BIOL 200 (Section 921)
Lecture # 12; July 5, 2006
Unit 9: Cell Cycle, Cell division
Readings:
•
I. Cell cycle: ECB 2nd ed., Chapter 19, pp. 637-40. Overview of the cell
cycle; Chapter 18, pp. 611-25. Good questions: 18-2, 18-3, 18-4; 18-7a, d, e;
18-10; 18-15; 18-16.
ECB 1st ed., Chapter 17, pp. 547-550. [Skim pp. 551-560] Overview of the
cell cycle; Chapter 18, pp. 571-581.
•
II. DNA replication: ECB 2nd ed., Chapter 6, pp. 196-208 DNA Replication;
Questions # 6-9, 6-10, 6-11 (very good).
ECB 1st ed., Chapter 6, pp. 189-197 DNA Replication; Questions # 6-14, 615, 6-16 (very good)
•
III. Chromosomes and Mitosis: ECB 2nd ed., Chapter 19 pp 639-55 for
Mitosis. Good questions: 19-3, 19-4; 19-5; 19-8; 19-10; 19-12; 19-14.
ECB 1st ed., Chapter 8, p 249-50 Telomeres: Specialized structures ensure
that chromosomes replicate efficiently; Chapter 17 pp 551-562 for Mitosis
Learning objectives
I. Introduction and Overview of Cell Cycle; Regulation of cell cycle by CDK-cyclin
• Understand overall organization of the cell cycle and its relation to cell division (mitosis and
cytokinesis):
• Be able to list cell cycle stages in order and indicate the location of the two major cell cycle control
points.
• Give examples of discrete and continuous cell cycle processes.
• Understand the checkpoint concept and how this allows Integration of continuous and discrete cell cycle
processes
• Understand the role of CDK-cyclin in regulation of cell cycle processes.
• Be able to explain how the CDK complex is regulated by protein phosphorylation, and by proteolysis.
II. DNA Replication
• Explain what is meant by semi-conservative replication of DNA.
• Explain how DNA replication occurs in eukaryotes through the independent operation of many replicons.
• Describe the roles of the following in DNA replication: Helicase, DNA polymerase, primer, Okazaki
fragments, ligase.
• Explain the terms lagging and leading strand and the significance of these terms for DNA replication.
• Understand the relation between chromatids and chromosomes in both pre- and post-replication stages
• Explain and understand replicon and replication fork.
III. Chromosomes and Mitosis
• Understand why telomeres are important and how telomere extension occurs
• List the stages of mitosis in order and explain what is happening to the cytoskeleton, the mitotic spindle,
the nuclear envelope and the chromosomes during this process.
• Explain how chromosomes are moved to the poles at mitosis.
• What are kinetochores and what are they good for.
Cell cycle
• Cells arise from pre-existing cells
• The reproduction of cells takes place in an orderly
fashion and is genetically regulated
• Cells reproduce by a process called the cell cycle
which show (i) cell growth and chromosome
replication, (ii) chromosome segregation and (iii)
cell division
• Each cell type has its own timing and built-in
memory
• For example, cell cycle time varies from about 0.5
hours in early frog embryo cells to about 1 year in
human liver cells
Four phases of the cell cycle [Fig. 18-2]
• M phase includes
nuclear division and
cytokinesis
• G1 – the interval
between M and S
phases
• S phase – replication
• G2 – the interval
between S phase and
mitosis
A central control system (like a washing machine) triggers
the major processes of the cell cycle [Fig. 18-3]
• Like a clotheswashing machine,
there is a feedback at
each stage
• The next stage can
not begin until the
previous one has
ended
• This is a system of
checkpoints
Two major checkpoints in the cell cycle
• There is a checkpoint
in G1 for cell size. If
the cell has not grown
sufficiently, it will be
held in G1 until that
happens
• Similarly, there is a
checkpoint in G2 that
ensures replication is
complete before
mitosis begins
• Checkpoints also take
into account signals
from other cells and
environment.
