ch 12: cell cycle control Flashcards
what must happen for cells to proceed to different phases of the cell cycle?
they have to pass checkpoints
G0 exit
differentiated cells exit the cell cycle and do specific functions in the tissue they reside
what are the 3 checkpoints in the cell cycle?
- G1 checkpoint
- G2 checkpoint
- Metaphase checkpoint
G1 checkpoint
cell passes checkpoint if:
- cell size is big enough
- nutrients are sufficient
- social signals are present
- DNA is undamaged
G2 checkpoint
cell passes checkpoint if:
- chromosomes have replicated successfully
- DNA is undamaged
- activated MPF is present
Metaphase checkpoint
cell passes checkpoint if:
- chromosomes have attached to spindle apparatus
- chromosomes have properly segregated and MPF is absent
when does the G1 checkpoint take place?
after the first growth phase and before S phase
when does the G2 checkpoint take place?
after S phase and before mitotic phase
when does the metaphase checkpoint take place?
in between metaphase and anaphase
why are the 2 reasons it is important for cells to get checked before dividing?
- if a cell with a mutation continues to divide, all new cells made from that cell will inherit that mutation
- maintaining specific cell density ensures organism can keep up with resource demands
what two proteins are needed to pass G1 checkpoint?
cyclin and E2F proteins
what are the steps of G1 checkpoint?
- growth factors arrive from other cells
- cells produce cyclin and E2F after stimulation by growth factors
- cyclin binds to Cdk and becomes phosphorylated. Rb inactivates E2F by binding to it
- Cdk phosphorylates Rb after inactivating phosphate is removed.
- phosphorylated Rb releases E2F
- E2F triggers production of S Phase proteins
is the cyclin dependent kinase (Cdk) always present?
yes but it is not active until cyclin proteins are made
what is the purpose of the Cdk?
it phosphorylates and activates target cells to reach S Phase
what happens if the DNA is damaged at the G1 checkpoint?
p53 creates Cdk inhibitors which does not allow the cell divide
what happens if the cells do not divide at the G1 checkpoint?
the cell pauses at G1 and does one of 3 things
1. DNA repair
2. exit cell cycle at G0 and never divide again
3. cell death (apoptosis)
how is DNA damage caused?
by mutagens such as chemicals, UV radiation, X-rays, etc that can cause changes to DNA called mutations
what does exposure to UV light do to the cell?
reduces and stops cell division
what is one type of DNA repair mechanisms?
nucleotide excision repair
steps of nucleotide excision repair
- proteins detect error in DNA
- enzyme nicks DNA on both sides of the damage
- DNA helicase unwinds and removes the region with damaged bases
- DNA polymerase fills in the gap in the 5 to 3 direction using the undamaged strand as a template
- DNA ligase links the newly synthesized DNA to the preexisting strand
what happens after DNA is repaired?
it can now pass G1 checkpoint
uvrA protein
protein that recognizes DNA damage and signals to start repair mechanisms
recA protein
protein that facilitates DNA repair
if an organism has extra copies of P53 gene, what happens?
they are more likely to trigger cell death instead of repair the DNA
- which can lead to lower cancer rate
