Lecture 32 - Cell Cycle Flashcards
Overview of the cell cycle
- in G1 phase, the cell synthesizes all of the proteins, ribosomes, and other machinery it will need
- in S phase DNA is replicated
- G2 is a gap phase
- in M2, the cell divides
S and M phase
- chromosomes duplicate in S phase
- duplicated chromosomes are segregated into a pair of daughter nuclei in M phase
- nuclear division (mitosis) and cell division (cytokinesis) are collectively called M phase
Why were temperature sensitive mutants in yeast the key to understanding the cell cycle?
- showed that certain proteins are important to control transition from 1 stage to another
- cdc mutants arrest before cytokinesis => cdc is important for cytokinesis
What was the evidence to show that cell-cycle control is similar in all eukaryotes?
- if we complement the yeast cdc mutant with the human gene, cell cycle functions as normal
- 1st human/ yeast complementation
- also called rescue experiments (cell recovers to normal size
The cell-cycle control system is based on what?
- cyclically activated protein kinases
- when cyclin forms a complex with Cdk (cyclin-dependent kinase), the protein kinase is activated to trigger specific cell-cycle events
- without cyclin, Cdk is inactive
How are different events/ stages of the cell cycle triggered?
by Cdk associating successively with different cyclins
How is Cdk activity terminated?
by cyclin degradation
What are the 3 main classes of cyclin that are required in eukaryotic cells?
- G1/S cyclins: activate Cdks at the end of G1 and commit the cell to DNA replication
- S cyclins: activate Cdks during S phase and are required for the initiation of DNA replication
- M cyclins: promote the events of mitosis
What is the fourth cyclin?
G1-cyclin => controls start of G1
- not always there
Do cyclin-Cdk complexes phosphorylate different sets of substrate proteins?
yes
- M-cyclin/Cdk phosphorylates different proteins than S -cyclin/Cdk
What does phosphorylation of the cyclin/Cdk complex do?
adds another layer of control
- Wee1 kinase adds a phosphate => inactivates complex
- Cdc25 phosphatase removes a phosphate => activates complex
- a wee mutant (i.e., no wee) results in small cells that transition into mitosis quick
- a Cdc25 mutant results in elongated cells because cell never knows its time to undergo mitosis (i.e., keeps growing but doesn’t divide)
What triggers the final stages of cell division?
protein degradation; anaphase promoting complex (APC) promotes degradation of M-cyclin
Anaphase promoting complex (APC)
- activated during metaphase and promotes degradation M-cyclin
- APC polyubiquitinates M-cyclin to mark it for protein degradation
What is the central problem for a cell in M phase?
to segregate and separate its chromosomes
What does each replicated chromosome consist of at the end of S phase?
- a pair of identical sister chromatids glued together by a centromere
What molecule acts like a ring to hold the sister chromatids together?
- cohesin
- cohesin is loaded into unreplicated chromosomes and then encircles the new sister chromatids
What happens during prophase?
- the replicated chromosomes condense
- mitotic spindle starts assembling outside the nucleus
- centrosomes start separating to break the nuclear envelope
Prometaphase
- nuclear envelope breaks
- without envelope, microtubules attach to the chromosomes at kinetochore
- one kinetochore is attached to one of the chromatids (kinetochore is like the glue between the microtubules and chromosomes)
Where are kinetochore microtubules attached?
- to sister chromatids by their plus ends
- kinetochore proteins interact with centromeric chromatin
Metaphase
chromosomes align to center of spindle (i.e., cell)
What is the mitotic spindle made of?
- microtubules and their associated proteins (astral and kinetochore microtubules)
- non-kinetochore microtubules do not connect to chromatids, but connect entire structure together
What do microtubules do?
pull the daughter chromosomes towards the poles of the spindle (i.e., to opposite ends of the cell)
How do motor proteins contribute to the assembly and function of the mitotic spindle?
- kinesin-5 slides microtubules oriented in opposite directions
- kinesin-14 crosslinks antiparallel microtubules at the center and moves one of them
- kinesin-4/10 are plus directed and push chromosomes to center
- dynein pulls microtubules back to centrosome (i.e., pulls from edge of cell towards center)
What happens during anaphase?
- sister chromatids are separated into 2 chromosomes
- shortening of microtubules (i.e., catastrophe) brings the chromosome to poles
- spindles also move apart
Metaphase to anaphase transition (how does the cell know the chromosomes are properly aligned and ready to be separated?)
- Aurora B is a kinase force sensor that tells the cells the chromosomes are oriented properly
- low tension => aurora B is active
- high tension => aurora B is inactive
What does Aurora B do?
- it prevents the stable attachment of microtubules to chromosomes
- when there is tension , Aurora B is inactive
- when there is high tension (from both sides), Aurora B cannot reach target to phosphorylate
Separase
- a cysteine protease responsible for triggering anaphase by hydrolyzing cohesin
- indirectly activated by anaphase promoting complex (APC)
Telophase
- chromosomes arrive to poles
- new nuclear envelope assembles
- mitosis ends and cytokinesis starts
How does cytokinesis separate the two cells?
by using a contractile ring made of actin and mysosin (microtubules have NOTHING to do with cytokinesis)
Centrosome vs centriole vs centromere
centrosome: an organelle that serves as the organizing centre of all microtubules in an animal cell => regulates cell cycle
centriole: paired barrel-shaped organelles located in the cytoplasm of animal cells near the nuclear envelope => microtubules arranged in a certain way
centromere: the central region of the chromosome which consists of highly constricted DNA
