Cell Cycle Flashcards
homeostasis in adult cells
- Adult (differentiated) cells - a steady state system
o DNA constantly reads out into a particular set of RNAs –> proteins
o Cell neither grows, shrinks or changes much
o Rate of cell death = rate of cell division - Cell proliferation controlled by the cell cycle - an ordered series of events leading to replication of cells - a dynamic system
- Cell death - achieved through apoptosis
cell differentiation
- Cells need to balance growth with differentiation
- Cells can undergo differentiation only when they are not cycling
- This is an important process during all stages of development
- embryo –> adults
- As cells age they need to be replaced- old cells die by apoptosis and new cells are replaced by proliferation
eukaryotic cell cycle
- Interphase - period between cell divisions
- cell grows, develops & prepares for division (replicates DNA)
- M Phase – mitosis
- process of nuclear division & cytokinesis (cytoplasmic division)
cell cycle summary
Interphase: Period between mitoses has 3 distinct phases: G1, S and G2
- G1: Gap phase between birth of new cell (M) and chromosome replication (S) − cell grows in preparation for the next mitosis
- S: Synthesis phase − DNA is replicated to make an identical copy of each chromosome known as sister chromatids (4n in diploids; 2n in haploids)
- G2: 2nd Gap phase − cell prepares for mitosis
- M Mitosis − cell segregates one copy of each chromosome into each new daughter cell
- Checkpoints prevent transition to the next phase until current phase steps have been completed and mistakes corrected.
G0 Phase
- During G1, cells may exit the cell cycle in response to regulatory signals & enter a non-dividing (quiescent) state where the cell usually maintains a constant size.
- Cells may stay in G0 for an extended period, or even indefinitely.
- Cells may re-enter G1.
- Not all cells enter G0.
G1 (Gap 1) Phase
- Gap phase before S phase
- Cell grows, undertakes normal metabolism & synthesizes proteins needed for cell division
- Contains ‘start’ checkpoint of cell division - also known as the restriction point
o Passing ‘start point’ commits cell to cell division - Cells can exit cell division prior to ‘start’
S Phase
- Synthesis phase
o DNA is replicated
o Amount of DNA doubles ie 2n –> 4n
o Nucleus becomes enlarged
G2 (Gap 2) Phase
- 2nd Gap phase after S phase
- Further growth of cell
- Cell prepares to divide
M Phase
Mitosis
o Chromosomes condense
o Nuclear envelope disintegrates
o Chromosomes attach to mitotic spindle & move to midpoint in cell
o Sister chromatids pulled to opposite poles of cell
o Nuclear envelope reforms
o Cytokinesis (C ) – 2 daughter cells are formed - has 4 discreet phases
stages of mitosis (PMATC)
- Prophase: Chromosomes condense, nuclear envelope breaks down by retracting into the ER, spindle poles duplicate, and microtubules form the mitotic spindle apparatus.
- Metaphase: Spindle microtubules from each pole attach to chromosome kinetochores and center sister chromatid pairs in the spindle.
- Anaphase: Spindle microtubule shortening and motor proteins pull each sister chromatid toward an opposite spindle pole.
- Telophase: Chromosomes decondense, and each presumptive daughter cell reassembles a nuclear membrane around its chromosomes.
- Cytokinesis: Cell divides into two daughter cells.
centrosomes
- Found in animal cells only
- Produces microtubules, also known as microtubule organising centre (MTOC)
- Contains two centrioles at right angles where microtubules assembled
- One centrosome per cell in interphase
kinetochores
- Microtubule binding structure
- One per chromatid
- Either side of centromere
metaphase plate
- Equidistant between two spindle poles
- Chromosomes are tugged back & forth until they are ‘centralised’ between the poles of the cell
cytokinesis
Animal cells: cleavage furrow forms at equator of cell & pinches inward until cell divides in two. Is formed by a system of actin filaments pinching in as a contractile ring.
checkpoints in the cell cycle
As the cell passes through the cell cycle, control is exerted at certain checkpoints:
1. R-restriction point (external checkpoint-late G1)
o Where the CC becomes independent of mitogens. Regulated by growth factors, environment, oxygen
o eg, if conditions are unfavourable, CC will arrest.
- After S phase-internal checkpoint (G2)
o Monitors whether DNA replication is complete & if DNA damage has been repaired. Mitosis will not proceed if the cell ‘fails’. - During mitosis (Metaphase)
o Checkpoint identifies whether chromosomes correctly attached to spindles & spindles formed properly.
