Chapter 12: The Cell Cycle

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Omnis cellula e cellulaFrom every cell a cell – Rudolf Virchow

• Cell division: reproduction of cells• Cell cycle: life of a cell from the time it is first

formed from a dividing parent cell until it divides into 2 daughter cells

• Mitosis: nuclear division within a cell, followed by cytokinesis

• Cytokinesis: division of the cytoplasm

Getting the right stuff • What is passed on to daughter cells?– exact copy of genetic material = DNA• mitosis

– organelles, cytoplasm, cell membrane, enzymes• cytokinesis

chromosomes (stained orange)in kangaroo rat epithelial cellnotice cytoskeleton fibers

Mitosis• Mitosis functions in– Reproduction of single-celled organisms– Growth– Repair– Regeneration

• In contrast, meiosis produces gametes in sexually-reproducing organisms– Contains half the number of chromosomes of a

somatic cell

amoeba

G2

S G1

Mmetaphase

prophaseanaphase

telophase

interphase (G1, S, G2 phases)mitosis (M)cytokinesis (C)

C

• Frequency of cell division varies by cell type– embryo

• cell cycle < 20 minute– skin cells

• divide frequently throughout life• 12-24 hours cycle

– liver cells• retain ability to divide, but keep it in reserve• divide once every year or two

– mature nerve cells & muscle cells• do not divide at all after maturity• permanently in G0

Frequency of cell division

Organization of the Genetic Material• Cell division results in genetically

identical daughter cells– This requires DNA replication

followed by division of the nucleus• Genome: genetic content of the

cell– Prokaryotic cells have circular DNA

Circular DNA: a single long molecule– Eukaryotic cells contain a number of

DNA molecules specific to different speciesOne eukaryotic cell has about 2 meters of DNA

Organizing DNA• DNA is organized in

chromosomes– double helix DNA molecule– wrapped around histone

proteins• like thread on spools

– DNA-protein complex =chromatin• organized into long thin fiber

– condensed further during mitosis

DNA

histones

chromatin

duplicated mitotic chromosome

ACTGGTCAGGCAATGTC

double-strandedmitotic humanchromosomes

Distribution of Chromosomes During Cell Division

• Non-dividing cells contain chromatin– In human cells there are 46 strands of chromatin, or

23 corresponding pairs of strands

• Dividing cells contain chromosomes– Chromosomes becomes condensed prior to mitosis– Chromosomes contain sister chromatids bound by a

narrow centromere• Kinetochore: a structure of proteins associated with

specific sections of chromosomal DNA at the centromere

Chromosome Structure0.5 µm

Chromosomeduplication(including DNA synthesis)

Centromere

Separation of sister chromatids

Sisterchromatids

Centromeres Sister chromatids

A eukaryotic cell has multiplechromosomes, one of which is

represented here. Before duplication, each chromosome

has a single DNA molecule.

Once duplicated, a chromosomeconsists of two sister chromatids

connected at the centromere. Eachchromatid contains a copy of the

DNA molecule.

Mechanical processes separate the sister chromatids into two chromosomes and distribute

them to two daughter cells.

Figure 12.4

Mitotic Phase Alternates with Interphase in Cell Cycle

• Mitosis: M Phase includes mitosis and cytokinesis– Prophase– Prometaphase– Metaphase– Anaphase– Telophase

• Interphase: 90% of cell cycle; cell grows, producing organelles and proteins– G1: First gap– S: Synthesis, chromosome

replication– G2: Second gap; completes

preparation for cell division

I’m working here!

Time to Divide

In 24 hours…• Mitosis: 1 hour• G1: 5-6 hours (most variable)

• S: 10-12 hours• G2: 4-6 hours

INTERPHASE

G1

S(DNA synthesis)

G2Cyto

kinesis

Mitosis

MITOTIC(M) PHASE

Mitotic Spindle is Necessary for Nuclear Division

• Mitotic spindle: used to segregate sister chromatids in anaphase

• Consists of microtubules and proteins

• Microtubules are able to change in length– Elongate by adding

Tubulin subunits– Shorten by loss of

Tubulin subunits

CentrosomeAster

Sisterchromatids

MetaphasePlate

Kinetochores

Overlappingnonkinetochoremicrotubules

Kinetochores microtubules

Centrosome

ChromosomesMicrotubules0.5 µm

1 µm

Mitotic Spindle• Interphase: the centrosome is

replicated and the two centrosomes remain paired near the nucleus– Centrosome or Microtubule Organizing

