What can cause the cell to be arrested in G2 during the cell cycle?
Incomplete replication because the Cdc25 phosphatase is inhibited and inactivates the M-Cdk
When the cell is arrested in G2 due to incomplete DNA replication, what is the binding of M cyclin accompanied by?
The phosphorylation of of the mitotic Cdk by Wee1, an inhibitory kinase
What does the phosphorylation of the mitotic Cdk by Wee 1 do?
Inactivates the M Cdk
When will Cdc25 phosphatase be activated?
Only if the DNA damage is repaired and if the replicated DNA is ok
What will happen if there are still double stranded nicks on the DNA at the end of G2?
The phosphatase is inhibited and the DNA will undergo homologous recombination to repair
What does Cdc25 do once it is activated after DNA damage repair?
It will remove the phosphate group from the M-Cdk and the M-Cdk/M-cyclin complex is active
How does M-Cdk activate more M-Cdk?
Indirectly through a positive feedback loop
What do active M-Cdks do that activates more M-Cdks?
Phosphorylate and activate Cdc25
Describe the positive feedback loop that activates more M-Cdks
Activated Cdc25 phosphatases dephosphorylate and activate M-Cdks, which in turn activate more Cdc phosphatases by phosphorylating them, and these Cdc phosphatases in turn activate even more M-Cdks
What do M-Cdks do to Wee1?
Inhibit Wee1 by phosphorylating it
What are the 2 processes in M phase (list in order)?
1) Mitosis 2) Cytokinesis
List the phases of mitosis:
1) prophase 2) prometaphase 3) metaphase 4) anaphase 5) telophase
What is mitosis?
Nuclear division
What is cytokinesis?
Cytosolic division
What is longer, M phase or Interphase (G1, S, G2)?
Interphase is longer
What structures does M phase require?
Two transient cytoskeletal structures
What induces the assembly of the mitotic spindle?
M-Cdk activity
What is the function of microtubules in the mitotic spindle?
Separation of sister chromatids between 2 daughter cells
What else does the cell use to separate the cytoplasm once the chromosomes have separated as the cell progresses through M phase?
A contractile ring composed of actin and myosin filaments
What is the contractile ring composed of?
Actin and myosin filaments
In what cells is the contractile ring present in?
Only animal cells, not plant cells
What is involved in plant cytokinesis instead of the contractile ring?
The Golgi
At what phase does centrosome duplication start and stop?
Starts at S phase and ends by G2: the molecules that are present in the centrosome will be transcribed or translated during S phase and continue into G2
When can transcription and translation occur during cell division?
In all three phases of interphase: G1, S, and G2
What does the mitotic spindle assemble between?
2 centrosomes
What is important about the centrosomes in relation to microtubules?
They are the major microtubule organizing center in the cell
How many condensins do most eukaryotes have and what type are they?
2: one nuclear and one cytoplasmic
What begins to happen during prophase?
The mitotic spindle starts to assemble between 2 centrosomes
What do condensins do in prophase?
Start to condense the chromosomes
What are condensins?
Proteins that condense chromosomes in prophase stage of mitosis
What do M-Cdks do to condensins and when?
During M phase, they phosphorylate condensins to trigger their assembly and condense chromosomes
What proteins are condensins similar to and how do they differ?
Similar to cohesins but different in that they have a different function than cohesins in preparing the chromosomes for mitosis. Cohesins will keep the sister chromatids together after DNA replication in S phase. Condensins pack DNA into smaller structure
How do condensins pack chromosomes together?
By introducing supercoils into the DNA
What protein has an opposite function than a condensin?
Topoisomerase because it removes supercoils that might have been caused by helicase activity by introducing a nick in a single DNA strand
How are centromeres different from telomeres?
They are more internal and are recognized by/attack to the mitotic spindle, while telomeres are at the end of DNA
What is broken down during prometaphase?
The nuclear envelope (in both plant and animal cells)
How is the nuclear envelope broken down during prometaphase?
The nuclear lamina right under the nuclear envelope (which normally reinforces the envelope) is composed of lamin intermediate filaments and nuclear pore proteins that are targeted by M-Cdks and phosphorylated. This phosphorylation of the nuclear envelope
What does the breakdown of the nuclear envelope allow for?
