Bio 1a03: Theme4&5
Ruthy Lim
Theme 4
Module 1 - DNA Replication and Mitosis
Cell proliferation in prokaryotes:
● Binary fission - asexual reproduction
○ DNA attaches to inside of plasma membrane by proteins → DNA replicate at
replication region → cells elongate → elongates until 2 DNA attachment sites are
at opposite ends of elongated cell
○ newly made DNA anchored to plasma membrane
○ When replication complete, bacterium is ~2x its size → cell constricts along
midpoint → cell divides
Eukaryotic Cell Cycle:
Eukaryotic DNA - larger, organized into linear chromosomes,
condensed in nucleus
E.g. for muscle regeneration, quiescent cells in basement membrane
(from dormant G0 phase) activate and divide → myoblasts →
myofibers
● Exit cell cycle and enter quiescent G0 phase (cells mature)
Mitosis
Walter Flemming (1882) - stages of mitosis based on chromosomal position and features
● Analyzed developing salamander stained embryos - visualize dividing cells
Interphase
Chromosomes are long thin chromatin fibers - can’t be identified due to structure
S phase
DNA replicated & centrosome fully replicated
Centrosomes - organizing centres, radiate long microtubules that will form mitotic spindle
,Bio 1a03: Theme4&5
Ruthy Lim
● Begin at opposite poles
Prophase -
● In nucleus: chromatin fibers contract and DNA condenses
(tightly coiled) → chromosome visible and has two identical
sister chromatids at centromere
● In cytoplasm: mitotic spindle forms, microtubules form in
centrosome , centrioles start to go to opposite poles
Prometaphase -
● Spindle fibers attach to sister chromatids at kinetochore regions - chromosomes not
aligned
Metaphase -
● Chromosomes line up in center of cell (metaphase plate)
Anaphase -
● Binding proteins between sister chromatids break → centromeres of sister chromatids
split (disjunctional segregation) → single chromosomes go to opposite poles
● Chromosomes move through centromere first (v-shaped)
Telophase -
● Nonkinetochore microtubules elongate cell, two daughter nuclei form, nuclear envelope
around each set of chromosomes form
● Chromosomes decondense
Cytokinesis -
● Animals - contractile ring (made of motor proteins) form → contract actin fibers along
midline → cleavage furrow forms → cell separates
● Plants - cell plate in middle → creates cell wall → divides
Controlling Progression
Tim Hunt (1970s) - measure protein level changes of dividing sea urchin embryos
● Procedure: added radioactive labelled methionine to sea urchin eggs, took samples of
dividing embryos every 10 mins with gel electrophoresis
● Results: protein bands on gel were darker as cell division progressed but one oscillated in
intensity
● Cyclin - Regulates cyclin dependent kinase (CDK) activity
○ proteins that oscillate in concentration during cell cycle
● Cyclin dependent kinases (CDK) - activate or inactivate proteins by phosphorylating
amino acids on proteins
○ ATP dependent
○ Attach to cyclin to activate
, Bio 1a03: Theme4&5
Ruthy Lim
● Together they control progression of cell cycle by making changes to events
● cyclin-CDK regulation:
1. G1/S cyclin - CDK complex: prepare for DNA replication, needed for transition
from G1 → S phase
2. S cyclin - CDK: initiate DNA synthesis
3. M cyclin - CDK: prepare for mitosis
Checkpoints
● “Cellular surveillance”, can block cyclin-CDK activity or pause cell division until
preparations are done
1. DNA damage checkpoint (end of G1) - only undamaged DNA will enter S phase
● Cell size, nutrients adequate, social signals present
2. DNA replication checkpoint (end of G2) - only when all DNA replicated will it enter
mitosis
● DNA undamaged, activated MPF present
● MPF - cyclin coupled w/ CDK
3. Spindle assembly checkpoint (before anaphase) - cell only complete mitosis if
chromosome attached to microtubule
Example:
● Genes that inhibit cell cycle OFF
○ P53 in nucleus is not phosphorylated and quickly exits nucleus to degrade
○ DNA damage activates protein kinases that phosphorylate p53 → block export
and degradation process
● As p53 levels in nucleus ↑, p53 acts as transcription factor → turn genes ON - inhibit cell
cycle (block G1/S cyclin-CDK complex activity)
● Inhibiting cycle gives time to repair damaged DNA
Spindle assembly checkpoint
As early as prometaphase, there are regulatory proteins to monitor degree to which sister
chromatids are attached to microtubules at kinetochore regions
● Unattached kinetochores → "wait" signal → spindle assembly checkpoint proteins
recruited
● Proteins are activated by lack of tension in centromere
● proteins removed when attached by a separate enzyme that breaks sister chromatid
