Module 8 - Cycles of division and growth in cell populations Flashcards
The cell cycle: the four stages
G1 - Cell grows
S - DNA replicates
G2 - Cell grows more
M - Mitosis occurs
Why do cells divide?
- Sustain life
- Propagate heritable traits
- Generate mass
- Generate diversity
Embryonic stem cells
Can skip long G phases and just do rapid S->M cycles
Cyclins: the four different ones, what phases of the cell cycle they’re involved with, and the Cdk’s they pair with?
Cyclin D - G1, Cdk4 Cdk6
Cyclin E - S, Cdk2
Cyclin A - G2, Cdk2
Cyclin B - M, Cdk1
When/how do cyclins/Cdk level change throughout the cell cycle?
Cdk levels remain relatively constant throughout the cell cycle
Cyclin D - Rises during G1, falls during the start of S
Cyclin E - Rises during the end of G1, falls during the end of S
Cyclin A - Rises during the start of S, falls during the M phase
Cyclin B - Rises during the start of the G2 phase, falls during the end of the M phase
How do proteins associated with the cell cycle become activated?
Cyclins bind to the appropriate Cdk and cause it to have a conformational change causing the active site to be available for substrate binding
The Cdk-cyclin complex can then be phosphorylated by CAKs (Cdk-activating kinases)
Wee1 kinase
Puts inhibitory phosphates on Cdk1
Cdc25 phosphatase
Removes inhibitory phosphates from Cdk
CAKs
Cdk-activating kinases - add excitatory phosphates
Cdk1 and its role in the cell cycle
Begins and continues mitosis
It is activated at the end of G2 and is active until the end of mitosis
Proteolysis of the Cdk-cyclin complex
The complex is ubiquitylated by APC which causes it to move into the proteasome for degradation
Degradation occurs instead of simply adding inhibitory phosphates as a means to keep the directionality of the process
The general process during mitosis
1) Cyclin b levels are increased at the end of the G2 phase and they bind to Cdk1
2) Cdk1 triggers mitosis (not sure how) after phosphorylation from CADs
3) Cdk1 causes activation of Cdc25 and inhibition of wee1 so that inhibition of Cdk1 is reduced
4) Once mitosis is complete/anaphase needs to begin, the APC ubiquitylates the Cdk1 complex
5) The Cdk1 complex moves to the proteasome where cyclin b is degraded
Mitosis stop techniques
If dividing is not a suitable option (ie the cell is damaged and/or there is replication stress) then there are two main methods:
- The activation of ATM/ATR which activates Chk1/2 which activates Wee1 and inhibits Cdc25 so that the Cdk1 complex is inactivated
- The addition of CKI (Cdk inhibitor) to the Cdk1 complex which inactivate the complex
APC: what does it do, when is it active/inactive, and why is it highly regulated?
Anaphase promoting complex, have a wild guess what it does… It, however, also inactivates the Cdk1 complex and causes the degradation of cyclin b
Active at the start of anaphase and inactivates at the beginning of G1
It causes the breakdown of cyclin b, something essential for the start of mitosis so if it was not regulated then mitosis wouldn’t occur
The SAC: what is it, what does it do, and when is it inactivated?
The spindle assembly complex - activated by unattached kinetochores and prevents the APC from activating until all chromosomes are correctly aligned
What happens when kinetochores are attached?
The SAC is inactivated and the APC is activated
APC: what is it and what does it do?
The anaphase-promoting complex
Two main functions:
* Ubiquitylates the Cdk1 complex and causes it to move to the proteasome for cyclin b degradation (which signals mitosis exit)
* Ubiquitylates the Securin-separase complex causing it to move to the proteasome for securin degradation causing separase to be free and cleave the sister chromatids and allow them to move to opposite sides of the cell
JAK/STAT pathway
Stimulated by erythropoietin - EPO binds to the JAK receptor protein which causes STAT to be phosphorylated and dimerise and move to the nucleus to affect cell expression
Types of external factors
Mitogen factors - Stimulate proliferation/cell cycle
Survival factors - Prevent apoptosis
Growth factors - Stimulate growth
Differentiation factors - Control cell lineage
The restriction point
In G1, there is a period in which the cell can be kept in G1 (and can enter the quiescence phase) without continuing the cell cycle; however, once the restriction pathway has been crossed, there is no pausing the cycle
The role of mitogens
1 - Bind their cognate receptors and activate RAS-MAPK signalling pathways, in turn, activating specific transcription factors
2 - This drives expression of the Cyclin D gene, in turn giving rise to a new Cyclin D protein which binds and activates Cdk4/6
3 - Active Cdk4/6-Cyclin D complexes then phosphorylate Rb, causing it to let go of a different transcription factor - E2F
4 - Active Cdk4/6-Cyclin D complexes then phosphorylate Rb, causing it to let go of a different transcription factor - E2F
5 - Positive feedback, whereby activate Cdk2-Cyclin E also phosphorylates Rb, gives rise to a surge in Cyclin E levels
6 - The burst in Cdk2-Cyclin E activity then drives entry into S-phase, for example by phosphorylating replication origins thus triggering DNA synthesis
Proliferation
Paneth cells secrete Wnt signals
These signals are received by the stem cells, causing them to induce Cyclin D and proliferate
Precursor cells then migrate up and out of the crypt - i.e. away from the Wnt signals
Once out of range, they stop proliferating and differentiate
The main type of tumour
Carcinomas - originate from epithelial cells and account for 80% of tumours
The two types of carcinomas
Squamous cell carcinomas - carcinomas arising from epithelial cells that form protective layers
Adenocarcinomas - carcinomas arising from epithelial cells that secrete that are first benign (adenomas) and then malignant (adenocarcinomas)
The stages of cancer
1- normal epitherlium
2 - hyperplastic epithelium
3 - benign (early/intermediate/late adenomas)
4 - malignent (carcinoma)
5 - invasion and metastasis
The 4 (10) hallmarks of cancer
- Sustaining cell proliferation signalling
- Evading growth suppressors
- Resisting cell death
- Genome instability and mutation
- Avoiding immune destruction
- Inducing angiogenesis
- Deregulating cellular energetics
- Enabling replicative immortality
- Activating invasion and metastasis
- Tumour-promoting inflammation
What is the basis of cancer?
