The Cell Cycle
Pages 1067-1071, 1087-1088, 1105-1107
S phase
-Functions
-Duplicate the DNA
-Protein packing must be reproduced
-2 issues facing DNA replication
-Accuracy of replication
-Copy DNA only once
Control of DNA Duplication
2 steps in Replication
1. Late M/Early G1
-Formation of
prereplicative complex
(pre-RC)
2. Start of S phase
-Preinitiation complex
Initiation of DNA Replication
ORCs are always
bound to the origins
pre-RC assembly:
-Inhibited by Cdk
activity
-Stimulated by
APC/C
Initiation of DNA Replication
-S-Cdk triggers the
preinitiation complex
-Diassembly of preRC
-ORC and Cdc6 is P
by Cdk
-Cdc6 is degraded
-Cdt1 is inhibited by
geminin
Initiation of DNA Replication
-DNA Replication is
completed
-In late M, APC/C the
degradation of geminin
-Cdk activity is
decreased, OCRs are
dephosphorylated
-Cdc6 is synthesized
-pre-RC assembly
Duplication of Chromatin Structure
-Cdks stimulate synthesis of the 4 histone subunits
-Nucleosome assembly factors typically associate with the
replication fork
-Histone modifying enzymes are thought to play a role
Cohesins and Sister Chromatids
-Cohesin – large protein complex which holds the sister
chromatids together
Smc (Structural Maintenance of Chromosomes)
Sister Chromatid Separation
Securin inhibits separase
Separase cleaves cohesin
P
Creation of DNA Lesions
DNA Damage Signaling Pathways
DNA Damage
Checkpoint
Mdm2 – ubiquitin ligase
p21 – CKI (Cdk Inhibitor Protein)
Check1/2
Chk1/2 phosphorylate Cdc25 thereby inactivating
the phosphatase activity resulting in inactive Cdk
DNA Damage
-Cell cycle is arrested until the damage is repaired
-If it’s not repaired:
1. Unicellular organisms will resume their cycle taking a
potential mutation over death
2. Multicellular organimsms will sacrifice a cell over the
health of the organism
Apoptosis
Pages 1115-1128
Apoptosis
Apoptosis is one type of programmed
Process of Apoptosis
-Cells shrink and condense
-Cytoskeleton collapses
-Nuclear envelope disassembles
-Nuclear chromatin fragments
-Cell surface belbs and may break up (apoptotic bodies)
-Cell surface is chemically altered
-Macrophage engulfs the cell
Process of Cell Necrosis
-Cell insult or injury
-Cells swell and burst
-Cell contents are released
-Inflammatory response
Examples of Apoptosis
1. Number of cells
-Nervous system
Examples of Apoptosis
2. Development
Paws of embryonic mouse
Examples of Apoptosis
3. Regulate cell numbers
-Liver
4. Quality control
-Eliminates abnormal, misplaced, nonfunctional,
or dangerous cells
-Developing T and B cells that do not produce
useful antigen receptors or that are self-reactive
5. Supply of cells
-Large numbers of neutrophils are produced and
stored awaiting infection
Biochemical Characteristics
DNA Cleavage
-Endonuclease cleaves
DNA into fragments
between nucleosomes
Biochemical
Characteristics
TUNEL Assay - TdT-mediated
dUTP nick end labeling
Phosphatidylserine localization
-movement from the inner
to outer membrane
-marks the cell for
macrophages
Caspases
-Enzymes responsible for apoptosis
-A family of proteases that cleave proteins at aspartic acid
residues
-C for cysteine (in active site) and ASP for aspartic acid
-Synthesized as inactive precursors, procaspases
-Not all caspases are involved with apoptosis
-ICE, interleukin-1-converting enzyme
Procaspase Cleavage
Caspases
-Initiator procaspase, start the proteolytic cascade
-Executioner procaspases, cleave and activate other
executioner procaspases and other targets
-Targets include: nuclear lamins, endonuclease inhibitor,
cytoskeleton components, cell-cell adhesion proteins
-The caspase cascade is:
-Very destructive
-Self-amplyfing
-Irreversible
Caspase Cascade
Caspases
-Initiator caspases have a caspase recruitment domain
(CARD) that enables them to bind to adaptor proteins into
activation complexes
-In the complex, the initiator caspases are close enough
to activate each other starting the cascade
-2 Apoptotic pathways
1. Extrinsic Pathway
2. Intrinsic Pathway
Extrinsic Pathway
DISC – death-inducing signaling complex
Inhibitors such as decoy receptors and intracellular blocking proteins
Intrinsic Pathway
Releases mitochondrial
proteins into the cytoplasm
Cytochrome C release can
trigger apoptosis through
interaction with the adapter
protein Apaf1
Intrinsic Pathway
Classes of Bcl2 Proteins
Bcl2 proteins –regulate apoptosis through controlling the
release of cytochrome c
Pro-Apoptotic BH123
Regulation of
Intrinsic
Pathway
BH3-only proteins activate
apoptosis through direct
binding with anti-apoptotic
proteins
-p53 activates BH3-only
proteins (Puma and Noxa)
-Bid – extrinsic and
intrinsic pathways
Model for
IAPs
Inhibition of Apoptosis
(Anti-apoptotic)
(BH3-only
pro-apoptotic)