Cell division, cell growth, cell Cycle

• Interphase and meiosis I

Centrosomes(with centriole pairs)

Sisterchromatids

Chiasmata

Spindle

Tetrad

Nuclearenvelope

Chromatin

Centromere(with kinetochore)

Microtubuleattached tokinetochore

Tertads line up

Metaphaseplate

Homologouschromosomes

separate

Sister chromatidsremain attached

Pairs of homologouschromosomes split up

Chromosomes duplicateHomologous chromosomes

(red and blue) pair and exchangesegments; 2n = 6 in this example

INTERPHASE MEIOSIS I: Separates homologous chromosomes

PROPHASE I METAPHASE I ANAPHASE I

Figure 13.8

1. Synapsis (聯會 )(synaptonemal complex)

2. cross over

TELOPHASE I ANDCYTOKINESIS

PROPHASE II METAPHASE II ANAPHASE II TELOPHASE II ANDCYTOKINESIS

MEIOSIS II: Separates sister chromatids

Cleavagefurrow Sister chromatids

separate

Haploid daughter cellsforming

During another round of cell division, the sister chromatids finally separate;four haploid daughter cells result, containing single chromosomes

Two haploid cellsform; chromosomesare still doubleFigure 13.8

• Telophase I, cytokinesis, and meiosis II

MITOSIS MEIOSIS

Prophase

Duplicated chromosome(two sister chromatids)

Chromosomereplication

Chromosomereplication

Parent cell(before chromosome replication)

Chiasma (site ofcrossing over) MEIOSIS I

Prophase I

Tetrad formed bysynapsis of homologouschromosomes

MetaphaseChromosomespositioned at themetaphase plate

Tetradspositioned at themetaphase plate

Metaphase I

Anaphase ITelophase I

Haploidn = 3

MEIOSIS II

Daughtercells of

meiosis I

Homologuesseparateduringanaphase I;sisterchromatidsremain together

Daughter cells of meiosis IIn n n n

Sister chromatids separate during anaphase II

AnaphaseTelophase

Sister chromatidsseparate duringanaphase

2n 2nDaughter cells

of mitosis

2n = 6

• A comparison of mitosis and meiosis

Cell cycle:

--- the life of a cell from the time it is first formed from a dividing parent cell until its own division into two cells. Smallest unit of life

all living things must reproduce Cells replicate for growth, replacement, and repair

Cell division functions in reproduction, growth, and renewal.

20 µm200 µm

Cell CycleThe Cell’s Time Clock

• Cell division requires Mitosis & Cytokinesis

• Phases of a dividing cell’s life – interphase

• cell grows• replicates chromosomes• produces new organelles &

biomolecules– mitotic phase

• cell separates & divides chromosomes

– mitosis• cell divides cytoplasm &

organelles– cytokinesis

Cytokinesis

MMitosis

G1Gap 1

G0Resting

G2Gap 2

SSynthesis

Cell cycle MMitosis

G1Gap 1

G0Resting

G2Gap 2

SSynthesis

• Cell has a “life cycle”cell is formed from a mitotic division

cell grows & maturesto divide again

cell grows & matures to never divide again

G1, S, G2, M G0

epithelial cells,blood cells,stem cells

brain nerve cells

liver cells

Interphase• Cell performs normal function

• Three subphases:– G1: cell duplicates most organelles– S: quantity of DNA in the cell is

doubled as chromosomes are replicated. Each chromosome has a pair of sister chromatids connected by a centromere that contains a kinetochore

– G2: chemical components stockpiled

• Nucleolus present

Mitosis

• Nuclear division without a reduction in chromosome number

• Each new cell (daughter cell) will have the same quantity of DNA as the parental cell

• Why is this important?