The mitotic spindle microtubules to gain access to the chromosomes and the chromosomes begin to attach to the spindle
What are centromeres recognized by and how?
The centromeric region of DNA has special sequences that are recognized by kinetochore proteins
What happens once the kinetochore proteins recognize the centromeric region of DNA in the chromosomes?
The kinetochore proteins either bind or be captured by the plus end of the spindle microtubules
What do kinetochore microtubules do?
Interact with and bind to the kinetochore proteins that are bound to the special sequences that define the centromeric regions of the chromosome
How many microtubules attach to each kinetochore complex?
Many attack to each complex, about 15 to 35 microtubules per kinetochore in mammals
What types of interactions exist between kinetochores and microtubules?
Dynamic interactions
What generally happens at the metaphase stage?
Chromosomes will start to align at the equator (middle) of the mitotic spindle (metaphase plate)
What checkpoint is present in metaphase?
The mitotic checkpoint, which is also the spindle assembly checkpoint
What does the cell check for at the mitotic spindle/spindle assembly checkpoint?
Whether or not all the chromosomes are attached to a spindle microtubule and are aligned at the metaphase plate at the equator of the mitotic spindle
What can delay the transition from metaphase to anaphase?
A delay of even a single chromosome arriving at the metaphase plate
What begins anaphase?
The cleavage of cohesins
While the sister chromatids are aligned at the equator of the mitotic spindle, what two enzyme are bound to each other?
Securin and separase
What is the function of securin?
Securin is an inhibitory protein that inhibits the separase enzyme
What is the function of separase?
Separase is an enzyme that is a protease and eventually degrades cohesin
Once the cell is beyond the mitotic checkpoint and knows that the chromosomes are aligned properly, what happens?
A ubiquitin ligase called APC (anaphase promoting complex) can ubiquitinate securin, thereby degrading secruin
What happens once secruin is degraded by APC?
Separase is released from inhibition, activated, and will cleave cohesin complexes
What happens one the cohesin complexes are cleaved?
Sister chromatids will separate and go towards each opposite spindle pole (sister chromatids called chromosomes) and anaphase is set to occur
What else does APC ubiquitinate and what is the result?
M cyclins and S cyclins, and the result is their degradation and the inactivation of Cdk
What happens at telophase?
The nuclear envelope is reassembled around the chromosomes at each spindle pole, and chromosomes decondense at the end of telophase
At what phase does cohesin activity occur?
S phase to keep the sister chromatids together after DNA replication
At what phase does histone duplication occur?
S phase
When are condensins active?
During prophase of M phase where DNA is condensed
What type of filaments are nuclear lamins?
Nuclear localized intermediate filaments and they are phosphorylated by M-Cdk during nuclear envelope breakdown
What triggers the reassembly of the nuclear envelope?
The dephosphorylation of the nuclear lamins and nuclear pores by a phosphatase after the mitotic spindle starts to disassemble
What happens once nuclear lamins and pores are dephosphorylated?
Nuclear envelope vesicles sequester and reassemble around chromosomes (at telophase)
What type of protein is required in nuclear envelope reassembly?
A phosphatase to dephosphorylate
What happens after vesicles cluster sequester and reassemble around chromosomes?
Nuclear pores are inserted into the nuclear envelope, nucleus expands, and chromosomes decondense. There is continued fusion until the cell is back into interphase.
When does cytokinesis occur?
At the end of telophase
What is cytokinesis?
Cytoplasm is being divided into 2 by a contractile ring composed of actin filaments and myosin filaments
How does the contractile ring divide the cytoplasm in two?
Forms a cleavage furlough that will divide the cytoplasm
What has already happened by the time cytokinesis occurs?
The nuclear envelopes are completely reformed around each daughter nucleus which will have the condensing chromosomes.
When does the contractile ring that will divide the cytoplasm during cytokinesis form?
Will form near the end of anaphase and continue to form through telophase and after telophase
What does the positioning of the contracting ring in cytokinesis depend on?
The mitotic spindle
When will the centrosome be duplicated and what does this process require?
