1. Meiosis and chromosome numberLife cycle and ploidy levels
2. Steps in meiosis
3. Source of genetic variationa. Independent alignment of homologuesb. recombination
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• Gametes are haploid, with only one set of chromosomes
• Somatic cells are diploid.
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• The human life cycle
• Meiosis creates gametes
• Mitosis of the zygote produces adult bodies
Figure 8.13
MEIOSIS FERTILIZATION
Haploid gametes (n = 23)
Egg cell
Sperm cell
Diploidzygote
(2n = 46)Multicellular
diploid adults (2n = 46)
Mitosis anddevelopment
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• Chromosomes are duplicated before meiosis, then the cell divides twice to form four daughter cells.
Meiosis reduces the chromosome number from diploid to haploid
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Figure 8.14, part 1
MEIOSIS I: Homologous chromosomes separate
INTERPHASE PROPHASE I METAPHASE I ANAPHASE I
Centrosomes(withcentriolepairs)
Nuclearenvelope
Chromatin
Sites of crossing overSpindle
Sisterchromatids
Tetrad
Microtubules attached tokinetochore
Metaphaseplate
Centromere(with kinetochore)
Sister chromatidsremain attached
Homologouschromosomes separate
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• In meiosis I, homologous chromosomes are paired– While paired, they cross over and
exchange genetic information
– homologous pairs are then separated, and two daughter cells are produced
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Figure 8.14, part 2
MEIOSIS II: Sister chromatids separate
TELOPHASE IAND CYTOKINESIS PROPHASE II METAPHASE II ANAPHASE II
Cleavagefurrow
Sister chromatidsseparate
TELOPHASE IIAND CYTOKINESIS
Haploiddaughter cellsforming
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• Meiosis II is essentially the same as mitosis– sister chromatids of each chromosome
separate
– result is four haploid daughter cells
MITOSIS MEIOSISDiploidsomatic cell
Diploidgameteprecursor
4
1
2
3
5
6
7
2n
2n
2n 2n
2n
2n 2n 1n 1n
2n
2n
2n
1n 1n 1n 1n
division
division
duplication
haploiddiploid
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Figure 8.15
MITOSIS MEIOSIS
PARENT CELL(before chromosome replication)
Site ofcrossing over MEIOSIS I
PROPHASE ITetrad formedby synapsis of homologous chromosomes
PROPHASE
Duplicatedchromosome(two sister chromatids)
METAPHASE
Chromosomereplication
Chromosomereplication
2n = 4
ANAPHASETELOPHASE
Chromosomes align at the metaphase plate
Tetradsalign at themetaphase plate
METAPHASE I
ANAPHASE ITELOPHASE ISister chromatids
separate duringanaphase
Homologouschromosomesseparateduringanaphase I;sisterchromatids remain together
No further chromosomal replication; sister chromatids separate during anaphase II
2n 2n
Daughter cellsof mitosis
Daughter cells of meiosis II
MEIOSIS II
Daughtercells of
meiosis I
Haploidn = 2
n n n n
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• Each chromosome of a homologous pair comes from a different parent
– Each chromosome thus differs at many points from the other member of the pair
Genetic variation among offspring is a result of 1) Independent orientation of chromosomes in meiosis 2) random fertilization
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Figure 8.16
POSSIBILITY 1 POSSIBILITY 2
Two equally probable
arrangements of chromosomes at
metaphase I
Metaphase II
Gametes
Combination 1 Combination 2 Combination 3 Combination 4
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Homologous chromosomes carry different versions of genes at corresponding loci
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Figure 8.17A, B
Coat-color genes Eye-color genes
Brown Black
C E
c e
White Pink
C E
c e
C E
c e
Tetrad in parent cell(homologous pair of
duplicated chromosomes)
Chromosomes ofthe four gametes
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• Crossing over is the exchange of corresponding segments between two homologous chromosomes
• Genetic recombination results from crossing over during prophase I of meiosis, which increases variation further
Crossing over further increases genetic variability
tetrad
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Figure 8.18A
TetradChaisma
Centromere
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• How crossing over leads to genetic recombination
Figure 8.18B
Tetrad(homologous pair ofchromosomes in synapsis)
Breakage of homologous chromatids
Joining of homologous chromatids
Chiasma
Separation of homologouschromosomes at anaphase I
Separation of chromatids atanaphase II and completion of meiosis
Parental type of chromosome
Recombinant chromosome
Recombinant chromosomeParental type of chromosome
Gametes of four genetic types
1
2
3
4
Coat-colorgenes
Eye-colorgenes
END OF INTERPHASE
PROPHASE I METAPHASE I ANAPHASE I
MEIOSIS I
TELOPHASE IIANAPHASE II
METAPHASE IIPROPHASE IITELOPHASE I
MEIOSIS
METAPHASE I METAPHASE I
TELOPHASE II
METAPHASE II
INDEPENDENT ASSORTMENT
egg
polarbody
spermatogonium
primaryspermatocyte
secondaryspermatocyte
oogonium
primaryoocyte
secondaryoocyte
polar bodies(will be degraded)
spermatids
meiosis ll
meiosis l
SPERMATOGENESIS OOGENESISa b
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• Abnormal chromosome count is a result of nondisjunction
– Either homologous pairs fail to separate during meiosis I
8.21 Accidents during meiosis can alter chromosome number
Figure 8.21A
Nondisjunctionin meiosis I
Normalmeiosis II
Gametes
n + 1 n + 1 n – 1 n – 1Number of chromosomes
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– Or sister chromatids fail to separate during meiosis II
Figure 8.21B
Normalmeiosis I
Nondisjunctionin meiosis II
Gametes
n + 1 n – 1 n nNumber of chromosomes
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• Fertilization after nondisjunction in the mother results in a zygote with an extra chromosome
Figure 8.21C
Eggcell
Spermcell
n + 1
n (normal)
Zygote2n + 1
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• This karyotype shows three number 21 chromosomes
• An extra copy of chromosome 21 causes Down syndrome
8.20 Connection: An extra copy of chromosome 21 causes Down syndrome
Figure 8.20A, B
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• The chance of having a Down syndrome child goes up with maternal age
Figure 8.20C
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• Nondisjunction can also produce gametes with extra or missing sex chromosomes
– Unusual numbers of sex chromosomes upset the genetic balance less than an unusual number of autosomes
8.22 Connection: Abnormal numbers of sex chromosomes do not usually affect survival
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Table 8.22
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• Chromosome breakage can lead to rearrangements that can produce genetic disorders or cancer
– Four types of rearrangement are deletion, duplication, inversion, and translocation
8.23 Connection: Alterations of chromosome structure can cause birth defects and cancer
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Figure 8.23A, B
Deletion
Duplication
Inversion
Homologouschromosomes
Reciprocaltranslocatio
n
Nonhomologouschromosomes
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• Translocation
Figure 8.23Bx
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• Chromosomal changes in a somatic cell can cause cancer
Figure 8.23C
Chromosome 9
– A chromosomal translocation in the bone marrow is associated with chronic myelogenous leukemia
Chromosome 22Reciprocaltranslocation
“Philadelphia chromosome”
Activated cancer-causing gene