Ch 12 The Cell Cycle

What is the cell cycle in eukaryotes?

Just as organisms have a life cycle(birth-->maturation-->reproduction->-death)

Each cell has an ordered sequence of activities.

Draw cell cycle on board.

Most cells spend 90 % of life span in interphase: growth & synthesis

Remaining ~10% in M phase: mitosis and cell division (aka cytokinesis)

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G1

G2

S(DNA synthesis)

INTERPHASE

Cytokin

esis

MITOTIC(M) PHASE

Mito

sis

Summary of Cell Cycle Phases

– Mitotic (M) phase: mitosis and cytokinesis

– Interphase• G1 phase (“1st gap”):mRNA & protein synthesis

• S phase: DNA synthesis aka DNA replication, DNA

doubling

• G2 phase (“2nd gap”):mRNA & protein synthesis

Regulation of cell cycle

Two families of proteins control when the cell moves to different phases

1. Cyclins: regulatory subunits

2. Cyclin-dependent kinases (Cdks): catalytic subunits What is a kinase?

One subunit of each complex together in a heterodimer

Example: Cdk1/ cyclin B aka M-phase promoting factor (MPF)

Cdk: cyclin-dependent kinase

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Degradedcyclin

G2

checkpoint

S

M

G 2G 1

Cdk

Cyclin isdegraded

MPFCyclin

Cdk

Molecular mechanisms that help regulate the cell cycle

accum

ulatio

nC

yclin

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MPF activity

G1 G2S MS MG2G1

M

Cyclin

Time

Fluctuation of MPF activity and cyclin concentrationduring the cell cycle

Rel

ativ

e co

nce

ntr

atio

n

Based on the data what correlates with the cell

Entering M- phase?

Exiting M-phase and entering interphase?

Cyclin is synthesized and complexed to Cdk1 to form active MPF --> M phase induced.

Exit from M-phase correlated to cyclin degradation andMPF inhibition.

• For unicellular organisms

- Division of one cell reproduces the entire organism

Focus on M phase and cell divisionPurpose of cell division

• For multicellular organisms - Embryonic and fetal growth - Growth

- Repair

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Reproduction

100 µm

Tissue renewalGrowth and development

20 µm200 µm

• In preparation• During S phase

cell duplicates genetic material (DNA)

M-phase• One DNA copy each moves to opposite ends of the cell

• Mother cell undergoes cytokinesis --> two genetically identical daughter cells.

Focus on M-phaseMitosis Cytokinesis

Cellular Organization of the Genetic Material

Genome– A cell’s endowment of DNA

All its genetic information

Chromosomes DNA packaged with associated proteins

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25 µm

Orange: DNA

Yellow/green:MT

Staining of all the chromosomes or the genome in a cell

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Chromosomeduplication(DNA synthesis)

0.5 µm

Centromere

Sisterchromatids

Separationof sister

chromatids

Centromeres Sister chromatids

Single chromosome

• All eukaryotic species– Characteristic number of chromosomes located

in nucleus

• Somatic cells- Nonreproductive e.g. muscle, epidermal cells- Two sets of chromosomes (diploid)- Undergo mitosis

• Germ cells or gametes– Reproductive e.g. sperm and eggs–Half as many chromosomes as somatic cells (haploid)–Undergo meiosis

• Mitosis is conventionally divided into five phases:– Prophase

– Prometaphase

– Metaphase

– Anaphase

– Telophase

• Cytokinesis is well underway by late telophase

[Animations and videos listed on slide following figure]

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G2 OF INTERPHASE PROPHASE PROMETAPHASE

Blue: ChromosomesYellow/green: MT

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METAPHASE ANAPHASE TELOPHASE AND CYTOKINESIS

10

µm

Video: Animal Mitosis

Video: Sea Urchin (time lapse)

Animation: Mitosis (All Phases)

