Cell Cycle Flashcards
What is the central dogma?
DNA –> RNA –> Protein
What is the importance of gene expression?
Dictates cell identity and function.
What are genes made up of?
Coding (exon) and non coding (intron) regions.
In diploid organisms there are two copies of every gene.
Homozygous = same alleles
Heterozygous = different alleles
Process of DNA to RNA
- transcription
- cleavage at poly(A) site
- polyadenylation
- RNA splicing
Process mRNA to Protein
- mRNA enters ribosome
- tRNA brings in corresponding amino acid to codon.
- amino acids begin to form polypeptide chains.
What are proteins?
Made up of a combination of 20 amino acids.
They are the workhorse of the cell, they do everything: structural, sensors, transporters, enzymes, transcription factors, cellular communication, signal transduction etc.
Amino acid sequence determines structure and structure determines function.
What is an exception to the central dogma?
Prion - infections protein with normal DNA sequences (Mad cow disease).
Occurs due to conformation changes in structure. Goes from Alpha-helix to beta-sheet.
This is an exception because even though the DNA sequence remains the same the protein changes.
What are the types of controls in an experiment?
Positive control: effect already known but to an extreme extent (not always needed).
Negative control: variable testing is removed.
Sham control: go through the entire procedure except for what you are testing for (placebo).
What are the different approaches to studying cells?
- Cell biology: direct observation
- Biochemistry: isolating and describing proteins
- Genetics: mutant genes, their mutant proteins and the effect on the cell/animal
- Genomics and Proteonomics: looking at all genes or proteins at the same time
- Developmental Biology: diferential gene expression an the signs that lead to a mature organism.
What are the stages of the cell cycle:
- G1 Phase: recovery from mitosis, growth.
- S Phase: DNA is duplicated.
- G2 Phase: pre-mitosis checkpoints.
- M Phase: mitosis, chromosome segregation and cell division.
- G0: temporary or permenent exit from cell cycle.
Note: everything except for mitosis is collectively known as interphase.
Define chromosome
The structural unit of genetic material consisting of double stranded DNA and proteins.
Define chromatid
one copy of a duplicated chromosome
Define sister chomatid
Identical copies of a chromosome joined by a centromere.
Define: cohesins
Holds sister chromatids together after DNA replication and are removed for separation of chromatids
Define: Homologous chromosomes
Chromosome pair that includes one from each parent.
What is the goal of mitosis?
Make two genetically identical daughter cells.
Chromosome segregation in mitosis
Homologous chromosomes line up independently on one another.
Sister chromatids separate.
What are the stages of mitosis?
- Interphase
- Propase
- Prometaphase
- Metaphase
- Anaphase
- Telophase
- Cytokinesis
What is interphase?
All stages of the cell cycle excluding mitosis.
Includes chromosome duplication, cohesion and centrosome duplication.
What is prophase?
Breakdown of interphase. Chromosomes condense
What is prometaphase?
Nuclear envelope breaks down. Chromosomes get capture by microtubles and brought to the equator
What is metaphase?
Chromosomes aligned at the equator.
Process of chromosome condensation
chromatin pacing aided by histones. Wraps to condense.
Define: centromere
DNA sequence that serves as a target for mitotic spindle
Define: kinetechore
protein complex that links the centromere to microtubules
Define: centrioles
contained within the centrosome and are made up of bundles of microtubules
What do microtubules do?
They connect to the kinetechore of chromosomes and also to the cell membrane. Creates tension and tracks that allow chromsomes to be moved. Motor proteins move along microtubules to move chomosomes
What is anaphase?
Cohesions degraded so sister chomatids split. Chromatids begin to move towards poles then spindle poles seperate.
What is telophase?
Nuclear envelope begins to assemble. Contractile ring forms
What is cytokinesis?
reformation of interphase microtubules. Contractile ring forms cleavage furrow and separates two daughter cells.
Differences between Mitosis and Meiosis?
Mitosis wants two identical daughter cells whereas meiosis wants 4 daughter cells that only have half the required DNA.
Mitosis: 2n
Mieosis: 1n
Process of Meiosis
- Meiosis 1 occurs the same as mitosis however recombination occurs between homologous chromosomes.
- Meiosis 2 the duaghter cells from Meiosis 1 seperate again without duplicating chromosomes.
Why bother with sex?
Evolution depends on novel mutations.
Mutations cannot be passed on to children unless they occur in gametes.
new combinations of existing alleles create new phenotypes.
