Cell Cycle I Flashcards
What phases collectively make up interphase?
G1, S, G2
What are the four phases of the cell cycle that occur sequentially and unidirectionally?
1 - Gap phase 1 (G1)
2 - Synthesis phase (S)
3 - Gap phase 2 (G2)
4 - Mitotic phase (M)
Cell reproduction begins with what?
the duplication of the cell’s components, including the exact duplication of each chromosome during S-phase
These are dividied equally between two daughter cells in M-phase
What are the G1 phase events?
- important regulatory period
- cells either commit to division during this time or exit the cell cycle (called G0 phase or qiuescence)
- most cell growth occurs during this phase, must be coordinated with cell division
When is the first major cell cycle checkpoint? What happens here?
the G1/S-checkpoint or Start (aka Restriction Point)
- this is the point of no return; cells commit to cell cycle progression after passage through Start
What are the events of the S-phase?
- important regulatory period
- human cells need a similar amount of time in S-phase as they do in G1
- chromosome replication
- synthesize histones needed to compact the newly made chromatin
- deposit cohesin along the chromosome arms to keep sister chromatid pairs together
- duplicate the centrosome
What is the centrosome?
the microtubule nucleating center of the cell
*two centrosomes then facilitate assembly of mitotic spindle in M-phase
What occurs in G2-phase?
- this is the transition period for the cell before committing to mitotic entry
- during this time, cells continue to grow
- G2 ends with a second major checkpoint, the G2/M checkpoint
When is the second major checkpoint of the cell cycle? What is the ‘commitment’?
- the G2/M checkpoint
- passage through this checkpoint is the next point of no return, and cells will end the division process
What are the six sub-phases of M-phase (mitosis) events?
1 - prophase 2 - pro-metaphase 3 - kinetochores on the chromosomes 4 - metaphase (within which is the 3rd major checkpoint - Spindle Assembly Checkpoint SAC) 4 - Anaphase 6 - Telophase 7 - Cytokinesis
Where does prophase stand in the M-phase? What occurs here?
- first step, before pro-metaphase
- chromosome condensation occurs through the action of the condensin complex
Where does pro-metaphase stand in the M-phase? What occurs?
- second step, after prophase, before kinetochores on the chromosomes
- nuclear envelope breakdown occurs and microtubules that grow from the two centrosomes invade the nuclear space in search of capturing
What happens after the nuclear envelope breakdown occurs and microtubules grow from the two centrosomes invading the nuclear space? What are the microtubules looking for?
The microtubules look for and capture kinetochores on the chromosomes
Where in the sequence of M-phase does metaphase occur? What are its events?
- after pro-metaphase and the capturing of kinetochores of the chromosomes by microtubules growing from the two centrosomes
In metaphase:
- a diamond-shaped mitotic spindle forms and microtubules (which make up the spindle) move all chromosomes to the metaphase plate in an event known as congression
- cells must pass the 3rd major checkpoint before transitioning to anaphase (Spindle Assembly Checkpoint, SAC)
When does the Spindle Assembly Checkpoint occur?
- during metaphase of the M-phase
- it is the third major checkpoint
- cells must pass through this checkpoint to move on to anaphase
What is congression?
the movement of all chromosomes (by microtubules) to the metaphase plate
When does anaphase occur in the M-phase? What are the events?
- after cells have gone through metaphase and the Spindle Assembly Checkpoint
- sister chromatids disjoin and move toward opposite spindle poles
When does telophase occur in the sequence of M-phase? What are its events?
- after anaphase (sister chromatids have separated and move toward opposite spindle poles) and before cytokinesis
- cells exit M-phase as chromosomes decondense and nuclear envelopes reform around them
When does cytokinesis occur during M-phase? What are its events?
- after telophase (chromosomes have decondensed and nuclear envelopes reformed around them)
- an actin-myosin ring (aka the cleavage furrow) forms at the cell equator and contracts to pinch the cell into two smaller daughter cells
How do small organelles (such as mitochondria and lysosomes) form and separate into the two new cells during cell division?
small organelles are abundant and evenly distributed during distribution
How do large organelles (endoplasmic reticulum, Golgi apparatus) form and separate into the two new cells during cell division?
large organelles are fragmented into smaller vesicles during M-phase and reform in the next G1 phase
how long does it take most human somatic cells to divide in culture? Where is most of this time spent?
- 18-20 hours
- most of time is spent in G1 and S phases
- less than an hour in M-phase
How have early embryos evolved to meet the demand for rapid cell division?
they have evolved to meet this need by eliminating the Gaph phase (G1 and G2)
*division necessary every hour or less
What is the significance of the first checkpoint (G1/S)?
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