Fundamentals of Cell Biology Chapter 13: The Birth and Death of Cells Dr. Saeb Aliwaini1

Fundamentals of Cell Biology

Chapter 13: The Birth and Death of Cells

Dr. Saeb Aliwaini 1

– How cells make the decision to begin moving through the stages of replication, and why some cells never make this journey

– How cells decide to die

Cell cycle

Dr. Saeb Aliwaini 2

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

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

New cells arise from parental cells that complete the cell cycle

• Key Concepts :– Cells divide by following carefully scripted program of molecular events

collectively called the cell cycle.

– The cell cycle is subdivided into five phases named G1, S, G2, M, and G0. Cells not actively dividing reside in G1 or G0 phase.

– Progression through the cell cycle is under the control of proteins that form checkpoints to monitor whether the proper sequence of events is taking place. Cells halt at these checkpoints until they complete the necessary steps to continue.

Dr. Saeb Aliwaini 5

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-strandedchromosomes

There’s no

turning back,

now!

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?

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

• 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?

“Point of no return”

Dr. Saeb Aliwaini 12

The G2/M checkpoint is the trigger for large-scale rearrangement of cellular architecture

• MPF

Early experiments characterizing the activity of Mitosis Promoting Factor.


New cells arise from parental cells that complete the cell cycle

– The G1/S checkpoint, called the restriction point or start point, is where cells commit to completing cell division.

– Proteins called cyclins play an important role in advancing cells through checkpoints.


Activation of cyclin-CDK complexes begins in G1 phase

Figure 13.04: Scientists discovered the first cyclins when they noted that high cyclin levels with the onset of mitosis in embryos. Cyclin levels drop sharply after this.


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

the cell cycle

“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

Control by cyclin/CDK complexes

Figure 13.05: Distinct cyclin-cdk complexes control progression through cell cycle checkpoints.


The cell cycle is divided into five phases

• “Resting” cells reside in G0 or G1 phase

• Several checkpoints define critical decision-making events in the cell cycle



Figure 13.02: Checkpoints control progression through the

cell cycle. Some of the major checkpoints are shown.

To start

• You need signals >>>>>


Cell cycle step 1: Signal transduction initiates cell cycle progression.

Figure 13.06: The family of mitogen activated protein kinases (MAPKs) and their

upstream regulatory proteins.

Figure 13.07: A simplified MAP kinase signaling pathway.



Cell cycle step 2: Changes in gene expression are required for progression through the restriction point

• progression through the restriction point in mammalian cells requires activation of at least two cyclin/CDK complexes: cyclin D1/CDK4 (or CDK6) and cyclin E/CDK2

• expression of most CDKs does not vary much throughout the cycle, but without their corresponding cyclins, they are not functional


Cell cycle step 3: Pro- and anti-growth signaling networks converge at the G1/S cyclin-CDK complexes

• Phosphorylation• Binding by inhibitory

kinases• Subcellular location• Protein degradation

Figure 13.08: Summary of the cyclin/cdk activation-inactivation cycle.


• The key function of G1-Cdk complexes in animal cells is to activate a group of gene regulatory factors called the E2F proteins, which bind to specific DNA sequences in the promoters of a wide variety of genes that encode proteins required for S-phase entry, including G1/S-cyclins, S-cyclins, and proteins involved in DNA synthesis and chromosome duplication.

• In the absence of mitogenic stimulation, E2F-dependent gene expressionis inhibited by an interaction between E2F and members of the retinoblastoma protein (Rb) family.


Cell cycle step 4: Active cyclin/CDKs phosphorylate pocket proteins, which activate E2Fs

Figure 13.09: The transcription factor E2F is inactivated by Rb

binding.

Figure 13.10: Examples of positive (green) and negative (red) feedback loops controlling E2F

function.



How E2Fs enhance expression of some genes while suppressing expression of others remains

unclear

Figure 13.11: A model of how E2F transcription factors can suppress or activate gene transcription.


Cell cycle step 5: The DNA replication machinery is activated by protein kinases

Figure 13.12: Assembly of the prereplication complex.


