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Meiosis & Sexual Reproduction
Asexual Reproduction
• The prokaryotic chromosome is a ring of DNA
• Binary fission– Splitting in two– Two replicate chromosomes are distributed to two
daughter cells– Produces two daughter cells identical to original cell –
asexual reproduction
Chapter 9 Section 4
Binary FissionCopyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
1. Attachment of chromosome toa special plasma membranesite indicates that thisbacterium is about to divide.
2. The cell is preparing for binaryfission by enlarging its cell wall,plasma membrane, and overallvolume.
3. DNA replication has producedtwo identical chromosomes.Cell wall and plasma membrane begin to grow inward.
4. As the cell elongates, thechromosomes are pulled apart.Cytoplasm is being distributedevenly.
5. New cell wall and plasmamembrane has divided thedaughter cells.
chromosome
cell wall
plasmamembrane
cytoplasm
SEM 2,345X© Dennis Kunkel Microscopy, Inc./Visuals Unlimited
Sexual Reproduction
• Two parents
• Genetic variation
• Haploid gametes
• Unique offspring– Similar to parents– Similar to each other
Haploid vs. Diploid
HAPLOID• 1n
• One copy of each chromosome
DIPLOID• 2n
• Two copies of each chromosome
• Homologous chromosomes
Homologous Chromosomes
6
a. Sister chromatids
duplicationduplication
chromosome
maternal chromosome
b.paternal chromosome
homologous pair
nonsisterchromatids
centromere
kinetochore
chromosome
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
a: © L. Willatt/Photo Researchers, Inc.
Overview of Meiosis
7
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
2n = 4
• Diploid parent cell divides twice to make haploid gametes
• Homologous chromosomes
• Sister chromatids
Overview of Meiosis
8
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
centrioles nucleoluscentromere
MEIOSISIHomologous pairs
synapse and then separate.
2n = 4 2n = 4
chromosomeduplication
Overview of Meiosis
9
centrioles nucleoluscentromere
sister chromatids
MEIOSISIHomologous pairs
synapse and then separate.
MEIOSISIISister chromatids separate,
becoming daughter chromosomes.
n = 2
Four haploiddaughter cells
Second divisionFirst division
synapsis
2n = 4 2n = 4
chromosomeduplication
n = 2
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Genetic VariationFor humans - 23 pairs of chromosomes:
• Independent Assortment – 50/50 chance that gamete gets maternal/ paternal homologous chromosome– # of possible gametes = 223
– >8 million possible gamete combinations
• Random Fertilization– (8 million)(8 million) = 64 trillion possible offspring
• Crossing over – increases genetic variation even more
Independent Assortment
11
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Combination 1
Combination 2 Combination 4 Combination 6 Combination 8
Combination7Combination 5Combination 3
Crossing Over Occurs During Meiosis I
12
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A
B
bB
C
c C
DD D
A Aa
b
B b
c
C c
dd d
a a
1 2 3 4 1 2 3 4 1 2 3 4
nucleoprotein lattice sister chromatidsof a chromosome
sister chromatidsof its homologue
chiasmata ofnonsisterchromatids1 and 3
a. b. c. d.
Daughterchromosomes
Crossing-overhas occurred
Bivalentforms
a: Courtesy Dr. D. Von Wettstein
Crossing Over
• Increases genetic variation
• Synapsis• Tetrad• Chiasma
• Happens during Prophase I
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
• Some offspring may have a better chance of survival
14
Karyotyping
Mitosis vs. Meiosis
Meiosis Compared to Mitosis
• Meiosis– 2 nuclear divisions– Chromosomes cross over– Halves chromosome
number– Produces 4 genetically
different daughter cells – Used only for sexual
reproduction
• Mitosis– 1 nuclear division– Chromosomes don’t cross
over– Preserves chromosome
number– Produces 2 genetically
identical daughter cells– Used for asexual
reproduction & growth
17
Life Cycles
Life Cycle of Humans
19
MEIOSISFERTILIZATION
2n
MITOSIS
2n
2n
2n
MITOSIS
egg
n
n
zygote
sperm
haploid (n)n = 23
2n = 46diploid (2n)
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Spermatogenesis and Oogenesis in Mammals
n
n
OOGENESISprimaryoocyte
Meiosis I
firstpolarbody
Meiosis II
Fertilization
secondaryoocyte
Meiosis II is completedafter entry of sperm
(fertilization)
egg
Sperm nucleus
fusion of spermnucleus andagg nucleus
zygote
2n
n
n
n
secondpolarbody
2n2n
n
n
n
Metamorphosisand maturation
Meiosis II
Meiosis I
SPERMATOGENESIS
primaryspermatocyte
secondaryspermatocytes
spermatids
sperm
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Changes in Chromosome Number & Structure
• Euploidy: correct number of chromosomes
• Aneuploidy: change in the chromosome number– Results from nondisjunction
• Monosomy - 1• Trisomy - 3
21
Nondisjunction
22
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pair ofhomologouschromosomes
nondisjunction
nondisjunction
pair ofhomologouschromosomes
normalMeiosis I
normalMeiosis II nondisjunction
Nondisjunction
23
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pair ofhomologouschromosomes
nondisjunction
a.
2n + 1 2n + 1 2n - 1 2n - 1
b.
2n 2n 2n + 1 2n - 1
pair ofhomologouschromosomes
normalMeiosis I
normalMeiosis II
Fertilization
Zygote
nondisjunction
Trisomy 21
24
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
Changes in Sex Chromosome
25
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
a. Turner syndrome missingchromosome X
b. Klinefelter syndrome extrachromosome X
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
Types of Chromosomal Mutation
26
c. Inversion d. Translocation
b. Duplicationa. Deletion
+
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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