1. Meiosis and chromosome number Life cycle and ploidy levels Steps in meiosis
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Transcript of 1. Meiosis and chromosome number Life cycle and ploidy levels Steps in meiosis
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