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◦ Mitosis produces genetically identical cells
for
– Growth
– Replacement
– Asexual reproduction
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MEIOSIS AND
GENETIC VARIATION
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◦ Somatic cells (not egg and sperm) have pairs
of homologous chromosomes (one from each
parent)
◦ Homologous chromosomes have the same:
1. Length
2. Centromere position
3. Gene locations
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Gene loci and alleles
Gene locations
– A locus is the position of a
gene
– Different versions of a gene
(alleles) may be found at the
same locus on maternal and
paternal chromosomes
Example: Chromosome 1
- contains over 3000 genes
- ~90% have been determined
◦ Sex chromosomes: X and Y
 1 pair in a human
 X and Y have different sizes and compositions
◦ Autosomes: are not X and Y
 22 pairs in a human
 Each pair has same size and composition
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Homologous pair of
chromosomes
Centromere
Sister chromatids
One duplicated
chromosome
– Diploid cells have two homologous sets of
chromosomes (chromosome number is 2n)
– Example: Somatic body cells like skin, liver, heart,
blood…
– Haploid cells have one set of chromosomes
(chromosome number is 1n)
– Example: Gametes—sperm and eggs
◦ Meiosis is a process that converts diploid
nuclei to haploid nuclei
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Haploid gametes (n = 23)
n
Meiosis
Egg cell
n
Sperm cell
Fertilization
Diploid
zygote
(2n = 46)
Multicellular
diploid adults
(2n = 46)
Mitosis and
development
2n
◦ Interphase
◦ Meiotic cell division
– Meiosis I: homologous chromosomes separate
– Produces 2 cells; the chromosome number is reduced
by half
– Meiosis II: sister chromatids separate
– Produces 4 cells; the chromosome number remains the
same
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MEIOSIS I: Homologous chromosomes separate
INTERPHASE
Centrosomes
(with centriole
pairs)
Nuclear
envelope
PROPHASE I
METAPHASE I
ANAPHASE I
Microtubules Metaphase Sister chromatids
remain attached
plate
attached to
Spindle kinetochore
Sites of crossing over
Sister
Chromatin chromatids
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Tetrad
Centromere
(with kinetochore)
Homologous
chromosomes separate
MEIOSIS II: Sister chromatids separate
TELOPHASE II
AND CYTOKINESIS
PROPHASE I
METAPHASE II
ANAPHASE II
TELOPHASE II
AND CYTOKINESIS
Cleavage
furrow
Sister chromatids
separate
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Haploid daughter
cells forming
◦ Outcome:
– Mitosis: two genetically identical cells, with the
same chromosome number as the original cell
– Meiosis: four genetically different cells, with
half the chromosome number of the original
cell
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MITOSIS
MEIOSIS
Parent cell
(before chromosome duplication)
Site of
crossing over
MEIOSIS I
Prophase I
Prophase
Duplicated
chromosome
(two sister
chromatids)
Tetrad formed
by synapsis of
homologous
chromosomes
Chromosome
duplication
Chromosome
duplication
2n = 4
Chromosomes
align at the
metaphase plate
Metaphase
Anaphase
Telophase
Sister chromatids
separate during
anaphase
2n
2n
Daughter cells
of mitosis
Tetrads
align at the
metaphase plate
Homologous
chromosomes
separate
(anaphase I);
sister chromatids remain
together
No further
chromosomal
duplication;
sister
chromatids
separate
(anaphase II)
Metaphase I
Anaphase I
Telophase I
Haploid
n=2
Daughter
cells of
meiosis I
MEIOSIS II
n
n
n
n
Daughter cells of meiosis II
Haploid gametes (n = 23)
n
Meiosis
Egg cell
n
Sperm cell
Fertilization
Diploid
zygote
(2n = 46)
Multicellular
diploid adults
(2n = 46)
Mitosis and
development
2n
1. Independent orientation during meiosis
(chance of getting chromosome from mom
or dad)
 Homologous chromosomes have different
“versions” or alleles of genes
Brown coat (C); black eyes (E)
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White coat (c); pink eyes (e)
Coat-color
genes
Eye-color
genes
Brown
Black
C
E
C
E
C
E
c
e
c
e
Meiosis
c
White
e
Pink
Tetrad in parent cell
(homologous pair of
duplicated chromosomes)
Chromosomes of
the four gametes
How do we get genetic variation??
