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CHAPTER 9: Sexual Reproduction and MeiosisBIO 121
Why Sex?
Section 9.1 Figure 9.1
Reproduction occurs in two main forms: asexual and sexual.
Human family: © Corbis RF; Amoeba reproducing by binary fission: © Carolina Biological Supply Company/Phototake; Mother cat and kittens: © Jane Burton/Dorling Kindersley/Getty Images
Why Sex?
Section 9.1 Figure 9.1
In asexual reproduction, an organism replicates its DNA and splits the contents of one cell into two. This amoeba is reproducing asexually.
Human family: © Corbis RF; Amoeba reproducing by binary fission: © Carolina Biological Supply Company/Phototake; Mother cat and kittens: © Jane Burton/Dorling Kindersley/Getty Images
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Why Sex?
Section 9.1 Figure 9.1
Asexual reproduction generates identical offspring.
Human family: © Corbis RF; Amoeba reproducing by binary fission: © Carolina Biological Supply Company/Phototake; Mother cat and kittens: © Jane Burton/Dorling Kindersley/Getty Images
Why Sex?
Section 9.1 Figure 9.1
In sexual reproduction, the DNA of the offspring comes from two parents. Humans, cats, and many other familiar organisms reproduce sexually.
Human family: © Corbis RF; Amoeba reproducing by binary fission: © Carolina Biological Supply Company/Phototake; Mother cat and kittens: © Jane Burton/Dorling Kindersley/Getty Images
Why Sex?
Section 9.1 Figure 9.1
A female parent contributes an egg, and a male parent produces sperm. These cells fuse, yielding the first cell of the next generation.
Human family: © Corbis RF; Amoeba reproducing by binary fission: © Carolina Biological Supply Company/Phototake; Mother cat and kittens: © Jane Burton/Dorling Kindersley/Getty Images
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Why Sex?
Section 9.1 Figure 9.1
But attracting a mate and producing sex cells are costly. Why is sexual reproduction so common?
Human family: © Corbis RF; Amoeba reproducing by binary fission: © Carolina Biological Supply Company/Phototake; Mother cat and kittens: © Jane Burton/Dorling Kindersley/Getty Images
Why Sex?
Section 9.1 Figure 9.2
Sexual reproduction produces genetic diversity among offspring, which increases the chance that some offspring survive in changing environments.
Question #1
Which two organisms are genetically the most similar?
Pair A
Pair B
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ANSWER
Which two organisms are genetically the most similar?
Pair A
Pair B
Pair A are identical to each other
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Karyotype
During cell division is when chromosomes are the most compacted and visible. The division process is stopped and stains are added. A photograph is taken and chromosome pairs are arranged.
This image is called a karyotype and can tell a lot about chromosome structure in a body cell.
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Karyotype
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Diploid Cells Contain Two Homologous Sets of Chromosomes
Section 9.2 Figure 9.3
Most cells in a sexually reproducing organism have two sets of chromosomes: one from the father and one from the mother.
All karyotype photos: © CNRI/Science Source
Diploid Cells Contain Two Homologous Sets of Chromosomes
Section 9.2 Figure 9.3
Cells with two sets of chromosomes are called diploidcells. The image to the right shows all of the chromosomes from a diploid human cell.
All karyotype photos: © CNRI/Science Source
Diploid Cells Contain Two Homologous Sets of Chromosomes
Section 9.2 Figure 9.3
The numbered chromosomes are called autosomes. These chromosomes are the same for both sexes.
All karyotype photos: © CNRI/Science Source
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Diploid Cells Contain Two Homologous Sets of Chromosomes
Section 9.2 Figure 9.3
The chromosomes denoted by a letter are the sex chromosomes. In humans, an individual with two X chromosomes is female, and an individual with one X and one Y chromosome is male.