Animation: Mitosis Overview

Animation: Late Interphase

Animation: Prophase

Animation: Prometaphase

Animation: Metaphase

Animation: Anaphase

Animation: Telophase

The Mitotic Spindle: A Closer Look

• Consists of MT– control chromosome movement during mitosis

• Assembly of spindle MT– at centrosome, the microtubule organizing

center (MTOC)

• Centrosome replicates (late G2)– migrate to opposite ends poles, as spindle

microtubules grow out from them

• Aster (a radial array of short microtubules) extends from each centrosome

• Spindle• includes the centrosomes, spindle MT, asters

• Some MT• attach to the kinetochores of chromosomes and

move chromosomes to metaphase plate

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Microtubules Chromosomes

Sisterchromatids

AsterCentrosome

Metaphaseplate

Kineto-chores

Kinetochoremicrotubules

0.5 µm

Overlappingnonkinetochoremicrotubules

1 µmCentrosome

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Chromosomemovement

Microtubule Motorprotein

Chromosome

Kinetochore

Tubulinsubunits

Movement of xhromosomes toward poles

MT shortens--> drags chromosome toward pole

• In anaphase– sister chromatids separate– move along the kinetochore microtubules toward

opposite ends of the cell

• MTs shorten by depolymerization of their kinetochore ends

• Nonkinetochore MT– overlap and push against each other from

opposite poles– Cause cellular elongation

–In telophase– nuclear envelopes reform around each chromosome set copy

–genetically identical daughter nuclei form at opposite ends of the cell

–Chromosomes decondense

–MT depolymerize

Cytokinesis: A Closer Look

• In animal cells– cleavage furrow forms by action of contractile ring (actinomyosin)

• In plant cells– cell plate forms by fusion and elongation of vesicles between daughter

cells

Animation: Cytokinesis

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Cleavage furrow100 µm

Contractile ring ofmicrofilaments

Daughter cells

Cleavage of an animal cell (SEM)

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1 µm

Daughter cells

Cell plate formation in a plant cell (TEM)

New cell wallCell plate

Wall ofparent cell

Vesiclesformingcell plate

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NucleusCell plateChromosomesNucleolus

Chromatincondensing 10 µm

Prophase. The chromatin is condensing.The nucleolus is beginning to disappear.Although not yet visible in the micrograph, the mitotic spindle is starting to form.

Prometaphase. Wenow see discrete chromosomes; each consists of two identical sister chromatids. Laterin prometaphase, the nuclear envelope will fragment.

Metaphase. The spindle is complete, and the chromosomes, attached to microtubules at their kinetochores, are all at the metaphase plate.

Anaphase. The chromatids of each chromosome have separated, and the daughter chromosomes are moving to the ends of the cell as their kinetochore micro- tubules shorten.

Telophase. Daughter nuclei are forming. Meanwhile, cytokinesis has started: The cell plate, which will divide the cytoplasm in two, is growing toward the perimeter of the parent cell.

Description of each mitotic phase

Prokaryotic Cell Division

• Binary fission– Usually one circular chromosome attached to

plasma membrane via origin of replication (ori)

– chromosome replicates

– Daughter chromosomes move apart as cell elongates

– Cell division

– Mechanism distinct from mitosis.

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Origin ofreplication

Cell wall

Plasmamembrane

Bacterialchromosome

E. coli cell

Two copiesof origin

Chromosome replication begins. Soon thereafter, one copy of the origin moves rapidly toward the other end of the cell.

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Origin ofreplication

Cell wall

Plasmamembrane

Bacterialchromosome

E. coli cell

Two copiesof origin

Chromosome replication begins. Soon thereafter, one copy of the origin moves rapidly toward the other end of the cell.

Replication continues. One copy of the origin is now at each end of the cell.

Origin Origin

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Origin ofreplication

Cell wall

Plasmamembrane

Bacterialchromosome

E. coli cell

Two copiesof origin

Chromosome replication begins. Soon thereafter, one copy of the origin moves rapidly toward the other end of the cell.