During S phase and continue into G2 phase: requires G1/S Cdk and S Cdk
When do the duplicated centrosomes begin to separate?
Beginning of M phase (prophase): one centrosome will go to one side of the nucleus and another goes to the opposite end
What does each centrosome at the opposite end of the nuclear start to form?
Each will nuclear/initiate the creation of an aster of microtubules
What happens to the aster of microtubules as prophase progresses?
They will start to form the mitotic spindle
How will some of the microtubules beginning to form the mitotic spindle interact with each other?
Some will interact to form a bipolar spindle
What is still present at the point where the spindle is just starting to be formed?
Still have the nuclear envelope and chromosomes that are condensing
What happens as the cell goes into prometaphase in regards to mitotic spindle assembly?
The nuclear envelope is broken down and some of the microtubules will capture the chromosomes by binding the kinetochores that recognize the centromeric regions of the chromosomes
What happens as the cell goes into metaphase with regards to mitotic spindle activity?
There is the formation of the metaphase plate/spindle where the chromosomes will align at the equator in the middle of the spindle
What does mitotic spindle assembly depend on?
The dynamic instability of microtubules
Describe the dynamic instability of microtubules:
Microtubules grow and contract at the plus ends rapidly and often. Microtubules will grow rapidly as they add tubulin dimers that are bound to GTP and they shrink rapidly as they undergo GTP hydrolysis
What happens at some point as the microtubules grow and shrink rapidly, exploring the cell interior?
Eventually some of microtubules will interact in the center and the interactions are stabilized by the activity of motor proteins and other proteins that will crosslink the microtubules and stabilize the plus end
What are interpolar microtubules?
Microtubules the are stabilized by motor and other proteins
Where do the interpolar microtubules interact with each other?
At the overlap zone
What are the microtubules not in the overlap zone and not stabilized by motor/other proteins called?
Aster microtubules
How are interpolar molecules stabilized at the overlap zone?
They crosslink the microtubules to stabilize the plus end products
What are the three classes of microtubules?
Aster, kinetochore, and interpolar microtubules
What do interpolar microtubules do?
Overlap at plus ends and stabilized at middle of spindle by motor proteins
Which motor proteins stabilize interpolar microtubules?
Kinesins (walk toward plus ends) and dyneins (walk toward minus ends)
What do aster microtubules do?
Grow and shrink through GTP hydrolysis
What do kinetochore microtubules do?
Capture chromosomes by binding to kinetochore proteins that recognize the centromeric regions of the chromosomes (chromosomes are also helping to form the spindle)
What processes are involved in mitotic spindle formation?
Continuous polymerization and depolymerization of microtubules
How is it known that microtubules continuously polymerize?
Addition of colchicine prohibits the polymerization of microtubules by prohibiting the addition of tubulin dimers to the plus end of the microtubule and ultimately the addition of colchicine leads to the loss of the metaphase spindle/plate
What does addition of colchicine do to the microtubules?
Prohibits the polymerization of microtubules by prohibiting the addition of tubulin dimers to the plus ends: loss of the metaphase spindle
What is involved in the alignment of chromosomes along the middle of the mitotic spindle?
A tug of war between kinetochore microtubules that generates tension
Describe the tug of war between kinetochore microtubules:
Each kinetochore will be tugging at sister chromatids or the chromosome as the pair of kinetochore microtubules position sister chromatids/the chromosome along the middle of the spindle. As soon as the chromosome is at the middle of the spindle, there is
What is a signal to the cell that the sister chromatids have attached to the mitotic spindle and are ready for anaphase?
The equal tension by the opposing kinetochore microtubules on the chromosome as it is aligned in the middle of the mitotic spindle
What will happen if there is even a single unattached chromosome on the mitotic spindle?
The cell cycle control system will arrest the cell at the mitotic spindle assembly checkpoint and anaphase does not occur.
When does the mitotic spindle begin to disassemble?
During anaphase and anaphase has 2 parts
What happens during anaphase A?
