Download Lab 4: Mitosis and Meiosis

Meiosis
Chapter 11
04/25/2016
Introduction to Meiosis: the Big
Picture
Starts with 1 original cell
Homologous chromosomes
cross over
Ends with 4 non-identical
cells
Each has half the DNA of the
original cell
Meiosis is Essential for Sexual
Reproduction
n
2n
meiotic
cell division
2n
2n
n
fertilization
diploid
parental
cells
haploid
gametes
diploid
fertilized
egg
With Sexual Reproduction,
Offspring Get Half of the DNA
from Each Parent
• 23 chromosomes
from maternal
(mom)
• 23 chromosome
from paternal
(dad)
Meiosis Allow For Genetic
Recombination
• Genetic
recombination
(during formation
of sperm and
eggs) is why most
siblings are not
identical
– Exception:
identical twins
Twins
http://www.embryology.ch/anglais/fplacenta/gemell01.html
Genetic Recombination by Crossing Over of Homologous
Chromosomes
Figure 13.8a
Prophase I
Centrosome
(with centriole pair)
Sister
chromatids
Chiasmata
Spindle
Telophase I and
Cytokinesis
Anaphase I
Metaphase I
Sister chromatids
remain attached
Centromere
(with kinetochore)
Metaphase
plate
Fragments
Homologous
chromosomes of nuclear
envelope
Homologous
chromosomes
separate
Microtubule
attached to
kinetochore
Cleavage
furrow
Each pair of homologous
chromosomes separates.
Chromosomes line up
Duplicated homologous
chromosomes (red and blue) by homologous pairs.
pair and exchange segments;
2n  6 in this example.
Two haploid
cells form; each
chromosome
still consists
of two sister
chromatids.
Figure 13.8b
Prophase II
Metaphase II
Anaphase II
Telophase II and
Cytokinesis
During another round of cell division, the sister chromatids finally separate;
four haploid daughter cells result, containing unduplicated chromosomes.
Sister chromatids
separate
Haploid daughter
cells forming
Spermatogenesis
46
46
92
92
46
23
Oogenesis
46
23
23
46
23
23
Chromosome Non-Disjunction
• Chromosomes do not properly separate
into daughter cells
• Can happen in mitosis; much more likely
in meiosis
Chromosome Non-Disjunction
• Abnormal gamete can participate in
fertilization with a normal gamete
• Offspring either…
– Have one copy of a chromosome
• Monosomy
• Nearly always fatal
• Exception: Turner Syndrome (XO)
– Have 3 copies of a chromosome
• Trisomy
• Usually fatal, some exceptions
Chromosome Non-Disjunction
• Trisomy 13 – Patau Syndrome
– 80% fatal in first year
• Trisomy 18 – Edward’s Syndrome
– 50% fatal in first year
• Trisomy 21 – Down Syndrome
– Survival to adulthood common
Is this karyotype normal?
Summary
Mitosis
Meiosis
Where it occurs
Body cells
Reproductive
organs
Why it occurs
Cell repair, growth,
asexual reproduction
Sexual
reproduction
Number of cells produced
2
4
Number of divisions
1
2
Number of chromosomes in daughter
cells
Same as mother cell
(diploid)
Half that of mother
cell (haploid)
Amount of variation in daughter cells
None
Lots
The human life cycle
mitotic cell division,
differentiation, and growth
mitotic cell division,
differentiation,
and growth
adults (2n)
baby (2n)
embryo (2n)
mitotic
cell division,
differentiation,
and growth
haploid (n)
diploid (2n)
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zygote (2n)
meiotic cell
division in
meiotic cell
ovaries
division in
testes
egg (n)
sperm (n)
fusion of gametes
Figure 9-1
Lifecycle of moss