unit 8 - cell cycle Flashcards
how long does it take to complete the cell cycle?
24 hours
* G1: 12 hours
* S: 7 hours
* G2: 4 hours
* M: 1 hour
how much of the cell’s life is spent in interphase?
90%
G1 phase (first gap)
kind of the default stage. after cells are produced by mitosis, they grow until they reach their mature size. acquire nutrients, oxygen, and signals from the environment. continue in G1 phase until a signal (growth factors and hormones, mitogens) is recieved to induce division (unless conditions are unfavorable– low oxygen, too much waste, not enough nutrients, high pH. in this case, they remain in G1 phase or enter G0)
G0 phase (resting phase)
- cells without growth factors can enter G0 and remain there for long periods of time without proliferating
- neurons, muscle cells are usually in G0, other cells switch on and off
S phase (synthesis phase)
G2 phase (second gap)
- cell replenishes energy stores
- synthesizing of proteins needed for chromosome condensation/movement
- some cell organelles are duplicated
- cell contents are reorganized in preparation for mitosis
M phase (mitotic phase)
- prophase
- prometaphase
- metaphase
- anaphase
- telophase
cytokinesis
separation into two daughter cells. happens via a cleavage furrow in animal cells, which is caused by a contractile ring of actin as it treadmills and pinches the membrane in two
cell division in embryonic cells
- early embryo cells divide much more rapidly, in only 30 minutes
- formation of blastocyst
- skip the G1 and G2 phases and alternate between M and S phases. since they don’t have time to grow in between the cells get smaller and smaller over the rounds of cell division
control of the cell cycle
- regulatory proteins
- cell-cycle checkpoints, with molecular breaks that can stop the cycle to prevent the next cell from proceeding too soon:
1. G1/S checkpoint: cell monitors size (has it grown big enough?), DNA integrity (is any DNA damaged and in need of repair?), environmental conditions (enough nutrients and oxygen, pH, waste) and signalling (presence of growth factors?). if something is wrong, there isn’t a ‘proceed’ signal and the cell goes into G0
2. G2/M checkpoint: cell monitors DNA synthesis (did all of the DNA copy?) and checks for damage and mutations. if something is wrong, cell attempts to complete DNA replication/repair, and undergoes apoptosis if that doesn’t work.
3. M checkpoint: determines if all sister chromatids are correctly attached to the spindle microtubules, pauses the cycle until the kinetochores of each pair of sister chromatids are anchored to at least two microtubules on opposite ends of the cell. does this by scanning for straggler chromosomes that are floating instead of attached to microtubules.
what is the most important regulator of the cell cycle?
cyclins and cyclin-dependent kinases
ADD MORE
how are cyclin-CDKs turned off?
- degrade cyclins - ubiquitin tags are attached to cyclin to target them to proteosomes, where they are degraded. cyclin levels fall and CDK returns to inactive form
- block cyclins/CDKs with inhibitors - also known as tumor supressor proteins, most commonly in Ink4 or Cip/Kip families
CDK inhibitor p21
production of p21 is controlled by transcription factor p53
* act on the G1 checkpoint
* if DNA damage is detected, p53 halts cell cycle and recruits enzymes to repair the DNA, or p53 triggers apopsosis if the DNA can’t be repaired
*missing or defective/mutated p53 leads to unregulated replication of damaged DNA, which can lead to cancerous cells or mutated proteins. defective p53 is found in half of all cancers.
apoptosis vs. necrosis
- necrosis: cell ‘explodes.’ happens due to outside threats like venom or burns. triggers inflamation when the cell contents spill out
steps of necrosis: - apoptosis: DNA condenses, organelles shrink, cell sends out an eat-me signal to phagocytes but stays intact
intrinsic pathway of apoptosis
1. Bcl family proteins
extrinsic pathway of apoptosis
1. FasR receptor
cytoskeleton degredation
DNA cleavage
FIX.
caspases
protease/degradase enzymes responsible for apoptosis.
* digest the cell from the inside
* cleaves nuclear lamina
* activates endonucleases that digest DNA and RNA
* degrades cytoskeleton, causes blebbing of plasma membrane and fragmentation, which triggers the formation of apoptotic bodies
*cleaves organelles
blebbing
activation of caspases - intrinsic pathway
activation of caspases - extrinsic
