Cell cycle - principle of regulation Flashcards
phase of cell cycle
G1 -> S -> G2 -> M phase (mitosis)
cell cycle timing and structure
varies in different cells and organisms
mechanism control
similar in all eukaryotes
common model system
unicellular yeast
embryo of frogs/ flies
mammalian cells
making model for the mechanism of cell cycle
combine data to show principles of regulation are same
cycle progression study methods
using flow cytometry - study individual cells
using fluorescence
based on DNA content
check point of cell cycle control system
making certain decision on whether to progress
control timing and coordination of cell cycle event
system in cell cycle control
robust and reliable biochemical timer but high adaptable - can be modified
first stage - start transition
is it environmentally favourable
before S phase
prevent progression if cell growth is insufficient
Cell growth being insufficient
DNA damage
if DNA damage occurs
cell enter prolonged non-dividing state until conditions are met
stage 2 - G1/M transition
all DNA replicated? environment being favourable?
regulatory transition controlled by various factors
factors the control regulatory transition
DNA damage or completion of DNA replication
stage 3 - metaphase to anaphase transition
all chromosomes attached to spindle
initiation of sister chromatid separation could be blocked
if chromosomes are not properly attached to MT of mitotic spindle
checkpoints if something goes wrong
arrest cycle at transitions
launch events in complete reversible fashion
cell cycle control system depends on
based on cyclically activated Cdk
Cdk
cyclin dependent kinase
without cyclin, Cdk is inactive
cyclin + Cdk
complex
cyclin-Cdk complex
protein kinase is activated to trigger specific cell cycle events
different types of cyclins
form different cyclin-Cdk complex which trigger different cycle events
G1/S - cyclin
activate at start of G1/S-Cdk - progression through start
Concentration of G1/S - cyclin
concentration decreases as S-Cdk activated
S - cyclin
activate Cdk for DNA synthesis therefore remains high
Concentration of S - cyclin
increases at start and decreases at metaphase-anaphase
M-cyclin
activate Cdk for entry into mitosis
Concentration of M-cyclin
increases to mitosis and peak in metaphase
G1 concentration
increases depending on rate of cell growth of promoting signals
G1
activates Cdk stimulating entry of new cell cycle at start
function of cyclins
activates Cdks partner and direct it to specific target protein therefore complex phosphorylates different set of substrate proteins
some complex function
induce different effect at different times in cycle
as some Cdk substrate accessibility changes during cycle
regulating activities of complex
CAK
Wee 1 Cdc 25 regulatory pathway
Cdk inhibitor protein
CAK - Cdk activating kinases
3 states : inactive, partially active, fully active
CAK - inactive stage
without cyclin bound
active site blocked by region of protein (T-loop)
CAK - partially active
cyclin bind therefore T-loop moves out of active site
CAK - fully active
phosphorylation of Cdk 2 at threonine residue in T-loop activate enzyme to change shape of T-loop
to improve ability of enzyme to bind to protein substrate
Wee 1-Cdc 25 regulatory pathway function
cyclin-cdk complex can be inhibited by phosphorylation at 1/2 sites in active site of enzyme
Wee 1-Cdc 25 - inactive
phosphorylation of Tr15 by wee/ both Thr 14 and Tyr by Myt 1
Wee 1-Cdc 25 - reactivates
dephosphorylation by phosphatase Cdc 252
Cdk inhibitor protein - CKI
bind to complexes therefore inactive complex
CKI binding
large rearrangement in structure of Cdk active site = inactive
use of CKI binding
govern activities of G1/S and S-Cdks early in cycle
CKI
interacts with Cdks or Cdk-cyclin complex blocks activity during G1 or as a response to inhibitory signals from environment/damage DNA
How mechanism regulate cell cycle - e.g DNA damage
signalling pathway activated
process of mechanism regulating cell cycle
Kinase transmitted to p53
p53 phosphorylates in response to DNA damage forming stable active p53
process of using stable active p53 in regulating DNA damage in cell cycle
binds in nucleus to certain promoter of gene and bind to regulating region such as p21
all transcription of translation of p21
p21
Cdk inhibitor protein
p53
protein for major cell cycle regulator
transcription regulator
what else is involved in regulation of cycle
signal transduction via multi-step signalling pathway
regulated proteolysis process of forming polyubiquitin chain
Ub transferred to E1 - activated
and then it is transferred to E2
covalently attached to target protein by E3 Ub ligase forming polyubiquitin chain
regulated proteolysis process after formation of polyubiquitin chain
26s proteasome recognises polyubiquitylated protein and destroys it
Ub
ubiquitin
E1
ATP dependent manner
process of destroying polyubiquitin chain
ATP dependent
E3 Ub ligase has 4 classes
HECT-type
RING-finger type
PHD-finger type
U-box type
RING finger type subfamily
cullin based E3s
such as SCF - SKP1-CUL1-F-box protein
or APC/C - Anaphase-promoting complex/ cyclosome
SCF
ubiquitylates substrate - late G1 to early M
SCF activators
Skp2
FBW7
beta-TRCP
APC/C
active in mid M (anaphase) to late G1
APC/C activators
Cdc20 Cdn1
control of mitosis by APC/C
regulated proteolysis
M-Cdk in controlling mitosis
promotes early mitosis and activation of APC/C(Cdc20) triggering anaphase and mitotic exit - stimulating destruction of regulatory proteins
cyclin destruction
Cdk inactivation APC/C(Cdc20) and triggers activation of APC/C(Cdn1) - continued APC/C activity in G1
APC activiation
triggers sister-chromatid separation at M to A tranisiton
what APC/C does in controlling mitosis
destruction of S and M cyclin therefore inactivation of Cdk
therefore completion of mitosis and cytokinesis
