M1 – Genetics
Cell Division
Colleen Jackson-Cook, Ph.D., FACMG
Sanger Hall, Room 5-007
[email protected]
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Cell Cycle
G0 – Period of
metabolic activity
G1 – Most variable
In length; metabolic
& prep for S
S – DNA synthesis
G2 – Post replication
& prior to mitosis
•DNA compaction
•Mitotic apparatus
assembled
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Cell Division
Karyokinesis
Division of the chromosomal material
to the two daughter cells
Cytokinesis
Division of the cytoplasmic material to the
Two daughter cells
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Mitosis Overview
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Prophase
From: Molecular Biology of the Cell 3rd Edition
• Nuclear envelope breaks down
• Chromosomes observed and consist of two sister chromatids
• Additional spindle fibers extend between the poles of the cell
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Metaphase
From: Molecular Biology of the Cell 3rd Edition
• Chromosomes have moved to a point midway between the cellular poles
and are attached to mitotic spindle apparatus
• Chromosomes are oriented in such a way that the sister kinetochores face
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opposite poles
Anaphase
From: Molecular Biology of the Cell 3rd Edition
• Sister kinetochores separate and sister chromatids move toward
opposite poles due to the shortening of the mitotic spindles
• The cell elongates in the direction of the poles
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Telophase
From: Molecular Biology of the Cell 3rd Edition
• Kinetochore microtubules disappear
• A new nuclear envelope forms around each set of daughter chromosomes
(karyokinesis complete)
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• Chromosomes begin to elongate
Cytokinesis
• The division of the cytoplasmic
material to the 2 daughter cells
– Initiated by a “cleavage furrow”
that first appears during
anaphase
– Alignment of furrow related
to position of centrosomes
– Actin filaments form a
contractile ring that eventually
pinches the cells apart
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From: Molecular Biology of the Cell 3rd Edition
Summary of chromosomal morphology throughout cell cycle
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Meiosis
• Cell division by which the
diploid cells of the germline
give rise to haploid gametes
• Consists of one round of DNA
synthesis followed by 2 rounds
of chromosome segregation
and cell division (MI and MII)
• Genetic recombination occurs
between homologous
chromosomes
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Meiosis I – The Reduction Division
Prophase I
• Longest of the meiotic phases
•Recombination occurs
Theoretically, as a result of meiosis an
individual can produce a minimum of
223 or 8.4 x 106 genetically different
gametes, but because of genetic
recombination the gametic pool is
much larger than that.
Metaphase I
Homologous chromosomes
align along equator and separate
Anaphase I
Chromosomes go to opposite poles
Sites of recombination (chiasmata)
have resolved
Telophase I
Chromosomes reach poles
Division of cytoplasm
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Meiosis II- Equitorial Division
There is NO DNA synthesis
Between MI and MII
Metaphase II
The sister kinetochores face
opposite poles
Anaphase II
Sister chromatids move to
opposite poles
Telophase II
Nuclear membrane reforms
and chromosomes decondense
After division of cytoplasm
there are haploid daughter cells
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Mitosis
vs.
• Somatic cells
• Diploid (2n=46) cells
produce diploid cells
• Single replication and
single division of genetic
material
• No recombination
Meiosis
• Germline cells
• Diploid cells produce
haploid (n=23) cells
• Single replication and two
successive divisions of
genetic material
(Meiosis I and Meiosis II)
• Recombination between
homologous
chromosomes occurs
during the early stages of
meiosis
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Meiotic
Recombination
•Leads to the formation of
new combinations of alleles
on the same chromosome
•Areas on chromosomes
that are close to one another
are less likely to be shuffled
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Gender Differences in Meiosis
Oogenesis
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•
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Female
Largely confined to prenatal
development (arrested in
Prophase I)
Cytokinesis is uneven
Completion of Meiosis II
occurs at fertilization
Spermatogenesis
•
•
•
•
Male
Ongoing through adult life
Cytokinesis is even
All stages of Meiosis are
complete before fertilization
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