‘COMMITMENT TO DIVN.’
‘START’
Xenopus egg, does not divide
unless activated [Fig. 18-8]
microinjection of cytoplasm extract
into egg activates mitosis [Fig. 18-9]
Inject cytoplasm
from m phase cell
Promotes
mitosis
Inject cytoplasm
From interphase cell
No mitosis, stays
in interphase
CONCLUSION: Some kind of cytoplasmic factor
promotes cell division – called m phase promoting
factor (MPF)
Cyclin-CDK complex [Fig. 18-5]
Cyclin-dependent
kinase (Cdk)
Cyclinamount
varies during
cell cycle
The major Cyclins and Cdks
[Table 18-2]
-------------------------------------------------------Cyclin-Cdk
Cyclin
Cdk partner
Complex
--------------------------------------G1-Cdk
cyclin D
Cdk4, Cdk6
G1/S-Cdk
cyclin E
Cdk2
S-Cdk
cyclin A
Cdk2
M-Cdk
cyclin B
Cdk1
--------------------------------------------------------
Changes in [M-cyclin] and M-Cdk activity
during the cell cycle [Fig. 18-6]
Factors affecting the activity of
Cdks
1.
2.
3.
4.
Cyclin degradation by ubiquitination
Phosphorylation and dephosphorylation
Positive feedback
Cdk inhibitor proteins
Cyclin degradation by ubiquitination inhibits the
activity of Cdks [Fig. 18-7]
18_07_cyclin_degradat.jpg
Phosphorylation and Dephosphorylation
• Cell cycle control is regulated by phosphorylation
and dephosphorylation
• Protein kinases and phosphatases form a switch
• Protein kinases that control cell cycle are present
throughout the cell cycle. Their activity rises and
falls.
• A second set of proteins CYCLINS have no
enzyme activity themselves. They bind to kinases
and activate them.
• The kinases of the cell cycle control system are
known as cyclin-depedent protein kinases or Cdks.
Protein phosphorylation [Fig. 4-41]
kinase adds
Phosphatase removes
phosphate group
Protein phosphorylation acts as a switch to
modify protein activity [Fig. 4-41]
Selective phosphorylation and dephosphorylation
activate M-Cdk [Fig. 18-11]
18_11_M_Cdk_active.jpg
Thr 14,
Tyr 15
Thr 161
Fig. 18-12: CDK phosphorylates activating phosphatase: further
activation of CDK via a positive feedback loop.
Different cyclin-Cdk
complexes trigger
different events in
cell cycle [Fig. 18-13]
18_13_Cdks_cyclins.jpg
M-Cdk effects
1. M-Cdk contains a single protein kinase, which
triggers mitosis
2. It phosphorylates MAPs and causes cytoskeleton
to reorganize – forming the spindle
3. It posphorylates the nuclear lamina and causes
the nuclear envelope to break down.
4. It phosphorylates non-histone proteins and
causes chromosome condensation
DNA damage arrests the
Cell cycle in G1 [Fig. 18-15]
18_15_cell_cycle_G1.jpg
The cell-cycle control system can arrest the cycle
at
various checkpoints [Fig. 18-17]
18_17_arrest_checkpt.jpg
Cell below
critical size
Cell below
critical size
DNA replication
Learning objectives
• Explain what is meant by semi-conservative replication of
DNA.
• Explain how DNA replication occurs in eukaryotes
through the independent operation of many replicons.
• Describe the roles of the following in DNA replication:
Helicase, DNA polymerase, primer, Okazaki fragments,
ligase.
• Explain the terms lagging and leading strand and the
significance of these terms for DNA replication.