2. Random fertilization
 Random egg and random sperm get together
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How do we get genetic variation??
3. Crossing over
 New combinations of genes are formed by
genetic recombination
 Material from non-sister chromatid are shared
at chiasma
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Coat-color
genes
C
Eye-color
genes
E
c
e
1
Breakage of homologous chromatids
C
E
c
e
2
C
Tetrad
(homologous pair of
chromosomes in synapsis)
Joining of homologous chromatids
E
Chiasma
c
e
C
E
Chiasma
e
c
3
Separation of homologous
chromosomes at anaphase I
C
E
C
e
c
E
c
4
C
e
Separation of chromatids at
anaphase II and
completion of meiosis
E
Parental type of chromosome
C
e
c
E
c
e
Recombinant chromosome
Recombinant chromosome
Parental type of chromosome
Gametes of four genetic types
WHEN THINGS GO WRONG…
ALTERATIONS OF
CHROMOSOME NUMBER AND
STRUCTURE
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◦ Karyotype: shows stained and magnified
versions of chromosomes
– Karyotypes are produced from dividing white
blood cells, stopped at metaphase
Centromere
Sister
chromatids
Pair of homologous
chromosomes
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5
◦ Trisomy 21: individual inherits three copies of
chromsome 21
– An imbalance in chromosome number causes
Down syndrome, which is characterized by
– Characteristic facial features
– Susceptibility to disease
– Shortened life span
– Mental retardation
– Variation in characteristics
– The incidence increases with the age of the
mother
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◦ Nondisjunction : failure of chromosomes or
chromatids to separate during meiosis
– During Meiosis I
– Both members of a homologous pair go to one pole
Nondisjunction
in meiosis I
Normal
meiosis II
Gametes
n+1
n+1
n–1
n–1
Number of chromosomes
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◦ Nondisjunction : failure of chromosomes or
chromatids to separate during meiosis
– During Meiosis II
– Both sister chromatids go to one pole
Normal
meiosis I
Nondisjunction
in meiosis II
Gametes
n
n+1 n–1
n
Number of chromosomes
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◦ Autosomes: usually VERY bad!
◦ Sex chromosomes: abnormalities tend to be
less severe because
1. Y chromosome is small (only ~200 genes vs
2000 on X)
2. X-chromosome inactivation
– In each cell of a human female, one of the two X
chromosomes becomes tightly coiled and inactive
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Sex linked genes/
diseases
Sex determination on
Y
Kleinfelter’s syndrome
Differences between Kleinfelter’s
syndrome and Turner’s syndrome
◦ Structure changes result from breakage and
rejoining of chromosome segments
–
Deletion is the loss of a chromosome segment
–
Cri du chat is deletion of part of chromosome 5
Deletion
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Example: Cri du chat is deletion of part of
chromosome 5
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◦ Structure changes result from breakage and
rejoining of chromosome segments
–
–
Duplication is the repeat of a chromosome
segment
Charcot-Marie-Tooth disease type I duplication
on chromosome 17
Duplication
Homologous
chromosomes
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Example: Charcot-Marie-Tooth disease type I
duplication on chromosome 17
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◦ Structure changes result from breakage and
rejoining of chromosome segments
–
–
Translocation is the attachment of a segment
to a nonhomologous chromosome; can be
reciprocal
Some Down’s syndrome
Reciprocal
translocation
Nonhomologous
chromosomes
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Example: Translocation Down’s syndrome
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