Cell Cycle Flashcards
name the stages in the cell cycle
G1, S phase, G2, M phase and G0
what happens at the G1 checkpoint?
- beginning of cell cycle
- preparation phase of the cycle
- preparing to enter S phase
- check for DNA damage and extracellular damage
what happens in S phase?
DNA synthesis
what happens at the G2 checkpoint?
- cells with duplicated chromosomes held together by centromere
- another preparation phase
- 2x number of organelles
- increase amount of cytoplasm
- checks DNA damage and if DNA replicated correctly
what happens in the M phase?
-mitotic division
what is G0?
the resting phase
- cell cycle re-entry not possible
- maintain in G0 unless stimulations to divide
what is interphase?
all stages except M phase
what are the phases of mitosis?
- prophase
- prometaphase
- metaphase
- anaphase
- telophase
- cytokineses
what happens in prophase?
- chromosomes condense
- centromeres move to opposite poles
- mitotic spindles form
what happens in prometaphase?
- breakdown of nuclear envelope
- chromosomes attach to mitotic spindle
what happens in metaphase?
- centrosome on opposite ends
- chromosome most condensed and line up on equator of mitotic spindle
what occurs at anaphase?
- sister chromatids separate synchronously
- each new daughter chromosome move to opposite spindle pole
what happens at telophase?
- chromosomes arrive at spindle poled
- chromosomes decondense
- nuclear envelope reforms
what is the last phase of mitosis and what happens?
- cytokineses
- cytoplasm divides to form 2 daughter cells
what are the 3 requirements for mitosis
1) each phase occurs once
2) phases must be in the right order
3) phases must be non overlapping
what is the mitotic spindle and what is its fuction?
a microtubule machine that assembles during prophase and separates duplicated chromosomes
what are the 3 mitotic spindles?
- astral microtubules
- kinetichore microtubules
- interpolar microtubules
what happens at cytokinesis?
- contractile ring divides cells by two
- cytoskeletal structure of actin myosin
- ring contracts to form cleavage furrow
how are cell organelles distributed un cell division?
- relies on organelles to be scattered randomly in cytoplasm
- number of copies of organelles are roughly equal
what is it called when meiosis goes wrong and what happens?
- non disjunction
- failure of homologous to separate in MI or MII
- non disjunction at MI: autosomal chromosomes
- non disjunction at MII: sex chromosomes
what are the consequences of non disjunction?
-usually the death of foetus or dies soon after
what are some non viable sex chromosome monosomies
sex chromosomes:
- XO (Turner’s syndrome)
- XXX (Triple X syndrome)
- XXY (Klinefelter’s syndrome)
what are the cell cycle regulators and what do they do?
Cyclin dependent kinases (Cdks) -enzymes that phosphorylate the target proteins -become active when bound to a corresponding cyclin
Cyclins
-regulators of Cdks
-different cyclins are produced at
each phase of the cell cycle
what re the basic principles of cell cycle control?
- Cdk levels fairly stable throughout cell cycle
- cyclin levels vary as part of cell cycle
- Cdk bound to Cyclin is active and phosphorylates a ‘target’ protein
- Cdk activation triggers the next step in the cell cycle, such as the entry into the S or M phase
- Cyclin degradation terminates Cdk activity
what normally happens at the G1 checkpoint to allow the cell to enter the S phase?
- induction/expression of Cyclin D which associates with CDK 4
- binding/ activation of Cdk4
- phosphorylation of pRB by CDK4
- release and activation of E2F
- S phase gene transcription
what happens at the G1 checkpoint if there is damage to the DNA?
- p53 (tumour suppression gene) is degraded quickly as it is unstable, and maintained at very low levels
- phosphorylated (active) p53 is not degraded
- active p53 promotes transcription of genes that induce cell cycle arrest,
- it binds to the promoter region of the p21 gene and stimulates p21 expression
- p21 binds and inhibits G1/S-Cdk/S-Cdk complexes
- cell arrests in G1 (allowing time to repair the damaged DNA)
what are the two families of Cdk Inhibitors (CKIs)?
Inhibitor of Kinase 4 family (INK4):
-specifically inhibit G1 CDKs (e.g. CDK4)
CDK Inhibitory Protein/Kinase Inhibitory Protein
(CIP/KIP) family
-inhibit all other CDK-cyclin complexes (late G1, G2 & M)
-gradually sequestered by G1 CDKs thus allowing
activation of later CDKs
how does the misregulation of the cell cycle cause cancer?
- cells escape the normal cell cycle checkpoint, leading to uncontrolled progression through the cell cycle
- many genes that regulate the cell cycle (eg. p53, p21) are often mutated in human cancers
