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Cell Division
Chapter 11
AP
Division in Prokaryotes
• Binary Fission
– Lack a nucleus
– Circular DNA attached to
plasma membrane
– At replication site 22 proteins
begin replication
– When complete, daughter
DNAs attached to PM next to
each other
– Plasma membrane grows
between DNA until divided in
two
Eukaryotic Chromosomes
• Discovered during mitosis
• Varied number between organisms
– Primitive plant (fern) has 500 pairs, but advanced
flowering plant has 1 pair
• Humans have 23 almost identical pairs
– Loss of one chromosome = monosomy (usually
death)
– Gain of one chromosome = trisomy (sometimes
death or developmental problems)
Structure of Chromosomes
• Chromatin
– Complex of DNA and protein
• Chromosome
– Composed of chromatin
– Long unbroken strands of DNA
– Can contain 140 million nucleotides
– Super-super-super-coiled
Supercoiling
• “String-of-beads”
– Every 200 nucleotides is wrapped around 8 histone proteins =
nucleosome
• DNA attracted to histones by opposite charge (“+” histones to “-”
phosphates)
– Heterochromatin
• Highly condensed chromatin that
does not uncoil, thus is never
expressed
– Euchromatin
• Condensed only during cell
division, but is uncoiled when not
dividing so genes can be
expressed
Karyotypes
• Particular array of chromosomes for an individual
• Chromosomes differ from each other within the same
cell
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Size
Staining properties
Location of centromere
Length of arms on either side of
centromere
• To view karyotype
– Induce cell division, stop cell division,
lyse cells, stain chromosomes, take
picture, cut out, then order largest to
smallest
Phases of Cell Cycle
• Phases
– Interphase
• G1 phase: primary growth
– G0 phase: resting phase
• S phase: synthesis of entire
genome
• G2 phase: prep for cell division
– (M) Mitosis
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Prophase
Metaphase
Anaphase
Telophase
– (C) Cytokinesis—division of
cytoplasm
Duration of Cell Cycle
• Cell cycle lengths
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Most embryos = 20 minutes
Fruit fly embryo = 8 minutes
Dividing mammalian cell = 24 hours
Human liver = 1 year
– M phase of cell cycle only about an hour for regular
cells
– Some cells enter G0 phase for days to years before
entering cellular division (some stay there
indefinitely)
Interphase
• G1
– Major cell growth
– Occurs directly after cell division, so cell must get bigger
and mature before dividing again
– Protein synthesis
• S phase
– Each chromosome is replicated producing two sister
chromatids attached by a centromere
• G2
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Second growth
Mitochondria and other organelles are replicated
Chromosomes are condensed (supercoiled)
Centrioles replicate (animal cells)
Mitosis
• Prophase
– Formation of mitotic apparatus
• Begins as chromosomes become visible w/ light microscope
• Condensation continues
– Assembling spindle apparatus
• Begins at end of G2 phase and continues into prophase
• Centrioles begin to move apart forming an axis of microtubules
called spindle fibers
– Bridge of spindle fibers called spindle apparatus
– Also in plant cells, but no centrioles
• Nuclear envelope disappears
– Microtubules from opposite poles begin to connect to
kinetochores of sister chromatids
Mitosis (cont.)
• Metaphase
– Chromosomes line up in center of cell at
metaphase plate
– Positioned by microtubules attached at
kinetochores
• Anaphase
– All centromeres divide at same time
– Separates sister chromatids
– Pulled apart to different poles
– Poles themselves move apart
– Centromeres move toward the poles
Mitosis (cont.)
• Telophase
– Spindle apparatus disassembles
– Nuclear envelope reassembles around each set of
sister chromatids (chromosomes, now)
– Chromosomes begin to uncoil
– rRNA genes expressed and nucleolus will soon be
reformed
Cytokinesis
• Mitosis over
• Nuclei at opposite ends of cell
• Actual cell division not over until two new
daughter cells separate = cytokinesis
• Involves cleavage of cell into two equal halves
• In animal cells
– Belt of actin filaments constricts and pinches creating
a cleavage furrow. Constriction continues until cells
separate
Cytokinesis
• In plant cells
– Cell wall too rigid to be pinched
– Assemble membrane components
from within
– Called cell plate
– Cellulose then placed between two
new membranes making cell wall
• In fungi and protists
– When mitosis is complete, nucleus
divides into daughter nuclei
Cell Cycle Control
• Need sufficient time for events to occur
– Internal clock
– End of last phase starts next phase
– “go/no-go” switches
• Control system
– Growth is assessed at G1 checkpoint
• Key decision whether cell should divide or not
– If favorable, cell goes into S phase
– If not, cell will continue to grow
Cell Cycle Control (cont.)
– DNA replication success assessed at G2
checkpoint
• Problems with DNA synthesis are fixed
• If passes inspection cell will enter M phase
– Mitosis assessed at M checkpoint
• At metaphase checkpoint initiates exit from mitosis and
cytokinesis and beginning of G1
Mechanisms of Cell Control
• Cyclin control system
– Cyclin-dependent protein kinases (Cdks)
• Phosphorylate particular amino acids in proteins
important to cellular division
• This initiates the cell cycle past the checkpoint
– G2 checkpoint is better understood
• G2 cyclin gradually increases
• Cyclin binds to Cdk forming MPF (mitosis promoting
factor)
• As more MPF accumulates there is a positive feedback
that phosphorylates more MPF
• The MPF threshold is reached and triggers mitosis and
the end of G2
Mechanisms of Cell Control
– MPF also activates proteins that destroy the very cyclin
that started the whole process
– As cyclin becomes less available to make MPF it initiates
the end of mitosis
• G1 checkpoint
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Less understood than G2
Thought to be regulated like G2
Cell size triggers DNA replication
Determinant for S phase is ratio of amount of cytoplasm
to genome size
– This triggers production of more cyclins, then triggering S
and G2 phases
Cell Cycle in Eukaryotes
• To maintain organization, only certain cells can
divide at certain times
• Cells use regulatory signals called growth factors
• Growth factors fit cell surface receptor
– Some very specific, others not so specific
– Many cells need many different types of growth
factors in order to overcome the controls that inhibit
cell division
• If cells do not get growth factors, they stop after
G1 and go into G0 phase (non-growing stage)
Cancer
• Unrestrained, uncontrolled cell growth
• Read about the p53 gene
• Proto-oncogenes (must be turned on to cause cancer)
– Genes that stimulate cell division
– If damaged they lead to oncogenes (cancer)
– myc, fos, jun
• myc expression prevents cell division even in presence of growth factors
• Tumor-suppressor genes (must be turned off or mutated to
cause cancer—recessive: both copies must be bad)
– Suppress cell division by preventing cyclins from binding to Cdk
– If mutated, can lead to uncontrolled cell division, but is recessive
– Read about retinoblastoma gene