attachments
Ruthy Lim
Theme 4
Module 1 - DNA Replication and Mitosis
Cell proliferation in prokaryotes:
● Binary fission - asexual reproduction
○ DNA attaches to inside of plasma membrane by proteins → DNA replicate at
replication region → cells elongate → elongates until 2 DNA attachment sites are
at opposite ends of elongated cell
○ newly made DNA anchored to plasma membrane
○ When replication complete, bacterium is ~2x its size → cell constricts along
midpoint → cell divides
Eukaryotic Cell Cycle:
Eukaryotic DNA - larger, organized into linear chromosomes,
condensed in nucleus
E.g. for muscle regeneration, quiescent cells in basement membrane
(from dormant G0 phase) activate and divide → myoblasts →
myofibers
● Exit cell cycle and enter quiescent G0 phase (cells mature)
Mitosis
Walter Flemming (1882) - stages of mitosis based on chromosomal position and features
● Analyzed developing salamander stained embryos - visualize dividing cells
Interphase
Chromosomes are long thin chromatin fibers - can’t be identified due to structure
S phase
DNA replicated & centrosome fully replicated
Centrosomes - organizing centres, radiate long microtubules that will form mitotic spindle
,Bio 1a03: Theme4&5
Ruthy Lim
● Begin at opposite poles
Prophase -
● In nucleus: chromatin fibers contract and DNA condenses
(tightly coiled) → chromosome visible and has two identical
sister chromatids at centromere
● In cytoplasm: mitotic spindle forms, microtubules form in
centrosome , centrioles start to go to opposite poles
Prometaphase -
● Spindle fibers attach to sister chromatids at kinetochore regions - chromosomes not
aligned
Metaphase -
● Chromosomes line up in center of cell (metaphase plate)
Anaphase -
● Binding proteins between sister chromatids break → centromeres of sister chromatids
split (disjunctional segregation) → single chromosomes go to opposite poles
● Chromosomes move through centromere first (v-shaped)
Telophase -
● Nonkinetochore microtubules elongate cell, two daughter nuclei form, nuclear envelope
around each set of chromosomes form
● Chromosomes decondense
Cytokinesis -
● Animals - contractile ring (made of motor proteins) form → contract actin fibers along
midline → cleavage furrow forms → cell separates
● Plants - cell plate in middle → creates cell wall → divides
Controlling Progression
Tim Hunt (1970s) - measure protein level changes of dividing sea urchin embryos
● Procedure: added radioactive labelled methionine to sea urchin eggs, took samples of
dividing embryos every 10 mins with gel electrophoresis
● Results: protein bands on gel were darker as cell division progressed but one oscillated in
intensity
● Cyclin - Regulates cyclin dependent kinase (CDK) activity
○ proteins that oscillate in concentration during cell cycle
● Cyclin dependent kinases (CDK) - activate or inactivate proteins by phosphorylating
amino acids on proteins
○ ATP dependent
○ Attach to cyclin to activate
, Bio 1a03: Theme4&5
Ruthy Lim
● Together they control progression of cell cycle by making changes to events
● cyclin-CDK regulation:
1. G1/S cyclin - CDK complex: prepare for DNA replication, needed for transition
from G1 → S phase
2. S cyclin - CDK: initiate DNA synthesis
3. M cyclin - CDK: prepare for mitosis
Checkpoints
● “Cellular surveillance”, can block cyclin-CDK activity or pause cell division until
preparations are done
1. DNA damage checkpoint (end of G1) - only undamaged DNA will enter S phase
● Cell size, nutrients adequate, social signals present
2. DNA replication checkpoint (end of G2) - only when all DNA replicated will it enter
mitosis
● DNA undamaged, activated MPF present
● MPF - cyclin coupled w/ CDK
3. Spindle assembly checkpoint (before anaphase) - cell only complete mitosis if
chromosome attached to microtubule
Example:
● Genes that inhibit cell cycle OFF
○ P53 in nucleus is not phosphorylated and quickly exits nucleus to degrade
○ DNA damage activates protein kinases that phosphorylate p53 → block export
and degradation process
● As p53 levels in nucleus ↑, p53 acts as transcription factor → turn genes ON - inhibit cell
cycle (block G1/S cyclin-CDK complex activity)
● Inhibiting cycle gives time to repair damaged DNA
Spindle assembly checkpoint
As early as prometaphase, there are regulatory proteins to monitor degree to which sister
chromatids are attached to microtubules at kinetochore regions
● Unattached kinetochores → "wait" signal → spindle assembly checkpoint proteins
recruited
● Proteins are activated by lack of tension in centromere
● proteins removed when attached by a separate enzyme that breaks sister chromatid
attachments
