BIOL 200 (Section 921)Lecture # 12; July 5, 2006

Unit 9: Cell Cycle, Cell divisionReadings:

• 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, 6-15, 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 objectivesI. 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 clothes-washing 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.

‘START’

‘COMMITMENT TO DIVN.’

Xenopus egg, does not divide unless activated [Fig. 18-8]

microinjection of cytoplasm extract into egg activates mitosis [Fig. 18-9]

CONCLUSION: Some kind of cytoplasmic factor promotes cell division – called m phase promoting factor (MPF)

Inject cytoplasmfrom m phase cell

Promotes mitosis

Inject cytoplasmFrom interphase cell

No mitosis, staysin interphase

Cyclin-CDK complex [Fig. 18-5]

Cyclin-amount varies

during cell cycle

Cyclin-dependent kinase (Cdk)

The major Cyclins and Cdks [Table 18-2]

--------------------------------------------------------Cyclin-Cdk Cyclin Cdk partnerComplex--------------- ----------- ---------------G1-Cdk cyclin D Cdk4,

Cdk6G1/S-Cdk cyclin E Cdk2S-Cdk cyclin A Cdk2M-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. Cyclin degradation by ubiquitination

2. Phosphorylation and dephosphorylation

3. Positive feedback

4. Cdk inhibitor proteins

18_07_cyclin_degradat.jpgCyclin degradation by ubiquitination inhibits the

activity of Cdks [Fig. 18-7]

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]

18_11_M_Cdk_active.jpgSelective phosphorylation and dephosphorylation

activate M-Cdk [Fig. 18-11]

Thr 14, Tyr 15

Thr 161

Fig. 18-12: CDK phosphorylates activating phosphatase: further activation of CDK via a positive feedback loop.

18_13_Cdks_cyclins.jpg

Different cyclin-Cdkcomplexes triggerdifferent events incell cycle [Fig. 18-13]

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

18_15_cell_cycle_G1.jpgDNA damage arrests the

Cell cycle in G1 [Fig. 18-15]

18_17_arrest_checkpt.jpgThe cell-cycle control system can arrest the cycle

at various checkpoints [Fig. 18-17]

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

06_05_replic.origin.jpg

Replication initiator proteins open up a DNA doublehelix at its replication origin [Fig. 6-5]

06_09_Replic.forks.jpg

Replication fork moves away in both directions

06_10_5prime_3prime.jpg

DNA synthesis in the 5’→3” direction

06_11_oppositepolarity.jpgThe two newly synthesized DNA strands have opposite polarity

06_12_asymmetrical.jpg

DNA replication forks are asymmetrical [Fig. 6-12]

06_14_polymerase2.jpg

DNA POLYMERASE CATALYZE BOTH DNA REPLICATION AND EDITING (PROOFREADING) [Fig. 6-14]

06_16_lagging strand.jpg

On the lagging strand, DNA is synthesized in fragments [Fig. 6-16]

06_17_group proteins.jpg

A group of proteinsact together at areplication 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

06_18_telomeres.jpg

Telomeres allow the completion of DNA synthesis at the ends ofChromosomes [Fig. 6-18]

Telomerase contains aShort piece of RNA whichacts as a primer for DNA synthesis

19_04_mediate_M.jpgThe cytoskeleton is involved in both mitosis and cytokinesis

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

picture?

Fig. 19-5: centrosomes duplicate to form mitotic spindle poles.

centrosome

duplication

Mitotic spindle poles

Fig. 19-7:Mitotic spindle

Selective stabilization of interpolar MT

prophase

Panel 19-1: prometaphase

Dynamic instability-MT’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

19_16_APC_triggers.jpg

The anaphase promotingfactor (APC) triggers theseparation of sisterchromatids by promotingthe destruction of cohesins[Fig. 19-16]

Fig. 19-19: cleavage furrow made by actin-myosin ring pulling PM in to divide

daughter cells

19_20_contractile_ring.jpg

Cytokinesis-division of cytoplasm due to actin-myosin contractile ring

Fig. 18-38: Golgi derived vesicles make a new cell wall in plants

mitotic spindleGolgi

Cell plate

Decondensing chromosomes of daughter nuclei

Fig. 19-22: cell plate forms during cytokinesis in plants