All karyotype photos: © CNRI/Science Source
Diploid Cells Contain Two Homologous Sets of Chromosomes
Section 9.2 Figure 9.3
Notice that each chromosome is paired with an equally‐sized chromosome (except for when the X chromosome pairs with the smaller Y chromosome).
All karyotype photos: © CNRI/Science Source
Diploid Cells Contain Two Homologous Sets of Chromosomes
Section 9.2 Figure 9.4
The chromosomes of a homologous pair look alike and carry the same sequence of genes for the same traits.
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Diploid Cells Contain Two Homologous Sets of Chromosomes
Section 9.2 Figure 9.4
However, the members of a homologous pair may carry different alleles, which are alternative versions of the same gene.
Question #2
The photo below shows two chromosomes from one cell. Which statement is true?
A. Both came from theperson’s mother.B. The 1‐2 pair is identicalto the 3‐4 pair.C. 1 is identical to 2.D. The chromosomes havenot been replicated yet.
1 2 3 4
ANSWER
The photo below shows two chromosomes from one cell. Which statement is true?
A. Both came from theperson’s mother.B. The 1‐2 pair is identicalto the 3‐4 pair.C. 1 is identical to 2.D. The chromosomes havenot been replicated yet.
1 2 3 4
Flower: © Doug Sherman/Geofile/RF
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MeiosisMeiosis is a reduction nuclear division that occurs only in the production of gametes (sex cells).
It is characterized by two consecutive nuclear divisions: meiosis I andmeiosis II
No Interphase (No DNA replication) between the two divisions
The daughter cells are produced with half the number of chromosomes as the parent cell and are called haploid (n).
Meiosis Is Essential in Sexual Reproduction
Section 9.3
Gametes, like the sperm and egg cells in this image, are haploid sex cells. A haploid cell has only one set of chromosomes.
Sperm contacting egg: © Francis Leroy, Biocosmos/Science Source Figure 9.15
Meiosis Is Essential in Sexual Reproduction
Section 9.3
When two haploid cells fuse at fertilization, one diploid zygote is formed. A zygote is the first cell of a new organism.
Figure 9.15Sperm contacting egg: © Francis Leroy, Biocosmos/Science Source
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Meiosis Is Essential in Sexual Reproduction
Section 9.3 Figure 9.5
The organism grows by mitoticcell division.
Mating frogs: © Nature Picture Library/Britain On View/Getty Images
Meiosis Is Essential in Sexual Reproduction
Section 9.3 Figure 9.5
In adults, specialized diploid cells called germ cells divide to form haploid gametes.
Mating frogs: © Nature Picture Library/Britain On View/Getty Images
Meiosis Overview
Section 9.3 Figure 9.6Human female karyotype: © James Cavallini/Science Source
Meiosis halves the chromosome number and scrambles alleles.
2n=4
n=2
n=2
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Overview of Meiosis
Parent CellInterphase
Meiosis I
DNA Replication
Meiosis II
Gametes
9.3 Mastering Concepts
How do haploid and diploid nuclei differ?
Ultrasound: © Chad Ehlers/Glow Images
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Meiosis I Chromosomes shorten and thicken.
Homologous chromosome pairs line up next to each other forming two rows of 23 instead of one row of 46.
The homologous chromosomes join together in a process called synapsis and twist into a unit called a tetrad.
Portions of one chromatid may be exchanged with another by crossing‐over. This act permits recombination of genes, which may produce new gametes.
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Meiosis I
• Meiosis I has four phases:
Prophase I, Metaphase I, Anaphase I, and Telophase I.
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Meiosis II
• Chromosome behavior in Meiosis II is like that of mitosis.
• Meiosis II has four phases:
Prophase II, Metaphase II, Anaphase II, and Telophase II.
In Meiosis, DNA Replicates Once, but the Nucleus Divides Twice
Section 9.4 Figure 9.11
This summary figure shows the nucleus dividing twice.
