Cellular Reproduction, Part 2:
Meiosis
Lecture 10
Fall 2008
1
Mitosis & Meiosis
Mitosis
• Form of cell division that
leads to identical daughter
cells with the full
complement of DNA
• Occurs in somatic cells
Karyotype
– Cells of body that are not
reproductive cells
– In humans, have 46
chromosomes
• 2 sets of 23 chromosomes
Fig. 13.3
2
Mitosis & Meiosis
• Homologous chromosomes
– Matched pair of chromosomes that carry potentially different
versions of the same genes
– Alleles: alternate forms of a gene
– Locus (Loci): a genes specific location on a chromosome
• Diploid (2n)
– Contains homologous pairs of chromosomes
Karyotype
Fig. 13.4
Fig. 13.3
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Mitosis & Meiosis
• Sex chromosomes (X, Y in mammals)
– Determines the sex of an organism
– In females, all 23 chromosomes are homologous
• XX = females
– In males, 22 are homologous, one pair does not match
• XY = male
• Autosomes
– Chromosomes that are not sex chromosomes
Karyotype
Fig. 13.3
Mitosis & Meiosis
Meiosis
• Form of cell division that leads to non-identical
daughter cells with one-half the complement of
DNA
• Forms gametes
– Reproductive cells (sperm cells & egg cells)
• Haploid (n)
– Contains only one member of each
homologous chromosome pair
– 23 chromosomes (human)
– 22 autosomes & 1 sex chromosome
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5
Meiosis
Meiosis
• Part of sexual reproduction
– Egg & sperm haploid (1n)
– Egg & sperm fuse into one
cell (fertilization)
– Fertilized egg (zygote) now
diploid (2n)
– Zygote grows by a series of
mitotic cell division
• Life cycle
– Generation to generation
sequence of stages in the
reproductive history of an
organism
Fig. 13.5
Meiosis
Differs from mitosis
• Halves the number of chromosomes
– Two rounds of cell division in meiosis
– Mitosis ends with 2 cells, meiosis with 4 cells
• Allows for exchange of genetic material
– Crossing over of homologous chromosomes
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Meiosis
2 rounds of cell division
Meiosis 1
• One cell divides into two cells
• Homologous chromosomes separate
–
–
–
–
Prophase 1
Metaphase 1
Anaphase 1
Telophase 1 & Cytokinesis
Meiosis 2
• Each of the above cells divides into two cells
• End with total of 4 cells
• Sister chromatids separate
–
–
–
–
Prophase 2
Metaphase 2
Anaphase 2
Telophase 2 & Cytokinesis
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• Activity
8
Meiosis
Fig. 13.7
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Meiosis
G2 of Interphase
Same as mitosis
• Chromosomes duplicated
– Each chromosome is two identical sister chromatids
• Chromosomes still uncondensed
• Centrosome
– Centrosome replicates into 2
In this example:
Red = chromosome from mother
Blue = chromosome from father
Meiosis 1: Homologous Chromosomes
Separate
Prophase 1
• Chromosomes condense
• Breakdown of nuclear envelope
• Homologous chromosomes
attached in pairs
– Each chromosome is made up of 2
sister chromatids
– 4 chromatids = tetrad
• Crossing over occurs
– Exchange of genetic material between
nonsister chromatids
• Formation of mitotic spindle
• Microtubules attach to tetrads &
move them towards the center of
the cell
Fig. 13.8
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Meiosis 1: Homologous Chromosomes
Separate
Metaphase 1
• Mitotic spindle fully formed
• Tetrads lined up at equator
of cell (metaphase plate)
• Both chromatids of one
homolog are attached to
microtubules from one pole
Fig. 13.8
12
Meiosis 1: Homologous Chromosomes
Separate
Anaphase 1
• Pairs of homologous
chromosomes are split up
– Homologous chromosomes
drawn to opposite poles of cell
• Cohesins broken down along
chromatid arms
– Still doubled - remain as sister
chromatids
• Cohesins remains at centromere
• Cell elongates
• Read Fig. 13.10 Inquiry
Fig. 13.8
13
Meiosis 1: Homologous Chromosomes
Separate
Telophase 1
• Chromosomes at poles of cell
• Two daughter nuclei begin to form in
the cell
• Chromosomes become less
condensed (depending on species)
• Mitotic spindle goes away
Cytokinesis
• Two haploid cells produced
– Have only one member of each
homologous chromosome pair
– That chromosome is still duplicated
(sister chromatids)
Fig.13.8
Meiosis II: Sister Chromatids Separate
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Meiosis II is basically the same process as Mitosis,
EXCEPT
• Starts with the two haploid cells produced at the
end of Meiosis 1, and
• Ends with four cells, each haploid, each
genetically different
Fig. 13.8
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A Comparison of Mitosis & Meiosis
Fig. 13.9
A Comparison of Mitosis & Meiosis
Fig. 13.9
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Genetic Variation in Sexual Life Cycles
Meiosis
• Form of cell division that leads to non-identical
daughter cells with one-half the complement of
DNA
Three forms of variation
• Independent assortment of chromosomes
• Random Fertilization
• Crossing Over
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Genetic Variation in Sexual Life Cycles
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Independent assortment of chromosomes
• 2n possibilities
• 223 = ~ 8.4 million possibilities
Fig. 13.11
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Genetic Variation in Sexual Life Cycles
Random fertilization
• Sperm cell ~ 8.4 million combinations
• Egg cell ~ 8.4 million combinations
• ~70 trillion (223 X 223) possibilities!!!