• Mitotic events can be categorized into discrete stages based on what is happening to structure of the cell

• Stage include:– Prophase

• Prometaphase– Metaphase– Anaphase– Telophase

Prophase(Including Prometaphase)

• Pro• Three things visibly

occur– Chromosomes condense

(shorten)– Centrosomes migrate to

the poles while producing spindle fibers

– Nuclear membrane fragments

Metaphase

• Meta• Chromosomes are

moved by growing spindle fibers to the equator of the cell (metaphase plate)

• Centrosomes are at the poles, nuclear membrane is gone

Metaphase Plate

Anaphase

• Ana• Centromere splits into two• Spindle fibers shorten from

kinetochore end separating sister chromatids

• Activated kinetochores “pull” chromatids along the spindle fibers and toward the poles

Telophase

• Telo• Nuclear membrane

reforms around each region of chromosomes

• Nucleolus reforms• Cytokinesis (division

of the cytoplasm) may occur

Cytokinesis May Vary Between Major Taxonomic Groups

Cytokinesis divides the cytoplasm

* Cleavage furrow * No cleavage furrow

Actin +Myosin

Daughter cells

1 µmVesiclesforming cell plate

Wall of patent cell Cell plate New cell wall

(b) Cell plate formation in a plant cell (SEM)

Cleavage furrow

Contractile ring of microfilaments Daughter cells

100 µm

(a) Cleavage of an animal cell (SEM)

2006-2007

Regulation of Cell Division

Coordination of cell division

• A multicellular organism needs to coordinate cell division across different tissues & organs– critical for normal growth,

development & maintenance• coordinate timing of

cell division• coordinate rates of

cell division • not all cells can have the

same cell cycle

• How do cells know when to divide? – cell communication signals

• chemical signals in cytoplasm give cue• signals usually mean proteins

– activators– inhibitors

Activation of cell division

experimental evidence: Can you explain this?

G2

S G1

M

metaphaseprophase

anaphasetelophase

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

Overview of Cell Cycle Control

• Two irreversible points in cell cycle– replication of genetic material– separation of sister chromatids

• Checkpoints – process is assessed & possibly halted

centromere

sister chromatids

single-strandedchromosomes double-stranded

chromosomes

There’s noturning back,now!

Cell Cycle Regulation

• Cell cycle events are triggered by the cell-cycle control system; a set of molecules found in the cytoplasm affected by internal and external controls

• Checkpoints in G1, G2, and M phases of the cycle• G1 checkpoint is most critical. May throw cells

out of cyclic phase into G0, never to divide again

Other Internal and External Factors

• Internal – M checkpoint does not proceed until signal is received that all

kinetochores are attached to spindle microtubules

• External– Growth factors: cycle will not proceed if requirements are not met– Social signals

• Density-dependent inhibition: under crowded conditions chemical requirements are insufficient to allow cell growth

• Anchorage dependence: some cells must be attached to a substrate in order to replicate

– DNA damage inhibits growth

External signals: ex. Growth factors

~ Cells fail to divide if an essential nutrient is left out of the culture medium.~ GFs trigger a signal transduction pathway that allows the cells to pass the G1 checkpoint and divide.

cell

PDGF receptor

Signal transduction

Cell division

PDGF

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

Density-dependent inhibition of cell division

~ Crowded cells stop

dividing

single layer

External signals: physical factor

Cells anchor to dish surface anddivide (anchorage dependence).

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

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

25 µm

Anchorage dependence

• Most animal cells exhibit anchorage dependence– In which they must be attached to a substratum to divide

* Cancer cells: ~ Exhibit neither density-

dependent inhibition nor anchorage dependence

25 µm

Cancer cells do not exhibit anchorage dependence or density-dependent inhibition.

25 µm

Normal cell ~ single layer

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

Internal signal 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 fibroblast (connective tissue)• heal wounds

Don’t forgetto mentionerythropoietin!(EPO)

The sequential events of the cell cycle are directed by a distinct cell cycle control system, a cyclically operating set of molecules in the cell that both triggers and coordinates key events in the cell cycle.

Control system

G2 checkpointM checkpoint

G1 checkpoint

G1

S

G2M

~ similar to a clock

The cell cycle is regulated at certain checkpoints by both internal and external controls.

