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Transcript of MEIOSIS AND CROSSING OVER Copyright © 2009 Pearson Education, Inc.
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MEIOSIS AND CROSSING OVER
Copyright © 2009 Pearson Education, Inc.
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Somatic cells have pairs of homologous chromosomes, receiving one member of each pair from each parent
Homologous chromosomes are matched in
– Length
– Centromere position
– Gene locations
– A locus (plural, loci) is the position of a gene
– Different versions of a gene may be found at the same locus on maternal and paternal chromosomes
Chromosomes are matched in homologous pairs
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The human sex chromosomes X and Y differ in size and genetic composition
Pairs of autosomes have the same size and genetic composition
Applying Your Knowledge
– Humans have 46 chromosomes; how many homologous pairs does that represent?
– If there is one pair of sex chromosomes, how many pairs of autosomes are found in humans?
Chromosomes are matched in homologous pairs
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Sister chromatids One duplicatedchromosome
Centromere
Homologous pair ofchromosomes
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Meiosis is a process that converts diploid nuclei to haploid nuclei
– Diploid cells have two homologous sets of chromosomes
– Haploid cells have one set of chromosomes
– Meiosis occurs in the sex organs, producing gametes—sperm and eggs
Fertilization is the union of sperm and egg
– The zygote has a diploid chromosome number, one set from each parent
Gametes have a single set of chromosomes
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Haploid gametes (n = 23)
nnEgg cell
Sperm cellFertilizationMeiosis
Multicellulardiploid adults
(2n = 46)
Mitosis anddevelopment
nn
22nn
Diploidzygote
(2n = 46)
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Like mitosis, meiosis is preceded by interphase
– Chromosomes duplicate during the S phase
Unlike mitosis, meiosis has two divisions
– During meiosis I, homologous chromosomes separate
– The chromosome number is reduced by half
– During meiosis II, sister chromatids separate
– The chromosome number remains the same
Meiosis reduces the chromosome number from diploid to haploid
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Events in the nucleus during meiosis I
– Prophase I
– Chromosomes coil and become compact
– Homologous chromosomes come together as pairs by synapsis
– Each pair, with four chromatids, is called a tetrad
– Nonsister chromatids exchange genetic material by crossing over
Meiosis reduces the chromosome number from diploid to haploid
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– Applying Your KnowledgeHuman cells have 46 chromosomes. At the end of prophase I
– How many chromosomes are present in one cell?
– How many chromatids are present in one cell?
Meiosis reduces the chromosome number from diploid to haploid
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Events in the nucleus during meiosis I
– Metaphase I
– Tetrads align at the cell equator
– Anaphase I
– Homologous pairs separate and move toward opposite poles of the cell
Meiosis reduces the chromosome number from diploid to haploid
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Events in the nucleus during meiosis I
– Telophase I
– Duplicated chromosomes have reached the poles
– A nuclear envelope forms around chromosomes in some species
– Each nucleus has the haploid number of chromosomes
– After Telophase I and Cytokinesis
– How many chromosomes are present in one human cell?
– How many chromatids are present in one human cell?
Meiosis reduces the chromosome number from diploid to haploid
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Centrosomes(with centriolepairs)
PROPHASE I
Microtubulesattached tokinetochore
INTERPHASE
Sites of crossing over Metaphaseplate
Spindle
MEIOSIS I: Homologous chromosomes separate
METAPHASE I
Sister chromatidsremain attached
ANAPHASE I
Nuclearenvelope
Sisterchromatids
Centromere(with kinetochore)
Homologouschromosomes separateChromatin
Tetrad
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Meiosis II follows meiosis I without chromosome duplication
Each of the two haploid products enters meiosis II
Events in the nucleus during meiosis II
– Prophase II
– Chromosomes coil and become compact
Meiosis reduces the chromosome number from diploid to haploid
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Events in the nucleus during meiosis II
– Metaphase II
– Duplicated chromosomes align at the cell equator
– Anaphase II
– Sister chromatids separate and chromosomes move toward opposite poles
Meiosis reduces the chromosome number from diploid to haploid
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Events in the nucleus during meiosis II
– Telophase II
– Chromosomes have reached the poles of the cell
– A nuclear envelope forms around each set of chromosomes
– With cytokinesis, four haploid cells are produced
– After Telophase II and Cytokinesis
– How many chromosomes are present in one human cell?
– How many chromatids are present in one human cell?
Meiosis reduces the chromosome number from diploid to haploid
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PROPHASE I
MEIOSIS II: Sister chromatids separate
METAPHASE II ANAPHASE II
Cleavagefurrow
TELOPHASE IIAND CYTOKINESIS
Sister chromatidsseparate
Haploid daughtercells forming
TELOPHASE IIAND CYTOKINESIS
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Which characteristics are similar for mitosis and meiosis?
– One duplication of chromosomes
Which characteristics are unique to meiosis?
– Two divisions of chromosomes
– Pairing of homologous chromosomes
– Exchange of genetic material by crossing over
Mitosis and meiosis have important similarities and differences
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What is the outcome of each process?