How is phosphorylation a mechanism of regulation?
Adding or removing a phosphate from a substrate is phosphorylation which causes the substrate to be activated/inactivated.
Kinase: adds phosphate
Phosphatase: removes phosphate
What are cyclin-dependent kinases?
CDK’s consist of a cyclin subunit and a cyclin dependent-kinase catalytic subunit.
Present throught the celly cycle but expressed at different stages.
CDK’s are inactive unless bound by a cyclin.
Cyclin determine CDK specificity.
There are different CDK’s which all regulate different things.
Types of CDK’s
- G1 CDKs: allow progression through G1.
- G1/S CDK: function at G1/S border –> degrades substrate to allow cells to enter s phase
- S CDK: drive DNA replication
- G2/M CDK: prepare for mitosis
- M CDK: important for mitosis
How do CDK’s work?
Cyclin binds to CDK to activate the substrate. The cyclin then acts on the target substrate and phosphorylates it.
What is Ubiquitination?
Ubiquitin-Protein ligases attach Ub to a target protein.
Occurs multiple times - polyubiquitination.
proteasome recognises polyub and destroys the protein.
What is the SCF complex?
Important in G1/S phase transition.
What is APC/C?
Degrades securin, securin is an inhibitor of separase. Separase cleaves cohesins to separate sister chromatids.
What happens in G1/S Transition
G1 CDK’s phosphorylate transcription factors . TF drive expression of genes for DNA replication. Genes include S-phase CDK.
G1 and G1/S CDK’s prep the cell for S phase. Induces expression of genes required for DNA replication.
SCF UB-Ligase degrades inhibitor of S-phase cyclin
What are temperature sensitive mutants?
Protein functions normally at permissive temperatures however a change in temperature will cause a protein to denature.
Why are temperature sensitive mutant important?
Able to analyse and study specific mutants.
Mutants go thorugh the cell cycle until the reach G2/M boundary and stop as they are unable to proceed at the restricitve temperature
What are the cell types of TS Mutants
Long obloong shape are mitosis-defective mutants. Caused by loss of CDC2
Small cells joined together are mutants that enter mitosis prematurely due to wee phenotype
What are the regulators of Mitotic Cyclin CDK
cdc25 drives mitosis
wee1 inhibits mitosis
What is the mitosis promoting factor
produced in G2 but kept in phosphorylated stated (inhibited). Once activated they phosphorylate: chromatin proteins, nuclear envelope proteins, microtubule proteins, kinetechore proteins etc.
What is a checkpoint?
Points within the cell cycle where the cell can stop if something has gone wrong … driven by porteins that inhibit cell cycle progression.
Allows cell to try and fix the problem before continuing.
What is the mitotic checkpoint?
ART1 recognises DNA replication forks. If DNA replication is incomplete ATR1 will be active and activate CHK1. CHK1 acts on cdc25 to inhibit mitosis.
How is the nuclear envelope broken down?
Nuclear lamina support nuclear envelope.
Nuclear lamina is the substrate for mitotic CDK’s. It gets phosphorylated and the meshwork breaksdown
What does APC/C participate in?
- induces anaphase. degrades cohesins.
Causes cdh1 to become active which elads APC/C to degrade mitotic cyclin - induced later steps in mitosis –> telophase and cytokinesis.
Why is degradation of mitotic cyclin important?
- chromatin condesation
2. depolymerisaion of microtubules
What is necrosis?
Unplanned death of a cell. Results from rupture of the cell and the spilling of its contents into the surrounding tissue.
Can cause neighbour cells to die causing a snowball effect.
What is necrosis caused by?
- toxins
- burns
- lack of circulation
- infection
- inflammation
What is apoptosis?
Programmed cell death. Cell is chopped up and packaged for removal. DNA is fractured and cell membranes pinch off into small structure.
Fragments are labeled for macrohasges so that they will be cleaned up thorugh phagocytosis.
What is the apoptosis signalling pathway
EGL1 –> inhibits –> CED-9 –> releases –> CED-4 dimer –> activates –> CED-3 –> destroys proteins
What does CED3 do?
- Cleaves lamins (nuclear envelope)
- Activates endonucleases (DNA digested)
- attacks dell structure components
- attacks cell-cell adgesion proteins
- celaves inteself
How do immune cells recognise dying cells?
Phophatidyserine normally on inside of cel is moved outside of cell causing a signal for macrophages.
What is cancer?
A disease of cell proliferation. Has a genetic basis. Almost always involves the cell cycle and apoptosis.