Mutations within oncogenes and tumour suppressor genes causing abnormal cell activity
Incorporation of retrovirus genes
RNA is injected and then converted into DNA by reverse transcription
v-src: what is it, what does it do, and what is its cellular counterpart?
An oncogene (the first to be discovered)
When expressed, it causes cellular transformation
c-src
How did src genes end up in viruses?
Viral infection -> viral genes incorporated into genes -> next to src gene -> co-transcription of src gene, adding it into the viral gene
Ras: its role in tumour formation
Ras mutation - uncontrolled Ras activation causes uncontrolled cyclin D production
Only one mutated allele is required
Retinoblastoma
Sporadic - no family history, one eye (unilateral), low risk of other tumours
Familial - family history, both eyes (bilateral), high risk of other tumours
In both situations, two mutated alleles are required (familial will be passed down, sporadic will not)
Neurofibramotis
Familial cancer syndrome that is caused by the NF1 tumour suppressor gene when both alleles are mutated
The inactive NF1 causes Ras to be constitutively active
Rb: its function in the cell cycle
Rb inhibits the E2F transcription factor which, when active, will drive the cell into the S phase and promote the cell cycle
If there is a complete loss of Rb function (both alleles mutated) then the cell cycle will repeat continuously
Wnt: its effect on cell proliferation
Wnt, when present, causes β-catenin and cyclin D1 to be switched on and cause cell proliferation to occur
Wnt, when absent, causes β-catenin and cyclin D1 to be switched off and cell proliferation does not occur
TGF-β: what does it do?
Inhibit the progression of the cell cycle - when its gene is mutated, its inhibition stops and the cell cycle may progress in an uncontrolled fashion
APC: what does it do?
Inhibit the progression of the cell cycle - when its gene is mutated, its inhibition stops and the cell cycle may progress in an uncontrolled fashion
Oncogenes
Oncogenes are genes that have the potential to cause cancer. They are derived from normal cellular genes called proto-oncogenes, which play essential roles in regulating cell growth, division, and differentiation. However, when certain alterations occur in proto-oncogenes, they can become oncogenes and promote uncontrolled cell growth and tumor formation.
The alterations that can convert proto-oncogenes into oncogenes include:
Gene amplification: Increased copy number of the oncogene, leading to overexpression of its protein product.
Activating point mutations: Specific changes in the DNA sequence of the proto-oncogene that result in a hyperactive or constitutively active protein. These mutations can lead to the deregulation of cellular signaling pathways involved in cell growth and proliferation.
Chromosomal translocations: Rearrangements of genetic material where a proto-oncogene is brought into close proximity or under the control of a different regulatory region. This can lead to increased expression or altered regulation of the oncogene.
When an oncogene is activated, it can drive abnormal cell growth and division, evade normal regulatory mechanisms, and promote tumor formation. Oncogenes often encode proteins involved in cell signaling pathways, cell cycle control, DNA repair, or other critical cellular processes.
It’s important to note that oncogenes are only one component of the complex process of cancer development. Other factors, such as tumor suppressor gene inactivation and environmental factors, also play significant roles in the progression of cancer.
Rb, Ran, RAS, and Rab proteins: what does each do?
Rb - inhibits E2F, inhibiting cyclin E production (preventing uncontrolled cell cycling)
Ran - small GTPase protein involved in nuclear transport
RAS - controls cyclin d production, mutations with it may cause cancer
Rab - bind with tethering proteins to fuse vesicles with target membranes