Center (MTOC): contains two centrioles• Prophase: spindle formation• Early Prometaphase: migration of

centrosomes to poles, spindle microtubules grow

• Late Prometaphase: centrosomes are at the poles, asters form – Aster: radial array of short microtubules– The mitotic spindle includes the

centrosomes, spindle microtubules and asters

Cellular Changes Indicate M- Phase Initiation

G2 of Interphase: nuclear envelope intact– Nucleus and nucleolus

present– Centrosome Replication

• Animal cells contain centrioles

– Chromosomes are not condensed and therefore not individually visible

Prophase• Nucleoli disappear• Chromosomes condense• Sister chromatids form– Two genetically identical

arms joined by the centromere and cohesin proteins

– Mitotic spindle forms using microtubules; asters form

– Centrosome migration as microtubules lengthen

Prometaphase• Nuclear envelope

fragments and microtubules invade nuclear area

• Nonkinetochore microtubules elongate

• Chromosomes condense further and gain kinetochore proteins

• Spindle fibers (microtubules) interact with kinetochores

Mitotic SpindlePrometaphase:• Kinetochore: protein structure

assiciated with specific sections of the chromosomal DNA at the centromere

• Kinetochore microtubules: attach kinetochore to spindle

• Metaphase Plate: created as microtubules attach to kinetochore, creation indicates Metaphase start

Metaphase

• Longest stage of mitosis– Can last up to 20 minutes

• Metaphase plate: site of chormosome alignment

• Centrosomes are at poles• Sister chromatid

kinetochores attach to kinetochore microtubules

Anaphase• Shortest stage of mitosis• Cohesins between sister

chromatids are cleaved by enzymes– Sister chromatids are now separate

chromosomes

• Spindle fibers shorten causing chromosomes to move to opposite poles– Spindles shorten (tubulin breakdown)

at kinetochore ends

• Nonkinetochore microtubules elongate cell

• Kinetochores use motor proteins that “walk” chromosome along attached microtubule– microtubule shortens

by dismantling at kinetochore (chromosome) end

Chromosome movement

Telophase

• Daughter nuclei form in cell• Chromosomes loosen and

become less dense• Nucleoli reappear

Cytokinesis• Occurs in animal cells only• Relies on cleavage marked by a

cleavage furrow• Cytoplasmic side of cleavage

furrow contains actin microfilaments and myosin– As the actin and myosin interact

the ring contracts and the cleavage furrow deepens

Mitosis in animal cells

Mitosis in whitefish blastula

Cellular Division in Plant Cells• Plant cells form a cell plate following mitosis• Golgi vesicles contain cell wall materials and

migrate toward the center of the cell – forming the cell plate, a new cell wall

Cytokinesis in plant cell

Binary Fission

• Asexual eukaryotes can utilize mitosis for reproductive purposes – this is called binary fission

• Asexual prokaryotes perform binary fission that does not involve mitosis

Evolution of Mitosis• Some proteins were

highly conserved• In prokaryotes, protein

resembling eukaryotic actin may help with cell division– Tubulin-Like proteins may

help separate daughter cells

Attach to nuclear envelope

Reinforce spatial arrangement of nucleus

Spindle inside nucleus

Cell Cycle Regulation•Cytoplasmic regulators as shown in Mammalian cell Fusion experiment by Johnson and Rao (1970)

Cell Cycle Control System• Control systems vary cell to

cell– Skin cells divide frequently,

liver cells only divide when needed (after injury), and nerve and muscle cells never divide

• Cytoplasmic molecules signal cell cycle as shown by Johnson and Rao

• Cell cycle events are regulated cyclicly– These are referred to as Cell

Cycle Checkpoints

Cell Cycle Control Systems• Cell Cycle Checkpoint: control

point in cell cycle where stop and go-ahead signals can regulate the cycle; relies on signal transduction pathways controlled by internal and external molecular signals– G1 checkpoint: acts as a

mammalian restriction point• “Go Signal” permits G1, S, G2 and M• “Stop signal” causes G0 phase: non-

dividing state

– G2 checkpoint– M checkpoint

Checkpoint control system• 3 major checkpoints:– G1/S• can DNA synthesis begin?

– G2/M• has DNA synthesis been completed

correctly?• commitment to mitosis

– spindle checkpoint• are all chromosomes attached to

spindle?• can sister chromatids separate

correctly?