Checkpoint control system

• Checkpoints– cell cycle controlled by STOP & GO chemical

signals at critical points– signals indicate if key cellular

processes have been completed correctly

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?

Cdk / G1cyclin

Cdk / G2cyclin (MPF)

G2

S

G1

CM

G2 / M checkpoint

G1 / S checkpoint

APC

ActiveInactive

ActiveInactive

InactiveActive

mitosis

cytokinesis

MPF = Mitosis Promoting FactorAPC = Anaphase Promoting Complex

• Replication completed• DNA integrity

Chromosomes attached at metaphase plate

Spindle checkpoint

• Growth factors• Nutritional state of cell• Size of cell

G1/S checkpoint

• G1/S checkpoint is most critical– primary decision point

• “restriction point”– if cell receives “GO” signal, it divides

• internal signals: cell growth (size), cell nutrition • external signals: “growth factors”

– if cell does not receive signal, it exits cycle & switches to G0 phase

• non-dividing, working state

G0 phase

MMitosis

G1Gap 1

G0Resting

G2Gap 2

SSynthesis

• 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 Checkpoints• If cell size inadequate

– G1 or G2 arrest• If nutrient supply inadequate

– G1 arrest • If an essential external stimulus is lacking

– G1 arrest (at R)• If the DNA is not replicated

– S arrest• If DNA damage is detected

– G1 or G2 arrest• If the spindle formation is improper,

chromosome misalignment– M-phase arrest

R

“Go-ahead” signals• Protein signals that promote cell growth &

division– internal signals

• “promoting factors”– external signals

• “growth factors”

• Primary mechanism of control– phosphorylation

• kinase enzymes• either activates or inactivates cell signals

Cell cycle signals

• Cell cycle controls– cyclins

• regulatory proteins• levels cycle in the cell

– Cdk’s• cyclin-dependent kinases• phosphorylates cellular

proteins– activates or inactivates proteins

– Cdk-cyclin complex• triggers passage through

different stages of cell cycle

activated Cdk

inactivated Cdk

Types of Cyclins and Cdks

• There are many types of cyclins, but the 4 main ones are:– Cyclin D (G1 cyclin)– Cyclin E (S-phase cyclin)– Cyclin A (S-phase and mitotic cyclin)– Cyclin B (mitotic cyclin)

• These are the 3 main cdks– Cdk4 (G1 Cdk)– Cdk2 (S-phase Cdk)– Cdk1 (mitotic Cdk)

• The complex of Cdk1 and cyclin B is called mitosis promoting factor (MPF) a.k.a maturation promoting factor

Rise and fall of cyclinsCy

clin

Con

cent

ratio

n

Mitosis

Cdks and cyclinsCyclin-dependent kinases (Cdks) are enzymes that are present in the cell cytoplasm at all times.

However, they are inactive unless they are bound by a specific partner-protein called a cyclin to form a Cdk-cyclin complex

The amount of cyclins in the cell changes – because they get degraded

A Cdk-cyclin complex will push the cell cycle forward.

Figure 19-35 Phosphorylation and Dephosphorylation in the Activation of a Cdk-Cyclin Complex

MPF: M-phase Promoting Factor

• MPF is composed of two key subunits: Cdc2 and Cyclin B. – Cdc2 is the protein that encoded by genes

which are required for passage through START as well as for entry into mitosis.

– Cyclin B is a regulatory subunit required for catalytic activity of the Cdc2 protein kinase.

What does MPF do?The complex of Cdk1 and cyclin B is called mitosis promoting factor (MPF)

MPF activity is dependent upon Cyclin B

• The cyclins were identified as proteins that accumulate throughout interphase and are rapidly degraded toward the end of mitosis.

• It is suggested that they might function to induce mitosis, with their periodic accumulation and destruction controlling entry and exit from M phase.

