Chapter 18 Flashcards
Describe G0 phase
Cells are metabolically active and alive, but do not divide
Most differentiated cells
Describe G1 phase. What stage of cell division does it correspond to?
First phase of the eukaryotic cell cycle
Between the end of cytokinesis and the start of DNA synthesis
Growth and normal metabolic roles
Part of interphase
Describe S phase. What stage of cell division does it correspond to?
DNA is synthesized/replicated
Part of interphase
Describe G2 phase. What stage of cell division does it correspond to?
Between the end of DNA synthesis and the beginning of mitosis
Growth and preparation for mitosis
Part of interphase
Describe M phase. What stage of cell division does it correspond to?
Mitosis/Meiosis and Cytokinesis
Includes prophase, prometaphase, metaphase, anaphase, and telophase
What is the engine of the cell cycle?
CDK/Cyclin Complexes
Describe the Cdk portion of CDK/Cyclin Complexes
enzymatic, removes phosphate from ATP and attaches it to substrate
Levels are constant during the cell cycle
Describe the cyclin portion of CDK/Cyclin Complexes
Regulatory, helps Cdk choose its protein targets
Levels change during the cell cycle
Describe the regulation of CDK/Cyclin Complexes
by phosphorylation
Two phosphorylation sites, an inhibitory site and an activating site
Must be phosphorylated on only the activating site
Default is phosphorylation on both sites
What controls which CDK/Cyclin Complexes are active? How does that change through the cell cycle?
Levels of each type of cyclin change during the cell cycle
This means different complexes are active during each phase
M cyclin is low until G2, where it begins to rise, peaking in the middle of M phase followed by a rapid drop
S cyclin is high in all phases except for G1 and the second half of M
Describe the degradation of cyclin
Triggered by ubiquitylation
Destroyed by the proteasome via proteolysis
How levels of cyclin can drop dramatically at the end of a cell phase
What are some effects of protein phosphorylation during the initiation of M phase?
Nuclear envelope breakdown Phosphorylation of nuclear pore proteins and lamins (a type of IF) Chromosome condensation Mitotic spindle formation Targeted protein degradation
Describe how the M-Cdk complex is activated
M-Cdk complex has two kinases associated with it
Wee1, an inhibitory kinase
Cak, an activating kinase
The activating phosphatase (removes inhibitory) is Cdc 25
Describe the positive feedback loop of M-Cdk
The active M-Cdk complex phosphorylates an inactive Cdc 25 phosphatase, activating it and leading to increased activation of M-Cdk
How is M-Cdk involved in the condensation of chromosomes? (2)
Mitotic chromosomes are made of centromere with 2 replicated sister chromatids
Cohesins hold the sister chromosomes together
M-Cdk complexes phosphorylate condensin proteins
This triggers assembly of condensin that help each
double helix to coil up into a more compact form
Put simply, how do cells stop the cell cycle at their checkpoints (if needed)?
Work by inhibiting cyclin-Cdk activity
Describe the G1/S phase checkpoint. What is the cell looking for? What is special about this checkpoint?
Occurs at the end of G1 phase
Called Start Checkpoint (yeast) or Restriction Checkpoint (Mammals)
Decide if the environment is favorable and it should divide and enter S phase
After this, it is committed to dividing
Describe the G2/S phase checkpoint. What is the cell looking for?
Occurs at the end of G2
Looks to see if all DNA is replicated and all DNA damage repaired
Describe the M phase checkpoint. What is the cell looking for (generally)?
Occurs in the middle of M
Called Spindle Assembly Checkpoint
Looks to see if all chromosomes are properly attached to the mitotic spindle
Checks metaphase to decide if it should enter anaphase
What are mitogens?
Mitogens are also called growth factors because they stimulate cells to grow and divide
Describe the involvement of mitogens and a cell protein called Rb in the G1 checkpoint
Cells that lack Rb will always progress through the G1 checkpoint
Normally, Rb is hypophosphorylated and inactivates various transcription regulators
Mitogens activate M1-Cdk and G1/S-Cdk through an intracellular signaling pathway
These phosphorylate/inactivate Rb until the end of M phase, leading to transcription and translation of proteins for cell proliferation
Describe the involvement of a cell protein called cdc 6 in DNA replication
Cdc 6 is normally found bound to an origin recognition complex (ORC) at an origin of replication
Phosphorylation of cdc 6 by S-cdk, leads to degradation and assembly of the replication fork
Replication of DNA is prevented until cdc 6 is synthesized in the following G1 phase
Describe the involvement of the cell proteins p53, p21, and Rb in DNA damage checkpoints
p53 is normally degraded in proteosomes
DNA damage leads to activation of protein kinases that phosphorylate/activate/stabilize p53
p53 then binds to the regulatory portion of the p21 gene, leading to transcription
p21 is a Cdk inhibitor protein that binds to G1/S-Cdk and S-Cdk, inactivating them and preventing them from phosphorylating/inactivating Rb
What happens in prophase? What would a picture of the cell look like?