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In Meiosis, DNA Replicates Once, but the Nucleus Divides Twice
Section 9.4 Figure 9.11
The cell on the left has two pairs of homologous chromosomes, whereas the cells on the right have two unpaired,
unreplicated chromosomes.
In Meiosis, DNA Replicates Once, but the Nucleus Divides Twice
Section 9.4 Figure 9.7
2n=4
All photos © Ed Reschke
In Meiosis, DNA Replicates Once, but the Nucleus Divides Twice
Section 9.4 Figure 9.7
n=2
All photos © Ed Reschke
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Question #3
A zygote of a ferret contains 40 chromosomes. How many chromosomes are in a ferret’s sperm cell?
A. 10B. 20C. 40D. 80
ANSWER
A zygote of a ferret contains 40 chromosomes. How many chromosomes are in a ferret’s sperm cell?
A. 10B. 20C. 40D. 80
The gametes produced in meiosis are different from one another because of:
•crossing over (prophase I)•independent assortment (metaphase I)
Meiosis Generates Variability
Section 9.5 Mother cat and kittens: © Jane Burton/Dorling Kindersley/Getty Images
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Meiosis Generates Variability
Section 9.5 Figure 9.8
During crossing over, two homologous chromosomes exchange genetic material during prophase I.
Figure 10.3b
Exchange of DNA during prophase I increases genetic variability. Chromatids are no longer exact duplicates.
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Crossing‐Over
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Meiosis Generates Variability
Section 9.5 Figure 9.9
In independent assortment, chromosome pairs align randomly during metaphase I.
Meiosis Generates Variability
Section 9.5 Figure 9.9
A germ cell with three pairs of chromosomes has four possible arrangements during metaphase I, yielding eight possible gametes (23 = 8).
During metaphase, chromosomes line up in PAIRS, but they line up randomly. This picture shows all the different possible arrangements for an organism with 6 chromosomes.
This is called INDEPENDENT ASSORTMENT
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Check for understanding
1. What phase directly follows metaphase I?
2. How many cells are present at the end of meiosis I ?
3. A cell has a diploid number of 60, what is the organism's haploid number?
4. Meiosis occurs in what type of cells?
5. In what phase do homologous chromosomes pair up and crossing-over can occur?
6. In what phase do the CHROMATIDS separate?
anaphase I
2
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gametes
prophase 1
anaphase 2
7. Identify the phase: 8. Identify the phase:
anaphase 1 anaphase 2
Meiosis Generates Variability
Section 9.5 Figure 9.10
If meiosis generates such great variability among gametes, how can siblings be identical?
Identical twins: © Barbara Penoyar/Getty Images RF
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Meiosis Generates Variability
Section 9.5
Sometimes, early in development, an embryo splits in two. Each embryo then develops independently.
Figure 9.10Identical twins: © Barbara Penoyar/Getty Images RF
Meiosis Generates Variability
Section 9.5
Fraternal twins are a testament to the variation among gametes; two sperm cells fertilize two separate egg cells, and
the offspring might look very different.
Figure 9.10Fraternal twins: © Image Source Black/ Getty Images RF
Question #4
Many sets of triplets consist of a “pair and a spare;” that is, two identical babies and one non‐identical baby. Producing this arrangement requires ___ sperm and ___ eggs.
A. 2 … 2B. 1 … 3C. 3 … 1D. 2 … 3
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ANSWER
Many sets of triplets consist of a “pair and a spare;” that is, two identical babies and one non‐identical baby. Producing this arrangement requires ___ sperm and ___ eggs.
A. 2 … 2B. 1 … 3C. 3 … 1D. 2 … 3
Flower: © Doug Sherman/Geofile/RF
Mitosis and Meiosis Compared
Section 9.6 Figure 9.11
Meiosis occurs in germ cells; mitosis occurs in body cells.
2n=4
n=2
2n=4
2n=4
Mitosis and Meiosis Compared
Section 9.6
Cytokinesis occurs once in mitosis, but twice in meiosis.
