
chromosomes
2n 2n 2n + 1 2n + 1 2n + 1 2n - 1
normal
normal
pair of
homologous
chromosomes
Meiosis I
Meiosis II
Fertilization
Zygote
nondisjunction
nondisjunction
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
2n - 1 2n - 1
40
Trisomy
Trisome 21
Occurs when an individual has three of a particular
type of chromosome
The most common autosomal trisomy seen among
humans
Also called Down syndrome
Recognized by these characteristics:
short stature
eyelid fold
flat face
stubby finger
wide gap between first and second toes
41
Trisomy 21
a. b.
extra chromosome 21
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
a: © Jose Carrilo/PhotoEdit; b: © CNRI/SPL/Photo Researchers
42
Changes in Sex Chromosome
Result from inheriting too many or too few X or Y chromosomes
Nondisjunction during oogenesis or spermatogenesis Turner syndrome (XO)
Female with single X chromosome
Short, with broad chest and widely spaced nipples
Can be of normal intelligence and function with hormone therapy
Klinefelter syndrome (XXY) – a male
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, presence of Y renders individual male
43
Changes in Sex Chromosome
a. Turner syndrome b. Klinefelter syndrome
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
a: Courtesy UNC Medical Illustration and Photography; b: Courtesy Stefan D. Schwarz, http://klinefeltersyndrome.org
44
Changes in Chromosome Structure
Changes in chromosome structure include:
Deletions
One or both ends of a chromosome breaks off
Two simultaneous breaks lead to loss of an internal segment
Duplications
Presence of a chromosomal segment more than once in the
same chromosome
Translocations
A segment from one chromosome moves to a non-
homologous chromosome
Follows breakage of two nonhomologous chromosomes and
improper re-assembly
Animation
45
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which is available at
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46
Changes in Chromosome Structure
Duplication
A segment of a chromosome is repeated in the
same chromosome
Inversion
Occurs as a result of two breaks in a
chromosome
The internal segment is reversed before re-insertion
Genes occur in reverse order in inverted segment
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48
Types of Chromosomal Mutation
c. Inversion d. Translocation
b. Duplication a. Deletion
+
a b c
d
e
f
g
a b c d
e
f
g
h
a b c
d
e
f
g
a b
e
f
g
l m n
o
p
q
r
a b c d
e
f
l m n
o
p
a b c
d
e
f
g
a b c
d
e
f
g
b c
d
e
f
g
g
h
q
r
d
e
a
c
d
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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Animation
50
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51
Types of Chromosomal Mutation
deletion lost
a. b.
+ h
a
b
c
d
e
f
g
h
a
b
c
d
e
f
g
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
b: Courtesy The Williams Syndrome Association
52
Types of Chromosomal Mutation
translocation
a. b.
a
b
c
d
e
f
g
h
s
t
u
v
w
x
y
z
a
b
c
d
e
f
g
h
s
t
u
v
w
x
y
z
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
b: American Journal of Human Genetics by N. B. Spinner. Copyright 1994 by Elsevier Science & Technology Journals. Reproduced
with permission of Elsevier Science & Technology Journals in the format Textbook via Copyright Clearance Center
53
Review
Reduction in Chromosome Number Meiosis Overview
Homologous Pairs
Genetic Variation Crossing-Over
Independent Assortment
Fertilization
Phases of Meiosis Meiosis I
Meiosis II
Meiosis Compared to Mitosis
Human Life Cycle
Changes in Chromosome Number and Structure
Sylv
ia S
. Ma
der
Copyright © The McGraw Hill Companies Inc. Permission required for reproduction or display
PowerPoint® Lecture Slides are prepared by Dr. Isaac Barjis, Biology Instructor
BIOLOGY 10th Edition
Meiosis & Sexual Reproduction
Chapter 10: pp. 169 - 188
54
egg
FERTILIZATION MEIOSIS
MITOSIS
MITOSIS
sperm
n
n
2n
2n
2n
2n
zygote
haploid (n)
n = 23
2n = 46
diploid (2n)
SPERMATOGENESIS
OOGENESIS
Metamorphosis
and maturation
Primary
spermatocyte
Primary
oocyte
zygote
egg
Secondary
oocyte
Meiosis II is completed
after entry of sperm
spermatids
sperm
Secondary
spermatocytes
First
polar body
Second
polar body
fusion of sperm
nucleus and
egg nucleus
sperm nucleus
2n
2n
2n
n
n
n
n
n
n
n
n
Meiosis I
Meiosis II
Meiosis I
Fertilization
cont'd
Meiosis II
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.