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Meiosis and genetic variation
Diploid cells have two copies
of each chromosome
•  Gametes are produced
through meiosis
•  Stages of meiosis
•  Homologous
chromosomes
contain copies of
the same genes
•  Meiosis produces haploid
cells that are genetically
unique via
–  Independent assortment
–  Crossing over
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Jan 20, 2017
•  One maternal,
one paternal
Diploid cells have two copies
of each chromosome
Siblings share 1/2
of their genes, on
average
•  Alleles are
different forms of
the same gene.
Each get one
chromosome from
mother and one
from father, but
they might not
both get the same
chromosome from
a parent.
Meiosis 1
Meiosis- key concepts
INTERPHASE
•  What is the purpose of meiosis?
•  What happens during meiosis?
•  How does meiosis generate diversity?
MEIOSIS I: Separates homologous chromosomes
PROPHASE I
METAPHASE I
ANAPHASE I
Sister chromatids
remain attached
Sister
chromatids
Chiasmata
Spindle
Tetrad
5
Chromosomes duplicate
Homologous chromosomes
(red and blue) pair and exchange
segments; 2n = 6 in this example
Homologous
chromosomes
separate
Pairs of homologous
Tertads line up
chromosomes split up
1
Meiosis 2
MEIOSIS II: Separates sister chromatids
TELOPHASE I AND
CYTOKINESIS
PROPHASE II
METAPHASE II
ANAPHASE II
Cleavage
furrow
TELOPHASE II AND
CYTOKINESIS
Haploid daughter cells
forming
Sister chromatids
separate
•  Mitosis produces an exact copy of
the parent cell.
–  Used for growth and asexual
reproduction.
•  Meiosis produces reduced
(haploid) gametes, which are
genetically unique.
–  Necessary only for sexual
reproduction.
chromosomes
are still double
Figure 13.8
Mitosis vs Meiosis
Not paired
Homologs pair in
meiosis 1
Mitosis vs. Meiosis
diploid
haploid
Sister
chromatids
separate in
meiosis 2
Meiosis produces genetic variation through
Independent Assortment
Crossing over
•  Mitosis
•  Meiosis
•  Conserves the number
of chromosome sets
•  Reduces the number of
chromosome sets from
two to one
•  Daughter cells
genetically identical to
their parent cell and to
each other
•  Daughter cells
genetically distinct
from parent cell and
from each other
Independent Assortment
•  Each pair of chromosomes sorts its maternal and
paternal homologues into daughter cells
independently of the other pairs
Key
Maternal set of
chromosomes
Paternal set of
chromosomes
Possibility 1
Possibility 2
Two equally probable
arrangements of
chromosomes at
metaphase I
Metaphase II
Daughter
cells
Figure 13.10
Combination 1
Combination 2
Combination 3
Combination 4
2
Crossing Over
Crossovers are random
Prophase I
of meiosis
Nonsister
chromatids
•  Produces Recombinant
Chromosomes that carry genes
derived from two different
parents
Tetrad
Chiasma,
site of
crossing
over
Metaphase I
This tetrad has 3
crossovers:
2&3
1&3
2&4
Metaphase II
Daughter
cells
Recombinant
chromosomes
Figure 13.11
Spindle fibers from one pole
attach to BOTH sister
chromatids
Spindle fibers
from the other
pole attach to
the other
homolog
Crossovers are
essential for
correct alignment
at metaphase 1
Chiasmata hold
the pair together
while
Tug of war
Aligns tetrads
The human life cycle
Key
Haploid gametes (n = 23)
Haploid (n)
Diploid (2n)
Gametes are the only
haploid cells
Ovum (n)
Sperm
Cell (n)
Meiosis occurs
during gamete
formation
FERTILIZATION
MEIOSIS
Ovary
Diploid
zygote
(2n = 46)
Testis
Mitosis and
development
Figure 13.5
•  Alternation of generations
Multicellular diploid
adults (2n = 46)
•  Mosses, fungi, and some protists have a
dominant haploid generation.
–  multicellular diploid and
haploid stages
Haploid multicellular
organism
Haploid multicellular
organism (gametophyte)
n
Mitosis
n
n
Mitosis
Mitosis
n
n
n
Spores
Mitosis
n
n
n
Gametes
Gametes
MEIOSIS
Diploid
multicellular
organism
(sporophyte)
Figure 13.6 B
MEIOSIS
2n
(b) Plants and some algae
n
FERTILIZATION
2n
Mitosis
FERTILIZATION
Zygote
2n
Zygote
3