Chromosomes condense
The mitotic spindle assembles
clump of chromosomes in the middle of the cell, unorganized mess of MTs
What happens in prometaphase? What would a picture of the cell look like?
Nuclear envelope breaks down
Mitotic spindle captures chromosomes
MTs organized into spindles, chromosomes kinda spread out
What happens in metaphase? What would a picture of the cell look like?
Chromosomes are bound in a line, cell checks before continuing
Super organized MTs, chromosomes in a pretty line
What happens in anaphase? What would a picture of the cell look like?
sister chromatids are pulled apart and migrate to opposite sides of the cell
chromosomes are pulled to sides, aster MTs are really grown like crazy
What happens in telophase? What would a picture of the cell look like?
Mitotic spindle breaks down
Chromosomes decondense
Two nuclei are formed
two clusters of chromosomes, interpolar MTs are really grown like crazy
What are the two cytoskeletal structures involved in mitosis and what are they made of?
Contractile Ring
Made of actin and myosin
Mitotic Spindle
Made of microtubules (and dynein/kinesin)
Describe the three components of the mitotic spindle
Aster microtubules
Radiate out from poles, have roles in anaphase and telophase
Kinetochore microtubules
Attach to kinetochores (protein complexes)
Interpolar microtubules
Overlap in the middle, where kinesin binds them together
How does polymerization help form the mitotic spindle?
Dynamic instability in microtubules increases at the beginning of mitosis, due to M-Cdk phosphorylation of MAPs
Bipolar spindles can form without centrosomes (chromosomes nucleate MT assembly and motor proteins organize)
How do cellular motors help form the mitotic spindle? Which are required?
Connected groups of dynein bind to the microtubules and move towards their minus end
This organizes the MTs so they radiate out from the centrosome (minus ends cluster at centrosomes)
Kinesin binds to interpolar microtubules and moves towards the plus ends
Sets of kinesin both moving towards the plus ends of opposite MTs makes them spread out and overlap slightly, stabilizing them (separates centrosomes)
Dynein is not required
Describe the two models of spindle assembly. Which is correct?
Search and Capture
During prometaphase, MTs are probing for kinetochores and are stabilized by finding them
Centrosomes control growth and shortening
Self-organization
Proteins associated with chromosomes can nucleate MT growth (such as Ran)
Motors and chromosome proteins control spindle assembly
Combined
Occurs in most cells, not always 50-50
More robust, with a lower failure rate
Describe the structure of kinetochores
Protein complex with trilaminar structure
Outer plate interacts with MT
Inner plate associates with chromatin
Proteins radiate out of fibrous corona
Some directly interact with MTs, others directly interact with DNA, some are in the middle of the complex, others are involved in mitotic checkpoints
How are kinetochores involved in metaphase?
Microtubule polymerization at kinetochores moves chromosomes to metaphase plate to equalize tension
What is the role of the APC (anaphase-promoting complex)?
Ubiquitinates an inhibitor of an enzyme that when active cleaves cohesin, which allows anaphase due to separation of sister chromatids
Also ubiquitinates M cyclin and targets it for destruction (M cyclin peaks at start of mitosis, then drops rapidly to entrance into G1)
What, specifically, is the cell looking for at the M (or spindle assembly) checkpoint? How is this checkpoint regulated?
Makes sure that all the chromosomes are bi-orientated (not mono or both on one pole)
Looks for attachment to kinetochores and tension at the kinetochore
APC is negatively regulated by Mad2 protein at unattached kinetochores
Mad2 is lost when microtubules attach to kinetochores
Unattached kinetochores attract Mad2 and convert it to something else that inhibits APC
Generally, what forces are involved in anaphase A? (focus on MTs)
Chromosomes are pulled poleward as kinetochore microtubules depolymerize, force is generated mainly at kinetochores
Generally, what forces are involved in anaphase B? (focus on MTs)
Two spindle poles move apart as the result of two separate forces
(1) The elongation and sliding of the interpolar microtubules past one another pushes the two poles apart
(2) Forces exerted on the outward-pointing astral microtubules at each spindle pull the poles away from each other, towards the cell cortex
Describe the two theories of chromosomal movement during anaphase A. Which is right? How can you tell?