Kinetochore microtubules pull apart the sister chromatids toward the opposite spindle poles
What happens to the kinetochore microtubules during anaphase A as they pull apart the sister chromatids?
there is a loss of tubulin subunits at both the plus ends of those kinetochore microtubules as they pull the chromosomes toward each pole (microtubule shortening)
What happens in anaphase B?
Kinesis push interpolar microtubules pull away from each other, essentially pulling the interpolar molecules toward each spindle pole. Then, dyneins pull the poles away from each other toward the cell cortex.
What do kinesins do during mitotic spindle disassembly?
Push interpolar microtubules away from each other toward each spindle pole
What do dyneins do during mitotic spindle disassembly?
Pull the poles away from each other toward the cortex
What also happens to interpolar microtubules as the process of mitotic spindle disassembly continues?
Microtubule growth at the plus ends
What type of forces are present as interpolar microtubules are pushed and pull apart?
Sliding forces and pulling forces
What does the mitotic spindle determine during cytokinesis?
The initial plane of cytoplasmic cleavage
What do the remaining interpolar microtubules do in regards to contractile ring formation that is needed for cytokinesis?
Activate proteins that signal the cell cortex to assemble the contractile ring during anaphase
What side of the plasma membrane is the contractile ring associated with?
The cytoplasmic face of the plasma membrane
What is the orientation of the cleavage furlough?
The furlough formed by the contractile ring is perpendicular to the mitotic spindle
What happens to the contractile ring as cytokinesis progresses?
Gets smaller as cytokinesis progresses and disassembles once the cell has been cut in two
What is involved in plant cytokinesis?
Formation of a new cell wall
What is absent in plant cytokinesis?
The presence of a contractile ring
What do remnants of interpolar microtubules become in plant cytokinesis and what do they do?
Phragmoplast microtubules that guide the Golgi-derived vesicles towards the equator (center) of the mitotic spindle
What do the Golgi-derived vesicles contain in plant cell wall cytokinesis?
Cell wall material
What happens to the Golgi-derived vesicles once they are aligned at the equator of the spindle?
Fuse to form the cell wall
What allows for the inheritance of enough mitochondria during cell division?
Biogenesis and fission
How is the ER distributed among daughter cells?
Cut in two during cytokinesis
How is the Golgi distributed among daughter cells?
Fragmented and then it hitches a ride along the spindle microtubules via motor proteins into the daughter cells
How are other organelles and soluble proteins distributed during cell division?
Randomly inherited
Why is cell shape change necessary for cell division?
Needed for detachment (during division) and attachment (at the end of division) to neighboring cells and the extracellular matrix
What is one mechanism of control of cell numbers?
Apoptosis
Why do the number of cells in multicellular organisms need to be controlled?
Needed during development: for example, the web between fingers consists of cells but those cells need to die at some point or else humans will have webbed fingers.
What else is apoptosis needed for?
Getting rid of damaged cells
What organelle is involved in apoptosis?
Mitochondria
What type of signals inhibit Bcl2? What else do these signals do?
Damage extracellular signal or tissue development factors; activate Bad and Bad will then bind to and inactivate Bcl2
What happens in the cell death process once Bcl2 is inactivated?
Bax/Bak are activated and cause the release of cytochrome C, a mobile electron carrier present in the intermembrane space of the mitochondria, into the cytoplasm
What happens once cytochrome C is in the cytoplasm?
It activates an adaptor protein by binding to is and this leads to the assembly of the apoptosome
What occurs as a result of the binding of the adaptor protein to the cytochrome C?
The formation of a larger complex of adaptor-protein/cytochrome C molecules and the recruitment of procaspases to the apoptosome
What is the function of the procaspases in apoptosome formation?
The procaspases (procaspase 9 specifically) become active once bound to the complex and now function as the initiator caspases that trigger a caspase cascade
What happens as a result of the caspase cascade?
Other caspases like Executioner caspases are activated and cleave cellular molecules that include DNA (leads to apoptosis)
What do survival factors suppress and what is an example?
Apoptosis; example Akt that inhibits the proapoptotic Bad protein
What can growth factors do?
Stimulate an increase in cell size and mass: cell growth coupled with cell division in many cases
What is the function of mitogens?
Promote cell division by overcoming the block between G1 and S phase