Cell Reproduction:
Mitosis and Meiosis
Big Idea # 3
Genetics and Information Transfer
Ecosystem
Population
Organism
Tissue
Cell
How does the process
effect each level of
organization?
Terms
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Anaphase
Binary fission
Cancer
Cell cycle
Cell plate
Checkpoint
Chromatin
Chromosome
Crossing over
• Cleavage furrow
• Cyclin
• Cyclin-dependent kinase
(Cdk)
• Cytokinesis
• Density dependent
inhibition
• Diploid
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G0
G1
G2
Gamete
Genome
Haploid
Homologous
chromosomes
• Interphase
• M phase
• Meiosis
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Metaphase
Mitosis
MPF
PDGF
Prophase
S phase
Sister chromatids
Somatic cell
Telophase
BIG Idea # 3 Genetics
and Information Transfer
3.A.2 – The Cell Cycle Is A complex
Set of Stages That Is Highly Regulated
With Checkpoints, Which Determines
the Ultimate Fate of the Cell
Cell Cycle
3.A.2 – The Cell Cycle
Is a Complex Set of Stages
• Mitosis passes a genome from the parent cells to
daughter cells
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Mitosis occurs after DNA replication
Produces two identical cells
Growth, repair, asexual reproduction
Continuous process with observable structures (order of
the processes – replication, alignment, separation)
• Mitosis alternates with interphase
Genome
• All the genes of an organism
• Genes – sections of DNA with code for making
proteins
– Humans - 25,000 genes*,
46 chromosomes
– Average gene - 3000 nucleotides
Cell Division
• Prokaryote – circular
DNA
– Bacteria divide by binary
fission
– No coiling of DNA
– No proteins to manage
the DNA
• Eukaryote = DNA
coils into multiple
chromosomes
– DNA ‘managed’ by
proteins
Binary Fission in Bacteria
• Plasma membrane grows
inward
• Mitochondria and
chloroplasts go through
similar processes
Eukaryotes - Chromatin
• DNA is making proteins most
of the time
• Chromatin - DNA + proteins
(histones)
• Before mitosis, chromatin is
replicated; chromatin
condenses, coils and folds into a
chromosome
Chromosomes
• Chromosome = two sister
chromatids connected by a
centromere
• Chromatids are pulled apart
into two new cells at the end of
mitosis/meiosis
Homologous Pairs
Chromosomes
• Each species has a characteristic number of
chromosomes
– Human somatic cells have 46 (diploid)
– Human gametes have 23 (haploid)
• Karyotype – arrangement of chromosomes
3.A.2 – The Cell Cycle
Is a Complex Set of Stages
• After specialization, a cell enters a non-dividing
state but may re-enter the cell cycle when given
appropriate cues
• Interphase - three
phases
– Growth
– Synthesis
– Preparation for
mitosis
Cell Cycle
• Interphase - cell growth
– Longest time of the cycle
– Three subphases:
• G1 (“first gap”) growth
• S (“synthesis”) DNA is
copied
• G2 (“second gap”) cell
completes preparations
for division
• G0 – some cells do not
re-enter ‘S’
3.A.2 – The Cell Cycle
Is a Complex Set of Stages
• The cell cycle is directed by internal control or
checkpoints internal and external signals provide
stop-and-go signs at the checkpoints.