G0 phase• G0 phase– non-dividing, differentiated state– most human cells in G0 phase

liver cells in G0, but can be “called

back” to cell cycle by external cues

nerve & muscle cells highly specialized arrested in G0 & can never

divide

Cell Cycle Clock• The rate of cell cycle is

controlled by two proteins:– Cyclins– Cyclin-dependent

kinases (Cdks)

Cell Cycle Clock

• Kinases: activate or inactivate other proteins via phosphorylation– Inactive most of the time– Kinases are activated by cyclins

• Cyclins: regulatory proteins that have a cyclic (fluctuating) concentration in the cell

• Cyclin-dependent kinases (Cdks)– Activity fluctuates with cyclin concentration

Cell Cycle Clock• MPF: maturation-

promoting factor or M-phase promoting factor– MPF is a Cyclin-Cdk

complex• Triggers cell passage from

G2 to M• G2 checkpoint: As cyclin

builds during G2 it binds with Cdk– Resulting MPF

phosphorylates proteins, initiating Mitosis

• During anaphase, MPF inhibits itself by destroying its own cyclin– Cdk persists in the cell

Stop and Go Signals

• Internal signals– M Phase checkpoint relies on kinetochore

signaling– Allows for enzymatic cleavage of cohesins

• External factors– Nutrients– Growth Factor Dependency– Density-dependent inhibition– Anchorage dependence

External signals• Growth factors– coordination between cells– protein signals released by body cells

that stimulate other cells to divide• density-dependent inhibition

– crowded cells stop dividing– each cell binds a bit of growth factor

» not enough activator left to trigger division in any one cell

• anchorage dependence – to divide cells must be attached to a

substrate» “touch sensor” receptors

E2F

nucleuscytoplasm

cell division

nuclear membrane

growth factor

protein kinase cascade

nuclear pore

chromosome

Cdkcell surfacereceptor

P

PP

P

P

E2FRb

Rb

Growth factor signals

Example of a Growth Factor• Platelet Derived Growth Factor (PDGF)– made by platelets in blood clots– binding of PDGF to cell receptors stimulates cell

division in connective tissue• heal wounds

Growth Factors and Cancer• Growth factors can create cancers– proto-oncogenes• normally activates cell division

– growth factor genes – become oncogenes (cancer-causing) when mutated

• if switched “ON” can cause cancer• example: RAS (activates cyclins)

– tumor-suppressor genes• normally inhibits cell division• if switched “OFF” can cause cancer• example: p53

Cancer & Cell Growth• Cancer is essentially a failure

of cell division control – unrestrained, uncontrolled cell growth

• What control is lost?– lose checkpoint stops– gene p53 plays a key role in G1/S restriction point

• p53 protein halts cell division if it detects damaged DNA – options:» stimulates repair enzymes to fix DNA » forces cell into G0 resting stage» keeps cell in G1 arrest » causes apoptosis of damaged cell

• ALL cancers have to shut down p53 activity

p53 discovered at Stony Brook by Dr. Arnold Levine

p53 is theCell CycleEnforcer

DNA damage is causedby heat, radiation, or chemicals.

p53 allows cellswith repairedDNA to divide.

Step 1

DNA damage iscaused by heat,radiation, or chemicals.

Step 1 Step 2

Damaged cells continue to divide.If other damage accumulates, thecell can turn cancerous.

Step 3p53 triggers the destruction of cells damaged beyond repair.

ABNORMAL p53

NORMAL p53

abnormalp53 protein

cancercell

Step 3The p53 protein fails to stopcell division and repair DNA.Cell divides without repair todamaged DNA.

Cell division stops, and p53 triggers enzymes to repair damaged region.

Step 2

DNA repair enzymep53protein

p53protein

p53 — master regulator gene

Development of Cancer• Cancer develops only after a cell experiences ~6 key

mutations (“hits”)– unlimited growth

• turn on growth promoter genes– ignore checkpoints

• turn off tumor suppressor genes (p53)– escape apoptosis

• turn off suicide genes– immortality = unlimited divisions

• turn on chromosome maintenance genes– promotes blood vessel growth

• turn on blood vessel growth genes– overcome anchor & density dependence

• turn off touch-sensor gene

It’s like anout-of-controlcar with manysystems failing!

What causes these “hits”? • Mutations in cells can be triggered by

UV radiation chemical exposure radiation exposure heat

cigarette smoke pollution age genetics

Tumors• Mass of abnormal cells– Benign tumor • abnormal cells remain at original site as a lump – p53 has halted cell divisions

• most do not cause serious problems &can be removed by surgery

– Malignant tumor• cells leave original site– lose attachment to nearby cells – carried by blood & lymph system to other tissues– start more tumors = metastasis

• impair functions of organs throughout body

Cancer Cells• Benign tumors: can be removed by surgery• Malignant tumors: invade surrounding tissues and organs,

and often metastasize– Metastasis: spread of cancer to far locations in the body

• Treatment options– Radiation: destroys cancer cells – Chemotherapy: medication that targets rapidly dividing cells

(including cancer cells)• Ex: Taxol – stops dividing cells by prevents microtubule depolymerization in

metaphase, but also affects cells that naturally divide often such as intestinal cells and skin cells of hair follicles

Traditional treatments for cancers• Treatments target rapidly dividing cells

– high-energy radiation • kills rapidly dividing cells

– chemotherapy• stop DNA replication• stop mitosis & cytokinesis• stop blood vessel growth