Types of mutations
- gain of function
- loss of function
- dominant negative
Contributions to Cancer
Growth factors stimulate cell growth - want signalling pathways always active.
Evading growth suppressors.
metastisis activation
activate replicative immortalitiy.
resist cell death
inducing angiogensis
Selection in Cancer
Selects for cells that can grown
cell gets mutate that allows it to grow better
mutant cells will grow quicker and overtake the area
selection of cell with growth advantage - snowball effect
Cncer cells will alays seek to get more mutation to increase cell growth
Two type of cancer gene?
- proto-oncogenes
2. tumor-suppressor genes
What are proto-oncogenes?
normally promote cell growth. When mutate (gain of function) become oncogenes
What are tumor-suppressor genes?
inhibit celly cycle. MUtations are loss of fuction to allow cells to divide out of control
How do mutations arise
Can be spontaneous or from environmental damage. Ageing is the biggest risk of cancer.
What doe sit mean by cancer is a multi-hit disease.
It wasn’t to effect as many pathways as possible to increase cell growth - not just one pathway.
Each change gives a different growth advantage.
What is metastasis
spread of cancer cells from their site of origin and establishment of growth in other areas.
G1 restriciton point pathway for cancer?
The restriction point with the point at which the cell is committed to another round of cell division.
RB is a transcriptional factor that acts as a repressor for E2F.
E2F is a transcriptional factor that drives the transition into S phase.
G1 CDK phosphorylates RB inactivating it. Cuases RB to release from E2F. E2F no longer inhibited so can becomes activated. Transcription factors are transcribed and cell begins to prepare for S phase.
Is there another commitment point in G1?
G1 CDK inactivates APC/Cdh1 thorugh phosphorylation.
Emi1 acts on APC/Cdh1 to destroy Cd1 via ubiquitination.
APC/Cdh1 now inactive and can’t be activated again –> able to proceed to S Phase.
How does p16 effect G1 CDK?
p16 binds CDK preventing cyclin/CDK dimerization.
If p16 is inactive it can’t activate G1 CDK thus G1 CDK can’t inactivate Rb.
Mutations in one pathway
There will only be one mutation in a single pathway
What is p53?
detects conflicts within the cell and can activate several pathways including:
- cell cycle arrest
- senescnce (exiting cell cycle)
- apoptosis
It is a tumour suppressor
How is p53 regulated?
ATR inhibits Mdm2 which inhibits p53. p53 becomes activated in response to cnflict (DNA damage, hyperpoliferation etc).
4 p53 need to be joined together to work. If one is mutated the whole structure won’t work.
what is diferentiation?
generates cells with unique structure, function and biochemistry.
Whe a cell leaves the cell cycle it specialises.
How does a cell specialise?
Needs to leave the cell in G1 phase otherwise will go through another round of mitosis before specialisisng.
Whats causes the cell to leave the cell cycle to specialise?
- developmental cues
- gene expression changes
- epigenetic changes
Does a specilised cell proliferate?
No as it has left the cell cycle
Where does the differences in our cells come from?
All cells have the blueprint for making every cell in our body - they all contain the same DNA.
There are ‘marks’ on the DNA that promote/maintain differentiation and inhibit adaptation of an alternative cell fates.
What is epigenetic regulation?
marks on DNA that regulate gene expression and are inherited by daughter cells but independent of DNA sequence.
What can effect cell differentiation?
- cell-cell interations
2. morphogen gradients
How do cell-cell interations effect differentiation?
Neighbouring cells can influence the cell fate
Morphogen Gradients
Involved in patters of cell differentiation.
Amount of morphogen a cell is exposed to effects the differentiation.
In single cells
Bicoid = head
nanos = tail
In multicellular organisms
Wnts, BMP’s and Shh act on cell membrane
Mechanisms of differentiation at molecular level
- transcriptional regulation
- histones
- epigenetic marks
- acetyl and methyltrasnferases
Transcriptional regulation
TF promote or inhibit gene expression
enhancers - TF binding site
Histones
package DNA can be modified to regulate gene expression
epigenetic marks
histone acetylation and methylation as well as DNA methylation
acetylated histones
open chromatin and transciptionally active
de-acetylated histones
closed chromatin - transcriptionally silent
Cell Cycle Exit
differentiated cells become refractory to prolferative signals
G1/S CD inhibitors and Rb plays role in cell cycle exit
Terminal differentiaon different to senescence
