Cell Division
Part 1
A Generalized Cell
Golgi
body
Nuclear
envelope
Nucleolus
Chromosomal
DNA
Nucleus
Polyribosomes
Ribosome
Rough ER
Cytoplasm
Membrane protein
Plasma membrane
Smooth ER
Lysosome
Microfilaments
Centrioles
Mitochondrion
Microtubules
(b) Animal cell
The Cell Cycle
DNA Synthesis
Interphase
S
Synthesis
G1
Gap 1
M
Mitosis
G2
Gap 2
Growth
Gene expression
Differentiation
Two
daughter
cells
Gene expression
Quality control
Actual division process
Three Little Words Geneticist Need to
Hear… Homolog, Loci, Allele
Gene loci (location)
Unreplicated chromosome pair
Homologous
pair of
chromosomes
Genotype:
A
b
c
A
B
c
AA
Bb
cc
Homozygous Heterozygous Homozygous
for the
for the
dominant
recessive
allele
allele
Replicated Chromosome
Pair of sister chromatids
Centromere
(DNA that is
hidden beneath
the kinetochore
proteins)
One
chromatid
(dark blue)
(a)
(b)
Kinetochore
proteins
One
chromatid
(light blue)
Chromatids, Chromosomes… What the…
• At the end of S phase, a cell has twice as many
chromatids as there were chromosomes in G1
phase
– i.e. - human cell
• 46 chromosomes in G1 phase
• 46 pairs of sister chromatids in G2 phase
• chromosome is therefore a relative term
– In G1, anaphase, & telophase it refers to the
equivalent of one chromatid
– In G2, prophase, & metaphase, it refers to a pair of
sister chromatids
Interphase
Two centrosomes,
each with centriole pairs
Nuclear
membrane
• Chromosomes are
decondensed
• chromosomes
replicate
• The centrosome
divides
Chromosomes
Prophase
Microtubules
forming mitotic spindle
• Nuclear envelope
dissociates
• Centrosomes move
to opposite poles
• mitotic spindle
apparatus forms
Centromere
Sister
chromatids
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Astral
microtubule
Metaphase
plate
Polar
microtubule
Kinetochore
proteins attached
to centromere
Kinetochore
microtubule
(d) METAPHASE
Spindle Apparatus
• Composed of microtubules originated from centrioles
• Microtubules are formed polymerization of tubulin
proteins
• 3 types of spindle microtubules
– Aster microtubules
• Important for positioning of the spindle apparatus
– Polar microtubules
• Help to “push” the poles away from each other
– Kinetochore microtubules
• Attach to kinetochore , at the centromere
Kinetochore Spindle
Fibers
Figure 3.8
Prometaphase
Nuclear membrane
fragmenting
• Spindle fibers bind
kinetochores
• The two kinetochores
on a pair of sister
chromatids are
attached to
kinetochore MTs from
opposite poles
Spindle pole
Mitotic
spindle
Metaphase
Astral
microtubule
• Pairs of sister
chromatids align
themselves at the
metaphase plate
Metaphase
plate
Kinetochore
proteins attached
to centromere
Polar
microtubule
Kinetochore
microtubule
Anaphase
Chromosomes
• Centromeres separate
• Each chromatid, is
linked to only one pole
• As anaphase proceeds
– Kinetochore MTs shorten
• Chromosomes move to
opposite poles
– Polar MTs lengthen
• Poles themselves move
further away from each
other
Telophase & Cytokinesis
• Chromosomes reach poles
& decondense
• Nuclear membrane reforms
• Quickly followed by
cytokinesis
– In animals
• Formation of a cleavage furrow
– In plants
• Formation of a cell plate
Some Key Points
• Mitosis ultimately produces two daughter
cells genetically identical to the mother cell
– Barring rare mutations
• Processes requireing mitotic cell division
– Development of multicellularity
– Organismal growth
– Wound repair
– Tissue regeneration