1. Meiosis and Sexual Reproduction
Chapter 10
Meiosis and Sexual Reproduction

2. 10.1 Halving the Chromosome Number
Biology, 9th ed,Sylvia Mader
Chapter 10
10.1 Halving the Chromosome Number
Meiosis & Sexual Reproduction
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

3. Halving the Chromosome Number
Biology, 9th ed,Sylvia Mader
Chapter 10
Halving the Chromosome Number
Meiosis & Sexual Reproduction
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

4. Homologous Chromosomes
Biology, 9th ed,Sylvia Mader
Chapter 10
Homologous Chromosomes
Meiosis & Sexual Reproduction
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. a.
Sister chromatids
nonsister
chromatids
duplication
duplication
kinetochore
centromere
chromosome
homologous pair
chromosome
paternal chromosome
maternal chromosome b.
a: © L. Willatt/Photo Researchers, Inc.

5. Halving the Chromosome Number
Biology, 9th ed,Sylvia Mader
Chapter 10
Halving the Chromosome Number
Meiosis & Sexual Reproduction
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)

6. Halving the Chromosome Number
Biology, 9th ed,Sylvia Mader
Chapter 10
Slide #6
Halving the Chromosome Number
Meiosis & Sexual Reproduction
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 6

7. Overview of Meiosis Meiosis & Sexual Reproduction 7
Biology, 9th ed,Sylvia Mader
Overview of Meiosis
Chapter 10
Slide #7
Meiosis & Sexual Reproduction
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
First division
Second division
Four haploid
daughter cells
centrioles
nucleolus
sister chromatids
centromere
synapsis
chromosome
duplication
2n = 4
2n = 4
n = 2
n = 2
MEIOSISI
Homologous pairs
synapse and then separate.
MEIOSISII
Sister chromatids separate,
becoming daughter chromosomes. 7

8. Biology, 9th ed,Sylvia Mader
Chapter 10
10.2 Genetic Variation
Meiosis & Sexual Reproduction
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

9. Crossing Over Occurs During Meiosis I
Biology, 9th ed,Sylvia Mader
Chapter 10
Crossing Over Occurs During Meiosis I
Meiosis & Sexual Reproduction
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
nucleoprotein lattice
sister chromatids
of a chromosome
sister chromatids
of its homologue A A a a A a B B b b B b
chiasmata of
nonsister
chromatids
1 and 3 c C C c C c D D d d D d 1 2 3 4 1 2 3 4 1 2 3 4
Bivalent
forms
Crossing-over
has occurred
Daughter
chromosomes a. b. c. d.
a: Courtesy Dr. D. Von Wettstein

10. Biology, 9th ed,Sylvia Mader
Chapter 10
Genetic Variation
Meiosis & Sexual Reproduction
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

11. Independent Assortment
Biology, 9th ed,Sylvia Mader
Chapter 10
Independent Assortment
Meiosis & Sexual Reproduction
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Combination 7
Combination 3
Combination 5
Combination 1
Combination 2
Combination 4
Combination 6
Combination 8

12. Biology, 9th ed,Sylvia Mader
Chapter 10
Genetic Variation
Meiosis & Sexual Reproduction
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

13. Biology, 9th ed,Sylvia Mader
Chapter 10
Genetic Variation
Meiosis & Sexual Reproduction
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

14. Biology, 9th ed,Sylvia Mader
Chapter 10
10.3 The Phases of Meiosis
Meiosis & Sexual Reproduction
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

15. Biology, 9th ed,Sylvia Mader
Chapter 10
The Phases of Meiosis
Meiosis & Sexual Reproduction
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

16. Biology, 9th ed,Sylvia Mader
Chapter 10
The Phases of Meiosis
Meiosis & Sexual Reproduction
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

17. Biology, 9th ed,Sylvia Mader
The Phases of Meiosis
Chapter 10
Meiosis & Sexual Reproduction
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