• Understand the relation between chromatids and
chromosomes in both pre- and post-replication stages
• Explain and understand replicon and replication fork
Semiconservative replication of DNA: Each
daughter strand has one DNA template from
parental strand [Fig. 6-3]
“S” PHASE
Replication initiator proteins open up a DNA double
helix at its replication origin [Fig. 6-5]
06_05_replic.origin.jpg
06_09_Replic.forks.jpg
Replication fork moves away in both directions
06_10_5prime_3prime.jpg
DNA synthesis in the 5’→3” direction
The two newly synthesized DNA strands have opposite polarity
06_11_oppositepolarity.jpg
DNA replication forks are asymmetrical [Fig. 6-12]
06_12_asymmetrical.jpg
DNA POLYMERASE CATALYZE BOTH DNA REPLICATION
AND EDITING (PROOFREADING) [Fig. 6-14]
06_14_polymerase2.jpg
On the lagging strand, DNA is synthesized in fragments [Fig. 6-16]
06_16_lagging strand.jpg
06_17_group proteins.jpg
A group of proteins
act together at a
replication fork
A protein machine in DNA replication
Key Players
•
•
•
•
•
Leading strand DNA
Lagging strand DNA
DNA Polymerase III
Helicase
RNA Primase and
RNA Primers
• Okazaki Fragments
• Ligase
• Single-stranded DNA
binding proteins
Becker et al. The World of the Cell]
Chromosomes and Mitosis:
Learning objectives
• Understand why telomeres are important and how
telomere extension occurs
• List the stages of mitosis in order and explain
what is happening to the cytoskeleton, the mitotic
spindle, the nuclear envelope and the
chromosomes during this process.
• Explain how chromosomes are moved to the poles
at mitosis.
• What are kinetochores and what are they good for.
3 DNA sequences needed to produce a
eukaryotic chromosome [Fig. 5-18]
1. telomere
2. origin of
replication
3. centromere
Telomeres allow the completion of DNA synthesis at the ends of
Chromosomes [Fig. 6-18]
06_18_telomeres.jpg
Telomerase contains a
Short piece of RNA which
acts as a primer for DNA
synthesis
The cytoskeleton is involved in both mitosis and cytokinesis
19_04_mediate_M.jpg
Mitosis separates sister chromatids
into daughter chromosomes [Fig. 19-6]
Panel 19-1: prophase
Mitotic chromosome [Fig. 19-3]
2 chromatids
centromere
What keeps the 2 chromatids together?
Cohesins hold sister chromatids together.
Condensins help pack chromatin into chromosomes
[Fig. 19-3]
Where would you expect to
find cohesins in this
centrosome
Fig. 19-5:
centrosomes
duplicate to form
mitotic spindle poles.
duplication
Mitotic spindle poles
Fig. 19-7:
Mitotic spindle
Selective
stabilization of
interpolar MT
prophase
Panel 19-1: prometaphase
Dynamic
instabilityMT’s
shoot out
of spindle
poles.
Fig. 17-9: Kinetochore is protein complex
at centromere region of chromosome
MT bind here
Panel 19-1:
Metaphase
Fig. 19-13: Mitotic spindle at metaphase
Kinetochore
microtubules
Interpolar
microtubules
Panel 19-1: Anaphase
Fig. 19-17: Anaphase A-sister
chromosomes separate by kinetochore MT
shortening
Fig. 19-17: Anaphase B-poles move apart by
polar MTs sliding past each other
motors slide
polar MTs
The anaphase promoting
factor (APC) triggers the
separation of sister
chromatids by promoting
the destruction of cohesins
[Fig. 19-16]
19_16_APC_triggers.jpg
Fig. 19-19: cleavage furrow made by actinmyosin ring pulling PM in to divide
daughter cells
Cytokinesis-division of cytoplasm due to actin-myosin contractile ring
19_20_contractile_ring.jpg
Fig. 18-38: Golgi derived vesicles make
a new cell wall in plants
mitotic spindle
Golgi
Cell plate
Decondensing
chromosomes
of daughter
nuclei
Fig. 19-22: cell plate forms during
cytokinesis in plants