MPF activity is dependent upon Cyclin B

• Accumulation and degradation of cyclins

Figure 19-34 Fluctuating Levels of Mitotic Cyclin and MPF During the Cell Cycle

MPF regulation

• Cdc2 forms complexes with cyclin B during S and G2. • Cdc2 is then phosphorylated on threonine-161, which

is required for Cdc2 activity, as well as on tyrosine-15 (and threonine-14 in vertebrate cells), which inhibits Cdc2 activity. Dephosphorylation of Thr14 and Tyr15 activates MPF at the G2 to M transition.

• MPF activity is then terminated toward the end of mitosis by proteolytic degradation of cyclin B.

MPF regulation• Demonstration of regulation of MPF

Figure 19-40 A General Model for Cell Cycle Regulation

Cyclins & Cdks• Interaction of Cdk’s & different cyclins triggers the stages of

the cell cycle

Leland H. Hartwellcheckpoints

Tim HuntCdks

Sir Paul Nursecyclins

1970s-’80s | 2001

• external signals is density-dependent inhibition, in which crowded cells stop dividing but lost of contact inhibition and outgrowth in cancer cells

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 tumors• 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

Tumors

• Benign - A spontaneous growth of tissue which forms an abnormal mass is called a tumor. A tumor that is noninvasive and noncancerous is referred to as a benign tumor.

• Malignant - A tumor that invades neighboring cells and is cancerous is referred to as a malignant tumor.

• Matastasis – Cancer that has spread to other tissues.

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– 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 censor gene

It’s like anout of controlcar!

MMitosis

G1Gap 1

G0Resting

G2Gap 2

SSynthesis

Cancer & Cell Growth

• Cancer is essentially a failure of cell division control – unrestrained, uncontrolled cell growth

• What control is lost?– checkpoint stops– gene p53 plays a key role in G1 checkpoint

• p53 protein halts cell division if it detects damaged DNA – 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 is theCell CycleEnforcer

p53 discovered at Stony Brook by Dr. Arnold Levine

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 p53

protein

p53 — master regulator gene

Growth Factors and Cancer

• Growth factors influence cell cycle– proto-oncogenes

• normal genes that become oncogenes (cancer-causing) when mutated

• stimulates cell growth• if switched on can cause cancer• example: RAS (activates cyclins)

– tumor-suppressor genes• inhibits cell division• if switched off can cause cancer• example: p53

What causes these “hits”?

• Mutations in cells can be triggered by UV radiation chemical exposure radiation exposure heat

cigarette smoke pollution age genetics

How we naturally fight cancer cells

• Tumor suppressor genes like p53– Can arrest the cell cycle– Can launch the apoptotic pathway, causing the

rogue cells to lyseA mutation in the p53 gene can lead to cancer

• Immune cells (WBCs) such as NK cells can attack and lyse tumor cells– Some immune cells can signal the rogue cells to

launch the apoptotic pathways

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

New “miracle drugs”

• Drugs targeting proteins (enzymes) found only in tumor cells– Gleevec

• treatment for adult leukemia (CML)& stomach cancer (GIST)

• 1st successful targeted drug

Any Questions??

Signal Transduction Pathways • What are they?

– Signal transduction refers to any process by which a cell converts one kind of signal or stimulus into another.

– A large number of proteins, enzymes and other molecules participate in a "signal cascade“

• What is the end result?– Either the activation or inhibition of a certain enzyme in

the cytoplasm– Either the expression or suppression of a particular gene

Just a few examples of Signal Transduction Pathways

• Cell Division signals• Apoptotic signals• Insulin pathways

Apoptotic Pathways

Insulin Signaling Pathway

The binding of insulin to its receptor on a cell starts a cascade of cellular events which finally leads to the uptake of glucose and the lowering of blood glucose levels.

“Go-ahead” signals• Protein signals that promote cell growth &

division– internal signals

• “promoting factors”– external signals

• “growth factors”

• Primary mechanism of control– phosphorylation

• kinase enzymes• either activates or inactivates cell signals