Molecular biology 2 Flashcards
describe what occurs in the G1/DNA damage checkpoint
- The G1 checkpoint is mediated by a transcription factor- p53
- in response to DNA damage, levels of p53 increase
- many cancers have mutations in p53 to bypass this checkpoint
what are the roles of p53
- causes the transcription of p21, a CDK inhibitor
- CDK inhibition by p21 prevents cell cycle progression
- p21 also binds to and inhibits PCNA, a component of the DNA replication machinery, preventing its activity - stops cell division to allow for DNA repair
- where DNA damage is irreparable, p53 initials signalling pathways that lead to apoptosis
describe what occurs in the G2 checkpoint
- mediated by the Chk1 kinase as well as p53
- in response to unreplicated or damaged DNA, Chk1 inactivates the Cdc25 protein
- without cDC25, CDK1/Cdc2 remains inactive and cells arrest
- some cancers overexposes CDC25
what are the stages of mitosis
- prophase
- prometaphase
- metaphase
- anaphase
- telophase
explain what happens in prophase
- chromosomal material condenses to form chromosomes composed of 2 chromatids attached together at the centromere
- cytoskeleton is dissembled and mitotic spindle assembled
- nuclear envelope disperses
describe what happens in prometaphase
- chromosomal microtubules attach to kinetochores of chromosomes
- chromosomes are moved to spindle equator
describe what happens in metaphase
- chromosomes are aligned along the metaphase plate, attached by microtubules to both poles
describe what happens in anaphase
- centromeres split and chromatids separate
- chromosomes move to opposite spindle poles
- spindle poles move further apart
describe what happens in telophase
- chromosomes cluster at opposite spindle poles
- chromosomes become dispersed
- nuclear envelope assembles around chromosome clusters
- daughter cells formed by cytokinesis
describe what happens in the M phase checkpoint
- also known as spindle assembly checkpoint (SAC)
- unattached kinetochores inhibit APC/C and therefore anaphase initiation
describe what occurs in the control of chromatid seperation
- a cohesion complex binds sister chromatids together
- cohesins need to be cleaved by separase before chromatids separate in anaphase
- separase is kept inactive by securing until its degraded by proteolysis via APC/C
- when cell is ready to proceed, M-CDK phosphorylates APC/C facilitating cdc20 binding, which activates the complex
describe the current state of cancer genomics
- cancer gene census currently lists 578 genes where mutations have been implicated in cancer
- many relate to cell cycle control and apoptosis
what happens when cell cycle control is lost
- alterations in cell proliferation
- alterations in DNA damage response
- alterations in cell growth
what are oncogenes
- drive abnormal cell proliferation
- may represent the overactive form of normal cellular genes (protooncogenes) or may enter cell as part of a virus
what are tumour suppressors
genes that normally inhibit cell proliferation and tumour development
- in tumours, these are often lost or inactivated
- this usually requires 2 mutational events
what type of mutation are oncogenes
- dominant
- a single mutation event in proto oncogene creates oncogene
- activating mutation enables oncogene to stimulate cell survival and proliferation
what are the 3 ways a proto oncogene can be made overactive and converted into an oncogene
- deletion or point mutation
- gene amplification
- chromosome rearrangement (translocation)
what does a normal human male karyotype consist of
22 pairs of autosomes and 2 sex chromosomes- X and Y
- Cancer genes often show karyotypic abnormalities
what is the Philadelphia chromosome and what is its role
- reciprocal chromosomal translocation event
- found in most chronic myeloid leukaemia patients
- the ABL1 gene from chromosome 9 and the BCR gene from chromosome 22 fuse
- codes a hybrid constitutively active tyrosine kinase
what are the functions of proto oncogenes
- growth factors
- growth factor receptors- eg receptor tyrosine kinases
- elements of intracellular signalling pathways- regulatory GTPases (Ras)
- transcription factors
- genes associated with angiogenesis, invasiveness and metastasis- VEGF and VEGFR
describe the effect of the G:C to T:A mutation in the genetic code
- Glycine >T transversion mutation in codon 12 of Ras protein causes impaired GTPase function, leaving Ras constitutively switched on
- cell division irrespective of growth factor signalling
what type of mutations do tumour suppressor genes cause
- recessive
2.mutation event inactivates tumour supressor gene
- no effect of mutation in one gene copy - second mutation event inactivates second gene copy
- 2 inactivating mutations functionally eliminate the tumour suppressor gene, stimulating cell survival and proliferation
what are the 3 types of tumour suppressor genes
- gatekeepers- monitor cell division and induce apoptosis
- eg Rb, CKIs, apoptosis genes - caretakers- promote genome stability and oppose mutation rates via checkpoints and DNA repair
- p53 - landscapers- control the cellular microenvironment
- genes relating to extracellular matrix proteins, integrins
describe the characteristics of Rb mutations
- can be inherited or non hereditary
- both copies of Rb need to be mutated for tumour formation
- sometimes one mutation can be inherited and the second acquired
describe the mutator phenotype hypothesis
- mutation inactivates tumour supressor gene
- cells proliferate
- if mutation inactivates DNA repair gene, the cells have acquired a mutation that encourages additional mutations so process accelerates
- mutation of proto oncogene can then create an oncogene
- mutation inactivates more tumour suppressor genes
- leading to cancer
what are BRCA 1/2 genes
- caretaker tumour suppressor genes
- increases risk of developing breast cancer
what are BRCA mutations
- double strand DNA breaks can form from replication stress, exogenous stress
- BRCA can help to repair these breaks so the cell cycle can continue
- when lost, DNA is repaired through less accurate pathways
describe the role of cadherins
- cadherins mediate mechanical attachment of neighbouring cells
- within adheren junctions, cadherins link to actin filaments, facilitated by B-catenin linkers
describe the role of B-catenin as a landscaper
- landscaper tumour suppressor gene
- bound to cytoplasmic domain
- roles in cell to cell adhesion but also in coordinating proliferation with the Wet signalling pathway
- B catenin binding to cadherins antagonises Wnt
describe the role of B catenin as a proto oncogene
- when B catenin is not bound to cadherin, forms part of a cytosolic pool
- free B catenin acts as a transcription factor, up regulating proliferation
- Apc protein forms a destruction complex
- Apc inactivated by Wnt signalling at Frizzled
when does cancer arise
when suppressive pathway components are overpowered by proliferative pathway components
describe the stepwise evolution of cancer
- genetic alteration and loss of APC leads normal epithelium to become hyper proliferative epithelium
- increased genetic instability and loss of P53 leads to early adenoma
- activation of K-Ras leads to intermediate adenoma
- loss of tumour suppressors leads to late adenoma
- further loss of p53 leads to carcinoma
- other unknown alterations leads to metastasis