Genetic Variation in Sexual Life Cycles
Crossing over
• The exchange of
corresponding segments
between two nonsister
chromatids
– Homologous chromosomes
carry different alleles
• Occurs during Prophase 1 of
Meiosis
• Synapsis
– Connection of homologous
chromosomes
• Synaptonemal complex
(proteins)
• Chiasma – site of crossing
over
– Connection remains after
synapsis ends
Fig. 13.12
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Genetic Variation in Sexual Life Cycles
Crossing over
• Recombinant
chromosomes
– Chromosomes that
carry genes derived
from two different
parents
• Affects multiple genes
• Can be multiple
crossovers
Fig. 13.12
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22
Sexual vs. Asexual Reproduction
Asexual reproduction
• Reproduction involving only one parent that
produces genetically identical (clone) offspring
• Process of mitosis
Sexual reproduction
• Fertilization of an egg by a sperm creating
offspring that are genetically different from the
parent
• Sperm & egg created by meiosis
Sexual vs. Asexual Reproduction
•
•
•
•
Benefits of sexual reproduction?
Costs of sexual reproduction?
Benefits of asexual reproduction?
Costs of asexual reproduction?
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Regulation of Cell Cycle
• Regulation of timing and rate of
cell cycle critical for normal
growth, development and
maintenance
• Cell cycle control system
– Cyclically operating set of
molecules in cell that triggers and
coordinates key events
• Highly conserved evolutionarily
in eukaryotes
– Same molecules found in many
species
• Read Inquiry Fig. 12.13
12.14
Regulation of Cell Cycle
• Checkpoints
– Regulatory point
– Stop or go-ahead signals
• May have built in “stop” that must be overridden
by “go-ahead” signal
• Cellular surveillance mechanisms
– Check that processes have been completed correctly
– Signal a go-ahead
• Some signals come from outside of cell
• Major checkpoints
– G1, G2, M
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Regulation of Cell Cycle
• G1 checkpoint – restriction point
• If go-ahead, then moves on to S phase
• If go-ahead not received, then switch to G0
phase
– Non-dividing state
• Most cells in G0 phase
– Nerve, muscle never divides
12.15
26
Regulation of Cell Cycle
• Control of cell cycle based on abundance of
regulatory molecules
• Concentration of molecules fluctuate cyclically
• Regulatory molecules
– Protein kinases
• Proteins that activate/inactivate other molecules through
phosphorylation
• Typically present in constant concentration in cells
– Inactive
– Cyclins
• Protein whose concentration fluctuates cyclically
• Attaches to kinases to activate them
– Cyclin-dependent kinases (Cdks)
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Internal Signals
• Control at the G2 checkpoint
• MPF (maturation-promoting factor)
– Initiates mitosis by phosphorylating many proteins
• Phosphorylates proteins in nuclear lamina
– promotes fragmentation of nuclear envelope
• Role in chromosome condensation
• Role in mitotic spindle formation
• Concentration of MPF tied to cyclin concentration
– Cyclin-CDK complex
• Read Fig. 12.6 Inquiry
and Interview on pgs.
92-93
Fig. 12.17
Internal Signals
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• Cyclin synthesis
begins in S phase
– Concentration levels
rise
• Cyclin and Cdk
molecules combine to
form MPF
• MPF promotes
mitosis
• Cyclin starts to
degrade
– Anaphase
• Cdk remains in cell
Fig. 12.17
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Internal Signals
• Control within Mitosis
– All chromosomes must have the mitotic
spindle properly attached to the kinetochore
before Anaphase will begin
– Go-ahead signal is non-cdk regulatory
proteins activated
• Cleavage of cohesins
• Sister chromatids can separate
Control by external factors
• Growth factors
– Protein released by some cells that stimulate other
cells to divide
– 50+ identified
– Growth factors act as go-ahead signals
• Density-dependent inhibition
• Crowded cells stop dividing
• Cells release inhibitors
• Anchorage dependence
– Cells must be attached to a substrate to divide
– Signal evolves communication between extra-cellular
matrix and cytoskeleton
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Cancer: Loss of Cell Cycle Controls
• Cancer
– Uncontrolled cell division
• Do not respond to density dependent inhibition
or anchorage dependence
• Do not respond to absence of growth factors
• Continue to divide indefinitely if supplied with
nutrients
– Normal cells die after 20-50 divisions
• If dividing stopped, it is at random location in
cycle, not at checkpoint
Cancer: Loss of Cell Cycle Controls
• Transformation
– Process that converts normal cell to cancer cell
– Alteration of genes influencing cell cycle regulation
• Benign tumor
– Abnormal cells remain at original site
• Malignant tumor
– Cells invade other tissue
• Metastasis
– Spread of cancer cells to locations distant from
original tumor
• Lymph or blood vessels
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Cancer: Loss of Cell Cycle Controls
• Chemotherapy
– Targets specific steps in cell cycle
– Damages any actively dividing cells
– E.g., Taxol
• Prevents microtubule depolymerization
• Microtubules cannot shorten, so cell stuck
in metaphase
• Radiation
– Damages cancer cells more than normal cells
– Reduced ability to repair from radiation
damage
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