A key player is a large, multiprotein complex called the origin recognition complex (oRc), which binds to replication origins throughout the cell cycle. In late mitosis and early G1, the proteins cdc6 and other proteins bind to the ORC at origins and help load a group of six related proteins called the Mcm proteins. The resulting large complex is the pre-RC, and the origin is now licensed for replication.

DNA replication occurs in S phase

• 3 key steps

Figure 13.13: Activation of the replication complex.


Cell cycle step 6: DNA integrity is ensured by the G1/S, S/G2, and G2/M checkpoints

Figure 13.14: A current model for DNA repair.

Figure 13.15: Growth arrest induced by Chk1 and Chk2.


Cell cycle step 7: Cells increase in size during G2 phase

Figure 13.17: Wee1 mutation affects cell size. Compared to normal ("wild-type," WT) yeast, Wee1 mutants grow to half normal size before dividing.


Cell cycle step 8: Cyclin B/CDK1 activation drives cells through the G2/M checkpoint

Figure 13.18: A model for cell size control of cell growth in yeast in G2.

Figure 13.19: A model for how adhesion to ECM promotes cell growth

in mammalian cells.


Figure 13.20: Phosphorylation of Cdk1 primes it for activation but also keeps it in an inactive state.


Cell cycle step 9: Chromosome alignment is ensured by the mitotic spindle assembly

checkpoint

Figure 13.21: A model for anaphase promotion by APC/C.


Cell cycle step 10: Onset of cytokinesis is timed to begin only after mitosis is complete

• Cytokinesis requires the contraction of the contractile ring that lies just beneath the plasma membrane, perpendicular to the long axis of the mitotic spindle.

• It is important that the myosin motors in the ring not activate until mitosis, including reconstitution of the nuclear membrane, is complete.


Multicellular organisms contain a cell self-destruct program that keeps them healthy

• Key Concepts:– Cells die either by traumatic injury (necrosis) or by a

self-destruct program called apoptosis. – Apoptosis begins through at least two molecular

mechanisms, called intrinsic and extrinsic pathways.– The family of proteins called caspases includes

proteases that promote the degradation of organelles and cytosolic proteins during apoptosis.


Cells die in 2 different ways: necrosis and apoptosis

Figure 13.22: Cellular damage can result in necrosis, as organelles swell and the plasma membrane ruptures.


Apoptosis is a property of all animal cells and some plant cells


Apotosis is voluntary

Figure 13.23: Sections of the interdigital web show cell death (dark-staining nuclei). This cell death has the characteristics of apoptosis.


Apoptosis is induced via at least 2 different pathways

Figure 13.24: Ligation of death receptors causes the recruitment of the adaptor

protein FADD to the intracellular region of the death receptor.

Figure 13.25: E2F1 lies at the heart of the growth-versus-death decision making

system.


Targets of pro- and anti-apoptotic transcription factors are members

of bcl-2 family

Figure 13.26: The Bcl-2 family proteins share up to four Bcl-2 homology domains

(BH) and can be antiapoptotic or proapoptotic.

Figure 13.27: The Bcl-2 family of proteins compete with members of the

antiapoptotoic group to access the apoptotic group in an elaborate hierarchy.


Mitochondrial Outer Membrane Permeabilization (MOMP)

Figure 13.28: Signals for the induction of apoptosis trigger changes in the Bcl-2 family proteins, which function to inhibit or promote apoptosis. Activation of caspase 9 by the apoptososme. Insert, three views of apoptosome

structure as determined by electron microscopy.


Apoptosis triggers the activation of special proteases: the caspases

Figure 13.29: Different types of vertebrate caspases are shown schematically.


The final changes

• Stereotypical morphological changes take place during apoptosis– karyorrhexis

• Apoptotic cells are cleared by phagocytosis


• The APC/C catalyzesthe ubiquitylation and destruction of two major proteins. The first is securln,which normally protects the protein linkages that hold sister chromatid pairs together in early mitosis. Destruction of securin at the metaphase-to-anaphasetransition activatesa proteasethat separatesthe sisters and unleashes anaphase.The S- and M-cyclins are the second major targets of the APC/c. Destroying these cyclins inactivatesmost cdks in the cell


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Fundamentals of Cell Biology Chapter 13: The Birth and Death of Cells Dr. Saeb Aliwaini1