Molecular biology of cancer 1 Flashcards
what is the cell cycle
the process by which a single mother cell gives rise to 2 identical daughter cells
why is cell cycle control studied
- to understand development
- stem cells
- differentiation
- opportunities for therapeutic interventions
how do we study cell cycle
- in whole organisms- yeast
- in cell free extracts- Xenopus oocytes
- in cell culture- mammalian cells
what are the events of the cell cycle which can be seen under a microscope
- mitosis and cytokines (M phase) can be easily observed but only occupy a small fraction of the cell cycle
- the remaining, longer part of the cycle is the interphase
describe the process of genetic dissection of the cell cycle using yeast models
involves spindle formation and nuclear envelopes divide
-can see the different cell cycles through microscope
what are the advantages in using yeast models for genetic dissection of the cell cycle
- rapid reproduction
- genome size <1% of mammals
- amenable to genetic manipulation- gene deletions, replacement or alteration
- can proliferate in haploid state- single copy of each gene present
- can make temperature sensitive mutants
how can cell cycle stages be identified
- by using radiolabelled nucleotides and x ray photography
- using artificial analogues and antibody staining
- flow cytometry
outline the cell cycle phases
- G1- first gap phase
- cell growth - S- synthesis phase
- all DNA in the cells genome is replicated
- cell growth continues - G2- second gap phase
- cell growth and checking for replication errors - M- mitosis phase, usually followed by cytokinesis
- chromosomes are partitioned into 2 daughter cells which finally divide
what is involved in flow cytometry
- cells stained with DNA dyes to allow discrimination of cells with variable DNA content
- cells in G1 phase contain half the DNA of cells after DNA replication in G2 and M
- cells in this process of replication (S phase) contain an intermediate quantity
what are the main 2 factors that the cell cycle is controlled by
- the point of entry into the cell cycle
- at several critical checkpoints
what is the entry into cell cycle controlled by
availability of growth factors
describe the effect of availability of growth factors
- growth factor availability controls the cell cycle at the restriction point in late G1, acting as mitogens
- induce synthesis of D type cyclins via the MAPK signalling pathway
- CDK4/6 with cyclin D then drive passage through restriction point - if growth factors aren’t available, cells enter a quiescent stage of the cycle called G0
- once the cell passes the restriction point, its irreversibly committed to dividing, even if the growth factor signal is removed
- cells have a limited proliferation capacity
- eventually enter G0 permanently
- known as replicative senescence
outline the cell cycle checkpoints
- ensure complete genomes are transmitted faithfully to daughter cells
- a checkpoint in G2 phase arrests cells in response to damaged or unreplicated DNA
- A checkpoint in M phase arrests mitosis if daughter chromosomes are not properly aligned on the mitotic spindle
- the G1 checkpoint detects presence of damaged DNA and leads to cell cycle arrest
what is cell cycle progression controlled by
cyclin dependent kinases
- CDKs
what are CDKs and how do they work
- protein kinases whose activity rises and falls during cell cycle
- phosphorylation of intracellular proteins initiate or regulate the major events of the cell cycle
- partially activated by cyclins and regulated by multiple processes
- cyclins undergo synthesis and degradation in each cycle
- CDK levels are constant
- cyclins accumulate throughout interphase and are rapidly degraded towards the end of mitosis
what is the key to cell cycle control
cyclin availability
- controls the activity of CDKs and promotes cycle progression
- when complexed with an appropriate M phase cyclin, the CDK triggers the mitosis machinery
- when complexed with an appropriate S phase cyclin, CDK triggers DNA replication
- there are specific CDK/cyclin complexes for different cell cycle phases
which complexes are essential for normal cell cycle
CDK1, cyclin B1 and cyclin A2
describe the mechanisms of CDK regulation
- association with cyclins
- phosphorylation events
- activating
- inhibitory - association with CDK inhibitors
- degradation of cyclins/CKIs
what occurs in positive CDK regulation by phosphorylation
CDK activation by cyclin and CAK at threonine 160
what occurs in negative CDK regulation by phosphorylation
- additional regulation of CDKs by inhibitory phosphorylation by Wee1 at threonine 14 and at tyrosine 15
- opposed by Cdc25 phosphatase
describe the CDK regulation by association with CDK inhibitors (CKIs)
- CKIs are inhibitory proteins
- bind to CDK and cyclin complexes
- distort active site
- insert into ATP binding site - p21 and P16 thought to be the dominant inhibitors of cell proliferation in senescent cells
what are the 2 families of CKIs in mammalian cells
- Ink4 family- p15, p16, p18, p19
- inhibit CDK4/CDK6 (G1 phase) - Cip/Kip family- p21, p27, p57
- inhibit CDK1/CDK2 (G1, S, G2 and M phase)
what are the cell cycle control processes
- control of cell cycle entry
- once per cycle DNA replication in S phase
- DNA damage and replication checkpoints
- the events of mitosis
what does MAPK stand for
mitogen activated protein kinase
what is the role of RTK
most RTKs activate Ras- a monomeric small GTP binding protein
describe the properties of Ras
- Ras feeds into many intracellular signalling pathways
- one of these is the MAPK, acting as phosphorylation cascade
- cascade eventually alters activity of target proteins and causes changes in gene expression
how does the MAPK pathway initiate cell division
- receptor tyrosine kinase interacts with growth factor
- MAPK phosphorylation cascade leads to expression of transcription factors
- this causes transcription of cyclin D, which activates CDK4/6
describe the properties of the retinoblastoma protein (pRb)
- a tumour suppressor protein
- usually binds to and inactivates E2F proteins
- when cell is ready to divide (G1 restriction point), phosphorylation of pRb frees E2F to act as a transcription factor
- E2F acts on many S phase genes, leading to cell cycle entry and DNA synthesis
what is the main target of CDK4/6 with cyclin D
Rb protein
when does non mutagenic replication of every nucleotide occur
occurs only once per cycle
describe how cancer chemotherapeutic agents work
- stop cell division by blocking DNA replication, causing DNA damage or blocking mitosis specifically
- this causes growth arrest by triggering cell cycle checkpoints
- cancer treatments can also act via inhibition of growth factor signals, reducing entry into cell cycle (herceptin)