2n=4
n=2
2n=4
2n=4
Figure 9.11
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Mitosis and Meiosis Compared
Section 9.6
In mitosis, daughter cells have the same chromosome number as the parent cell. In meiosis, chromosome number is halved.2n=4
n=2
2n=4
2n=4
Figure 9.11
Mitosis and Meiosis Compared
Section 9.6
In meiosis, homologous chromosome pairing allows for crossing over. This process does not occur in mitosis.
2n=4
n=2
2n=4
2n=4
Figure 9.11
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Mitosis Compared to Meiosis
• Occurrence –throughout the body.
• Process – 1 cell division.
• Daughter cells – are diploid (2n) or identical to the mother cell.
• Results in – somatic cells (body cells)
• Occurrence – only in the reproductive organs (testes or ovaries).
• Process ‐ 2 cell divisions.
• Daughter cells ‐ are haploid (n).
• Results in – gametes (germ cells, sex cells, sperm or ova)
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9.6 Mastering Concepts
In what ways are mitosis and meiosis different?
Ultrasound: © Chad Ehlers/Glow Images
Errors Sometimes Occur in Meiosis
Section 9.7
If chromosomes fail to separate during anaphase I or anaphase II, then abnormal gametes form.
(Nondisjunction)
Figure 9.12
Errors Sometimes Occur in Meiosis
Section 9.7
This image shows how nondisjunction in meiosis I produces gametes with either an extra or a missing chromosome.
Figure 9.12
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Errors Sometimes Occur in Meiosis
Section 9.7
This image shows how nondisjunction in meiosis II produces two gametes with either an extra or a missing chromosome,
and two normal gametes.
Figure 9.12
Errors Sometimes Occur in Meiosis
Section 9.7
Abnormal gametes lead to zygotes with an improper chromosome number.
Figure 9.12
Errors Sometimes Occur in Meiosis
Section 9.7
Individuals with missing chromosomes often cease developing before birth. An extra chromosome often causes fewer
problems than a missing chromosome.
Figure 9.12
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Errors Sometimes Occur in Meiosis
Section 9.7 Figure 9.13
An extra copy of a chromosome—a trisomy—causes genetic disorders such as Down syndrome.
Karyotype: © CNRI/Science Source; Child with Down syndrome © George Doyle/Stockbyte/Getty Images RF
Errors Sometimes Occur in Meiosis
Section 9.7 Table 9.1
Extra or missing sex chromosomes cause sex disorders.
Errors Also Occur Within Chromosomes
Section 9.7 Figure 9.14
Mutations in chromosome structure can have major effects on an organism.
Chromosomes: © Addenbrookes Hospital/Science Source
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Question #5
In Turner syndrome, a female is born with only one X chromosome. This disorder could arise as a result of nondisjunction in ___.
A. sperm formationB. egg formationC. either sperm or egg formation
ANSWER
In Turner syndrome, a female is born with only one X chromosome. This disorder could arise as a result of nondisjunction in ___.
A. sperm formationB. egg formationC. either sperm or egg formation
9.7 Mastering Concepts
How can nondisjunction during meiosis lead to gametes with extra or missing chromosomes?
Ultrasound: © Chad Ehlers/Glow Images
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Meiosis Produces Human Gametes
Section 9.8 Figure 9.15
Sperm cells form in the testes. Egg cells form in the ovaries.
Sperm contacting egg: © Francis Leroy, Biocosmos/Science Source
Meiosis Produces Human Gametes
Section 9.8 Figure 9.16
Figure 10.9a
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Meiosis Produces Human Gametes
Section 9.8 Figure 9.17
During OOGENESIS, cytoplasm divides unevenly during each cytokinesis,
resulting in only ONE viable egg cell.
3 small polar bodies are formed
1 large OOCYTE has potential to be fertilized
Meiosis Produces Spores in Plants
Section 9.8 Figure 9.18