The Pacman Theory: The kinetochore causes depolymerization at the positive end of microtubules, and also holds onto the microtubules
The Flux/Traction Theory: Depolymerization occurs only at the minus end, kinetochore only has to hold on
Laser bleaching can make marks on MTs, so scientists can see if the length between the mark and the pole is shortening (Flux/Traction) or between the mark and the kinetochore (Pacman)
Which method is used depends on the cell, some do Pacman, some do Flux/Traction, some do both
Describe the involvement of cellular motors in anaphase A to modify MTs
In anaphase A, kinesin family molecules use ATP to depolymerize the MT the kinetochore and at the cell cortex
Describe the involvement of cellular motors in anaphase B to modify MTs
In anaphase B, kinesin moves towards the plus end of interpolar microtubules as they polymerize at those ends, resulting in a sliding force that pushes the poles apart (keep length of overlapping region the same)
In anaphase B, dynein attached to the cell cortex tries to walk towards the minus end of aster microtubules as they depolymerize at their plus ends, pulling the poles apart
Describe the contractile ring, including structure and location. How does it generate force?
A contractile ring made of myosin and actin filaments forms at the cleavage furrow, at the center of the remaining interpolar microtubules from the central spindle
Ring is attached to membrane-associated proteins on the cytoplasmic face of the plasma membrane and generates force by sliding the actin filaments past the myosin filaments
Describe the reformation of the nucleus after telophase. (3)
Vesicles containing the old nuclear membrane cluster around individual chromosomes and fuse to re-from the nuclear envelope
Nuclear pore proteins and nuclear lamins are dephosphorylated
Once reformed, the pores pump in nuclear proteins, the nucleus expands, and the chromosomes decondense
Describe necrosis
Due to injury, hypoxia, or infection by a virus
Involves lysing that causes an inflammatory response
Generally describe apoptosis. What happens to the cell membrane?
Orderly, with an identical series of events (condensed nuclei, fragmented DNA, membrane blebbing, change in asymmetry of membrane lipids, cytoskeleton collapse) that leads to engulfment by a phagocytic cell and recycling by a lysosome
Cytosolic phospholipids are flipped to the extracellular side, which sends a signal to the phagocytic cell
What is cytochrome C?
Cytochrome C is the small, mobile electron carrier right before the last respiratory complex, and usually lives in the mitochondrial membrane
What are the things that release cytochrome C?
Factors like DNA damage or triggering by white blood cells can cause activation of Bax/Bak molecules that release cytochrome C
Bax/Bak molecules are the major determinants of apoptosis, others regulate them
How does release of cytochrome C lead to activation of the apoptotic enzymes?
Once released, it binds to other proteins to form the apoptosome, or wheel of death, that cleaves an initiator procaspase into its active form
Cleavage of a peptide bond causes two inactive procaspase molecules to form an active homodimer caspase molecule, which is a protease (cleave proteins)
What happens after cytochrome C activates the first apoptotic enzyme?
That initiator caspase then cleaves an executioner caspase and begins a caspase cascade
Each new level of the cascade leads to the production of more caspase molecules
Executioners cleave other executioner procaspases and/or cleave proteins such as the nuclear lamin and nuclear pore proteins
How can immune cells promote apoptosis?
Killer lymphocytes have Fas ligands in their plasma membrane that, when bound to Fas death receptors in the target cell, trigger the assembly of a DISC (rather than the apoptosome) that activates (different) initiator procaspases
How are survival signals involved in development?
Example: only one neuron per cell is needed, so target cells only release enough survival signal for one neuron, causing others to apoptose
This makes sure they is at least and only one neuron per cell
What is Bcl2? What are the two ways it can be activated?
Bcl2 is an anti-apoptotic protein that keeps cytochrome c in the mitochondria
Survival signals activate a RTK (receptor tyrosine kinase) that phosphoralates and activates a transcription regulator that promotes the synthesis of Bcl2
Survival signals can also active Bcl2 already present in the cell, by activation of an Akt
This causes phosphorylation and inactivation of a protein called Bad, which is an inhibitor of Bcl2