Replication continues. One copy of the origin is now at each end of the cell.

Origin Origin

Replication finishes. The plasma membrane grows inward, and new cell wall is deposited.

Two daughtercells result.

My genome hurts!

It’s time to ask some questions.

Regulation of Eukaryotic Cell Cycle

Checkpoint controls

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G1 checkpoint

G1

S

M

M checkpoint

G2 checkpoint

G2

Controlsystem

(pauses if DNA is damaged or incompletely replicated)

(pauses if DNA is damaged)

(pauses if any kinetochoresare not attached to MT)

(aka spindle checkpoint)

• For many cells• G1 checkpoint most important one

• Go-ahead signal (no damaged DNA) at the G1 checkpoint -->• cell completes the S, G2, and M phases and divides

• e.g. skin cells, cells of gut lining and mouth

•No go-ahead signal

•Cells exits G1 and enters quiescent state of G0

•No cell division•e.g. nerve cells

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G1

G1 checkpoint

G1

G0

If a cell receives a go-ahead signal at the G1 checkpoint, the cell continues on in the cell cycle.

If a cell does not receive a go-ahead signal at the G1 checkpoint, the cell exits the cell cycle and goes into G0, a nondividing state.

Internal and External Signals at the Checkpoints Examples:

• Internal – Attachment of spindle to kinetochore (spindle checkpoint)

– What happens if no attachment occurs?

Arrest in metaphase; cell delays anaphase

• External–Growth factors secreted by neighboring cells–e.g. platelet-derived growth factor (PDGF) stimulates the division of human fibroblast cells in culture

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Petriplate

Scalpels

Without PDGF

With PDGF

Without PDGF

With PDGF

10 mm

Day 0

Day 4

Dissociate tissue into cells

What did PDGF induce?

• Other examples of external signals– Density-dependent inhibition (or contact inhibition):

-crowded cells stop dividing

–Anchorage dependence: -cells must be attached to a substratum in order to divide

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Cells anchor to dish surface anddivide (anchorage dependence).

When cells have formed a completesingle layer, they stop dividing(density-dependent inhibition).

If some cells are scraped away, theremaining cells divide to fill the gap andthen stop (density-dependent inhibition).

25 µmNormal mammalian cells

Note: each cell is anchoredto plastic substratum.

• Cancer cells are NOT:– Density or anchorage dependent

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Cancer cells do not exhibitanchorage dependenceor density-dependent inhibition.

Cancer cells25 µm

Loss of Cell Cycle Controls in Cancer Cells

• Do not respond normally to the body’s control mechanisms

• Form tumors, masses of abnormal cells within otherwise normal tissue

- Benign if stay at original site

- Malignant if migrate into other parts of body (metastisis)

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Cancer cell

Bloodvessel

LymphvesselTumor

Glandulartissue

Metastatictumor

A tumor grows from asingle cancer cell.

Cancer cells invadeneighboring tissue.

Cancer cells spreadthrough lymph andblood vessels toother parts of thebody.

A small percentageof cancer cells maysurvive and establisha new tumor in anotherpart of the body.

What triggers cancerous growth?

Chemicals e.g. in smoke and soot, ionizing radiation, certainRNA- and DNA containing viruses e.g. HIV, hepatitis B, papilloma virus; normal metabolic damage

Common property:Mutagenic: induce changes in the genome

that cause:

- over-expression of oncogenes, which encode proteins that push cell cycle forward

- under-expression of tumor-suppressor genes, which encode proteins that normally slow cell cycle down

-Chemotherapy

-Radiation

-Surgery

-Inhibitors of oncogenes

-Gene therapy

-Immunotherapy

-Anti-angiogenesis therapy (block blood supply to tumor)

How do these treatments stop cancer? Can they have negative side-effects,too?

Tumor Treatments: