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Fundamentals of Cell Biology Chapter 13: The Birth and Death of Cells


Fundamentals of Cell Biology Chapter 13: The Birth and Death of Cells Dr. Saeb Aliwaini * – PowerPoint PPT presentation

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Title: Fundamentals of Cell Biology Chapter 13: The Birth and Death of Cells

Fundamentals of Cell BiologyChapter 13 The
Birth and Death of Cells
Cell cycle
  • 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

Coordination of cell division
  • A multicellular organism needs to coordinate cell
    division across different tissues organs
  • critical for normal growth, development
  • coordinate timing of cell division
  • coordinate rates of cell division
  • not all cells can have the same cell cycle

Frequency of cell division
  • Frequency of cell division varies by cell type
  • embryo
  • cell cycle lt 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

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

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

Theres noturning 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
  • external signals growth factors
  • if cell does not receive signal, it exits cycle
    switches to G0 phase
  • non-dividing, working state

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

Activation of cell division
  • How do cells know when to divide?
  • cell communication signals
  • chemical signals in cytoplasm give cue
  • signals usually mean proteins
  • activators
  • inhibitors

experimental evidence Can you explain this?
Point of no return
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
Cyclins Cdks
  • Interaction of Cdks different cyclins triggers
    the stages of the cell cycle

Go-ahead signals
  • Protein signals that promote cell growth
  • 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
  • Cdks
  • cyclin-dependent kinases
  • phosphorylates cellular proteins
  • activates or inactivates proteins
  • Cdk-cyclin complex
  • triggers passage through different stages of cell

inactivated Cdk
activated Cdk
Control by cyclin/CDK complexes
Figure 13.05 Distinct cyclin-cdk complexes
control progression through cell cycle
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

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To start
  • You need signals gtgtgtgtgt

Cell cycle step 1 Signal transduction initiates
cell cycle progression.
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Cell cycle step 2 Changes in gene expression are
required for progression through the restriction
  • 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
    inter- action between E2F and members of the
    retinoblastoma protein (Rb) family.

Cell cycle step 4 Active cyclin/CDKs
phosphorylate pocket proteins, which activate E2Fs
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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
Cell cycle step 5 The DNA replication machinery
is activated by protein kinases
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 pro- teins. The
resulting large complex is the pre-RC, and the
origin is now licensed for replication.
Figure 13.12 Assembly of the prereplication
DNA replication occurs in S phase
  • 3 key steps

Figure 13.13 Activation of the replication
Cell cycle step 6 DNA integrity is ensured by
the G1/S, S/G2, and G2/M checkpoints
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 13.20 Phosphorylation of Cdk1 primes it
for activation but also keeps it in an inactive
Cell cycle step 9 Chromosome alignment is
ensured by the mitotic spindle assembly checkpoint
Figure 13.21 A model for anaphase promotion by
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

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
  • The family of proteins called caspases includes
    proteases that promote the degradation of
    organelles and cytosolic proteins during

Cells die in 2 different ways necrosis and
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
Targets of pro- and anti-apoptotic transcription
factors are members of bcl-2 family
Mitochondrial Outer Membrane Permeabilization
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 pro- teins. 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