– Mitosis:
– 2 genetically identical cells, with the same chromosome number as the original cell
– Meiosis:
– 4 genetically different cells, with half the chromosome number of the original cell
Mitosis and meiosis have important similarities and differences
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Independent orientation at metaphase I– Each pair of chromosomes independently
aligns at the cell equator– There is an equal probability of the maternal or
paternal chromosome facing a given pole– The number of combinations for chromosomes
packaged into gametes is 2n where n = haploid number of chromosomes
Random fertilization– The combination of each unique sperm with
each unique egg increases genetic variability
Independent orientation and random fertilization lead to varied offspring
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Two equally probablearrangements ofchromosomes at
metaphase I
Possibility 1 Possibility 2
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Two equally probablearrangements ofchromosomes at
metaphase I
Possibility 1 Possibility 2
Metaphase II
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Two equally probablearrangements ofchromosomes at
metaphase I
Possibility 1 Possibility 2
Metaphase II
Combination 1
Gametes
Combination 2 Combination 3 Combination 4
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Homologous chromosomes can carry different versions of genes
Separation of homologous chromosomes during meiosis can lead to genetic differences between gametes
– Homologous chromosomes may have different versions of a gene at the same locus
– One version was inherited from the maternal parent, and the other came from the paternal parent
– Since homologues move to opposite poles during anaphase I, gametes will receive either the maternal or paternal version of the gene
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Tetrad in parent cell(homologous pair of
duplicated chromosomes)
Coat-colorgenes
Chromosomes ofthe four gametes
Meiosis
PinkWhite
BlackBrown
Eye-colorgenes
C
e
E
c
C
e
E
c
C
e
E
c
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Genetic recombination is the production of new combinations of genes due to crossing over
Crossing over involves exchange of genetic material between homologous chromosomes
– Nonsister chromatids join at a chiasma (plural, chiasmata), the site of attachment and crossing over
– Corresponding amounts of genetic material are exchanged between maternal and paternal (nonsister) chromatids
Crossing over further increases genetic variability
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Centromere
ChiasmaTetrad
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Breakage of homologous chromatids
Coat-colorgenes
Eye-colorgenes
C
(homologous pair ofchromosomes in synapsis)
E
c e
Tetrad
C E
c e
Joining of homologous chromatids2
C E
c e
Chiasma
1
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Separation of homologous chromosomes at anaphase I
C E
c e
Chiasma
Separation of chromatids at anaphase II andcompletion of meiosis
C E
c e
c E
C e
c e
c E
C E
C e
Parental type of chromosome
Gametes of four genetic types
Recombinant chromosome
Parental type of chromosome
Recombinant chromosome
4
3
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ALTERATIONS OF CHROMOSOME NUMBER AND
STRUCTURE
Copyright © 2009 Pearson Education, Inc.
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A karyotype shows stained and magnified versions of chromosomes
– Karyotypes are produced from dividing white blood cells, stopped at metaphase
– Karyotypes allow observation of
– Homologous chromosome pairs
– Chromosome number
– Chromosome structure
A karyotype is a photographic inventory of an individual’s chromosomes
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Packed redand white bloodcells
CentrifugeBloodculture
Fluid1
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Packed redand white bloodcells
CentrifugeBloodculture
Fluid1
Hypotonicsolution
2
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Packed redand white bloodcells
CentrifugeBloodculture
Fluid1
Hypotonicsolution
2
3
Fixative
Whitebloodcells
Stain
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4
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Centromere
Sisterchromatids
Pair of homologouschromosomes
5
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Trisomy 21 involves the inheritance of three copies of chromosome 21
– Trisomy 21 is the most common human chromosome abnormality
– An imbalance in chromosome number causes Down syndrome, which is characterized by
– Characteristic facial features
– Susceptibility to disease
– Cardiac defects
– Lower intelligence
– Great variation
– The incidence increases with the age of the mother
An extra copy of chromosome 21 causes Down syndrome
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Infa
nts
wit
h D
ow
n s
ynd
rom
e(p
er 1
,000
bir
ths)
Age of mother
90
70
60
50
40
30
20
10
0
80
20 40353025 5045
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Nondisjunction is the failure of chromosomes or chromatids to separate during meiosis
– During Meiosis I
– Both members of a homologous pair go to one pole
– During Meiosis II
– Both sister chromatids go to one pole
Fertilization after nondisjunction yields zygotes with altered numbers of chromosomes
Accidents during meiosis can alter chromosome number
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Nondisjunctionin meiosis I
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Nondisjunctionin meiosis I
Normalmeiosis II
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Nondisjunctionin meiosis I
Normalmeiosis II
n + 1
Gametes
Number of chromosomes
n + 1 n – 1 n – 1
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Normalmeiosis I
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Nondisjunctionin meiosis II
Normalmeiosis I
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Nondisjunctionin meiosis II
Normalmeiosis I
Gametes
Number of chromosomes
n + 1 n – 1 n n
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Sex chromosome abnormalities tend to be less severe as a result of
– Small size of the Y chromosome
– X-chromosome inactivation
– In each cell of a human female, one of the two X chromosomes becomes tightly coiled and inactive
– This is a random process that inactivates either the maternal or paternal chromosome
– Inactivation promotes a balance between the number of X chromosomes and autosomes
Abnormal numbers of sex chromosomes do not usually affect survival
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Polyploid species have more than two chromosome sets
– Observed in many plant species
– Seen less frequently in animals
Example
– Diploid gametes are produced by failures in meiosis
– Diploid gamete + Diploid gamete Tetraploid offspring
– The tetraploid offspring have four chromosome sets
New species can arise from errors in cell division
Copyright © 2009 Pearson Education, Inc.
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Structure changes result from breakage and rejoining of chromosome segments
– Deletion is the loss of a chromosome segment– Duplication is the repeat of a chromosome
segment – Inversion is the reversal of a chromosome
segment– Translocation is the attachment of a segment
to a nonhomologous chromosome; can be reciprocal
Altered chromosomes carried by gametes cause birth defects
Chromosomal alterations in somatic cells can cause cancer
Alterations of chromosome structure can cause birth defects and cancer
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Deletion
Inversion
Duplication
Homologouschromosomes
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Reciprocaltranslocation
Nonhomologouschromosomes
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Chromosome 9
“Philadelphia chromosome”
Activated cancer-causing gene
Reciprocaltranslocation
Chromosome 22
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