18. Biology, 9th ed,Sylvia Mader
Chapter 10
Meiosis & Sexual Reproduction
Meiosis I in Plant and Animal Cells

19. Biology, 9th ed,Sylvia Mader
Chapter 10
Meiosis & Sexual Reproduction
Meiosis I in Plant and Animal Cells

20. Biology, 9th ed,Sylvia Mader
Chapter 10
Meiosis II in Plant and Animal Cells
Meiosis & Sexual Reproduction

21. Biology, 9th ed,Sylvia Mader
Chapter 10
Meiosis II in Plant and Animal Cells
Meiosis & Sexual Reproduction

22. 10.4 Meiosis Compared to Mitosis
Biology, 9th ed,Sylvia Mader
Chapter 10
10.4 Meiosis Compared to Mitosis
Meiosis & Sexual Reproduction
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

23. Meiosis Compared to Mitosis
Biology, 9th ed,Sylvia Mader
Chapter 10
Meiosis Compared to Mitosis
Meiosis & Sexual Reproduction

24. Biology, 9th ed,Sylvia Mader
Chapter 10
Meiosis Compared to Mitosis
Meiosis & Sexual Reproduction

25. Biology, 9th ed,Sylvia Mader
Chapter 10
Meiosis Compared to Mitosis
Meiosis & Sexual Reproduction
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
Metaphase
Chromosomes align
at the metaphase plate.
Anaphase
Sister chromatids separate and
become daughter chromosomes.
MITOSIS

26. Biology, 9th ed,Sylvia Mader
Chapter 10
Meiosis Compared to Mitosis
Meiosis & Sexual Reproduction
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.
Four haploid daughter cells.
Their nuclei are genetically
different from the parent cell.
n = 2
n = 2
MEIOSIS I cont'd
MEIOSIS II
Daughter cells
Telophase
Daughter cells
are forming.
Two diploid daughter cells.
Their nuclei are genetically
identical to the parent cell.
MITOSIS cont'd

27. Biology, 9th ed,Sylvia Mader
Chapter 10
The Cycle of Life
Meiosis & Sexual Reproduction
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
Others wither away as polar bodies

28. Biology, 9th ed,Sylvia Mader
Chapter 10
The Cycle of Life
Meiosis & Sexual Reproduction
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

29. Meiosis & Sexual Reproduction
Biology, 9th ed,Sylvia Mader
Life Cycle of Humans
Chapter 10
Slide #29
Meiosis & Sexual Reproduction
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 29

30. Biology, 9th ed,Sylvia Mader
Spermatogenesis and Oogenesis in Mammals
Chapter 10
Meiosis & Sexual Reproduction
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
SPERMATOGENESIS
OOGENESIS
primary
spermatocyte
primary
oocyte 2 n 2n
Meiosis I
Meiosis I
first
polarbody
secondary
spermatocytes n
secondary
oocyte n
Meiosis II n
Meiosis II
spermatids
Meiosis II is completed
after entry of sperm (fertilization) n
second
polarbody n
Metamorphosis
and maturation
Fertilization
egg n
sperm
Sperm nucleus n n
fusion of sperm
nucleus and
agg nucleus
zygote 2n

31. 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

32. Nondisjunction 32 pair of homologous chromosomes pair of homologous
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
pair of
homologous
chromosomes
pair of
homologous
chromosomes
nondisjunction
Meiosis I
normal
Meiosis II
normal
nondisjunction
Fertilization
Zygote
2n + 1
2n + 1
2n - 1
2n - 1 2n 2n
2n + 1
2n - 1 a. b. 32

33. 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

34. a: © Jose Carrilo/PhotoEdit; b: © CNRI/SPL/Photo Researchers
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

35. 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

36. 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

37. 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
extra
chromosome X
a(top): Courtesy UNC Medical Illustration and Photograph; b(top): Courtesy Stefan D. Schwarz, a, b(bottom): © CNRI/SPL/Photo Researchers, Inc

38. 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 non-homologous chromosome
Follows breakage of two non-homologous chromosomes and improper re-assembly

39. 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 re-insertion
Genes occur in reverse order in the inverted segment

40. Types of Chromosomal Mutation
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. a a a b b b b c c c c + a d d d d e e e e f d f f e g g g f g
a. Deletion
b. Duplication a a a a b b l b b l c m c c m d d n d n d c e o e o e e f p f p f f g q q g g g h r r h
c. Inversion d.
Translocation