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MPF
Platelet-derived growth factor (PDGF)
Cancer results from disruptions of the cycle controls
Cyclins and cyclin-dependent kinases control the cell
cycle
Regulation of the Cell Cycle
• Frequency of cell division depends on the type of
cell
– Skin cells - frequent
– Liver cells do not divide unless damaged (Go)
– Nerve and muscle cells do not divide after maturity
(permanent Go )
• Cell cycle is controlled by chemical signals:
– Evidence: fuse a cell in S phase with a cell in G1, the G1
cell will start S
– Evidence: fuse a cell in mitosis with one in interphase
causes the cell in interphase to start mitosis
Cell Cycle Control
• Checkpoints in cycle are control points
– Checks to be sure all ‘steps’ are completed
– Hormone signals from outside also help control the
cycle
• 3 checkpoints; G1, G2, and M
phases
Cell Signaling
Cell Cycle Control
• G1 checkpoint (restriction point) is most important
– Go signal = completes cell cycle and divides
– No Go = cell exits cycle
Cell Cycle Control
• Rhythmic fluctuations of proteins controls the cycle
– Kinases - activate or deactivate other proteins
• Constant amount
– Cyclins – levels fluctuate
• Kinases and cyclin form
cyclin-dependent kinases
(Cdks)
Cell Cycle Control
• Cyclin increases during interphase, then decreases
during mitosis
• MPF – composed of cyclin-Cdk
– “Maturation-Promoting Factor” triggers cell past G2
checkpoint to M phase
Internal and External Cues
• M phase checkpoint - ensures that the chromosomes
are attached to the spindle at metaphase plate before
anaphase begins so that daughter cells do not end up
with missing or extra chromosomes
– APF
• Mitosis is a continuous process:
– Mitosis is ‘usually’ broken into four subphases:
• Prophase
• Metaphase
• Anaphase
• Telophase
Prophase - Formation
• Chromatin coils up to form chromosomes
• G2 checkpoint
– MPF builds to a peak
Metaphase - Alignment
• Spindle fibers push the chromatids until they are all arranged
at the metaphase plate
• M – phase checkpoint
– APF builds up,
MPF degrades
Anaphase - Separation
• Centromeres divide
– Separates chromatids
• Each chromatid is pulled toward the pole by spindle
fibers
Telophase:
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Cell elongates
Two nuclei envelopes begin to reform
Chromatin uncoils
Cytokinesis begins
Cytokinesis
• Animal cells:
– Cleavage furrow - contractile ring of
actin and myosin forms
• Plants have cell walls
– Cell plate - vesicles from Golgi
coalesce at the metaphase plate
– Plate enlarges until fused with the
plasma membrane
Internal and External Cues
• Density-dependent inhibition
– Normal cultured cells divide until they form a
single layer
– Cells will grow to fill a gap
• Anchorage dependence –
cells must be anchored
(extracellular matrix)
Cancer Cells
• Do not respond to density-dependent inhibition or
anchorage dependence
• Do not stop dividing when growth factors run out
• May be ‘immortal’
– Normal cells - 20 to 50 times in vitro
– HeLa cells - Henrietta Lacks
Terms
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Allele
Chromosome
Crossing over
Fertilization
Gamete
Gametogenesis
Gene
Locus
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Meiosis I
Meiosis II
Sex chromosome
Sexual reproduction
Synapsis
Tetrad
Zygote
• Meiosis, a reduction division followed by
fertilization, ensures genetic diversity in sexually
reproducing organisms
– Ensures each gamete receives one complete haploid
(1n) set of chromosomes
– Homologous chromosomes are paired with one
homologue originating from the maternal parent and
one from the paternal parent. Orientation of the
chromosome pairs is random with respect to the cell
poles
– Separation of the homologous chromosomes ensures
that each gamete receives a haploid set of chromosomes
composed of both maternal and paternal chromosomes
– Homologous chromosomes may exchange genetic
material via ‘crossing over’ which increases genetic
variation
– Fertilization involves the fusion of two gametes
increasing genetic variation in populations by providing
for new combinations of genetic information in the
zygote and restores the diploid number of chromosomes
Asexual Reproduction
• Single parent
• No genetic variation
– Cloning, plant cuttings
– Binary fission in bacteria
– Spores – plants, fungi
• Very rapid, energy efficient method
• Few mutations - ‘bad’ or ‘weak’ genes also passed
along
• Genes susceptible to environmental change
Sexual Reproduction
• Sexual reproduction:
– Requires more energy
– Slower
– Gametogenesis - meiosis
• Genetic variation***
• Survival of the fittest
• Somatic cell - any cell other than gamete
• Gamete - sex cell, haploid
Homologous Chromosomes
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Homologues - same size, same genes
Same gene loci – location of gene on the chromosome
Autosome - non-sex chromosomes (22 pairs)
Sex chromosome – carries gene that determines
gender; dissimilar
Heredity: Meiosis
• Mitosis:
• Meiosis:
• 4 stages
• 2 identical daughter cells
• Diploid (2n)
• 8 stages
• 4 non-identical cells
• Haploid (1n)
Meiosis – Crossing Over
• Prophase I:
• Synapsis - homologous
chromosomes come together as
pairs
• Tetrads
• Crossing over may occur
Nondisjunction
• Chromatids fail to separate during meiosis
• Gametes are ANEUPLOID (have incorrect number of
chromosomes)