w11 txtbk Flashcards
cell cycle
process where cell duplicates its contents and divides into 2
M phase
period of the eukaryotic cell cycle during which the nucleus and cytoplasm divide to produce 2 daughter cells
interphase
long period of the cell cycle b/w one mitosis and the next
-includes G1 phase, S phase and G2 phase
S phase
s=synthesis
period during euk. cell cycle where DNA is synthesized
-flanked by 2 “gap” phases called g1 and g2 during which the cell continues to grow
g1 phase
falls b/w end of cytokinesis and start of DNA synthesis
g2 phase
falls b/w the end of DNA synthesis and the beginning of mitosis
what would happen if interphase lasted only long enough for DNA replication
the cell wouldn’t have time to double its mass before it divided
cell-cycle control system
guarantees that the events of the cell cycle
(DNA rep., mitosis…) occur in a set sequence and that each process finishes before next one starts
-to control this, the system is regulated at certain critical points of the cycle by feedback from the process currently being performed
–cannot be delayed
how is mitosis or cell division delayed
if needed, control system employs a set of molecular brakes called checkpoints, to pause the cycle at certain transition points
-this way, it doesn’t trigger next step until cell is properly prepared
late g1 phase
control system confirms that the environ. is favorable for proliferation before moving on to replicate DNA
if conditions unfavorable:
-cells can delay entry into cell cycle and can enter specialized resting state called G0
sometimes referred to as start
transition from G2 to M phase
control system confirms the DNA is undamaged and fully replicated
-ensuring the cell doesn’t enter mitosis unless dna is intact
transition midway through mitosis
cell cycle control machinery confirms the duplicated chromosomes are properly attached to cytoskeletal machine (called mitotic spindle)
-before the spindle pulls the chromosomes apart and segregates them into 2 daughter cells
why is the g1 phase important in animals
point in cell cycle where control system is regulated by outside influences
-signals from other cells stimulate cell proliferation when more cells are needed and block it when not needed
explain the different parts of the cell cycle
s phase- cell replicates dna
m phase- where mitosis occurs, followed by cytokinesis
g1 and g2- 2 gap phases between S and M phases, when the cell continues to grow and make preparations for the next phase
cell-cycle control system
network of regulatory proteins that govern the orderly progression of a eukaryotic cell thru stages of cell division
how does the cell cycle control system govern the cell cycle
by cyclically activating and then inactivating the key proteins and protein complexes that initiate or regulate dna rep, mitosis and cytokinesis
this regulation carried out thru the phosphorylation and dephosphorylation of proteins (switching the activity of a protein on and off)
phosphorylation rxns that control cell cycle
carried out by protein kinases, while dephosphorylation is performed by a set of protein phosphatases
kinases at the core of the cell-cycle are present in proliferating cells
-activated only when needed
-activity of each occurs in cyclical fashion
cyclin function
regulatory protein whose conc. rises and falls at certain times during cycle
-help control progression from one stage of cell cycle to next by binding to cyclin-dependent protein kinases (dks)
cyclin-dependent protein kinase (Cdk)
enzyme that when complexed with regulatory cyclin protein can trigger various events in the cell-division cycle by phosphorylating specific target proteins
-the cyclical changes in cyclic concentrations help drive the cyclic assembly and activation of the cyclin - Cdk complexes
-once activated, cyclin-Cdk complexes help trigger cell cycle events (eg entry into S or M phase)
what occurs in transition points throughout cycle
control system monitors cell’s internal state and external conditions before allowing cell to continue thru cycle
For example, it allows entry into the cell cycle and initiation of S phase only if environmental conditions are appropriate; it triggers mitosis only after the DNA has been completely replicated; and it initiates chromosome segregation only after the duplicated chromosomes are correctly aligned on the mitotic spindle
how does the control system monitor and slow progress at transition points
at start transition in late G1 phase, it uses Cdk inhibitors to keep cells from entering cell cycle
at G2 to M transition, it suppresses activation of M-Cdk by inhibiting the phosphatase required to activate Cdk
-can also delay chromosome segregation in mitosis by inhibiting the activation of APC/C
=prevents degradation of M-cyclin and other
regulators
how do cyclin-Cdk complexes trigger different stages of cell cycle
kinase activity of cdk subunit increases at different stages of cycle
g1 phase
important time of decision-making for cell
-based on intracellular signals that provide info
about the size of cell and extracellular signals reflecting conditions in the environ,
machinery can either hold cell transiently in G1, direct it to withdraw into a more prolonged nonproliferative state(G0)
or
allow it to prep entry into another cell cycle
if errors occur during dna rep, how does the cell keep from dividing with dna thats incorrectly or incompletely replicated
cell control system uses mechanism that can delay entry into M phase
-for cell to progress into mitosis, inhibitory phosphates must be REMOVED by activating protein phosphatase called Cdc25
–if DNA rep stalls, the presence of single-stranded DNA at replication fork triggers DNA damage response
-part of this response includes inhibition of the phosphatase Cdc25, which PREVENTS the removal of the inhibitory phosphatases from M-Cdk
–as a result, M-Cdk remains inactive and M phase delayed until DNA rep is complete and any DNA damage is repaired
