MCO 22-33 Flashcards
What happens in G2
2 centrosomes become visible
How are chromosomes made visible
By FISH (fluorescent in situ hybridisation)
- cells are fixed and permeabilised by detergent
- Incubated with fluorescent primers
- Primer become hybridised and “painted”
What happens in prophase
- Centrosomes move to opposite poles of the cell
- Chromosomes condense and become long
- Nuclear membrane disintegrates
- Spindle fibres attach to chromatids
What happens in metaphase
- Chromosomes line up along the metaphase plate
- metaphase has diagnostic value, you can compare the lined up chromosomes
What happens in anaphase
- Sister chromatids are separated into separate chromosomes
- cleavage furrow forms
What happens in telophase
- Chromosomes uncoil
- Nuclear membrane forms around the daughter nuclei
- Spindle fibres break down
What is the controller of mitosis
cyclin
Experiment that identified cyclin
- Some sea urchin eggs which were growing but not dividing and some which were growing and dividing were taken
- They were then placed separate lanes during different times
- An electric current was run through to separate proteins
What is cyclins partner called
cyclin dependent kinases (CDK)
How are cyclin/CDK complexes controlled
By destructive phases which reduces concentration
Why are there different cyclin/CDK complexes formed in different stages
To phosporylate different targets and perform different functions
How many cyclin and CDKs are there
At least 4 of each
Can the cycle survive on one cyclin and one CDK
Yes, cell grows slowly
- Low concentrations of cyclin/cdk fuse to G1 targets, they have a high affinity
- High concentrations of cyclin/cdk fuse to G2 targets, they have a low affinity
When are sorting signals needed in protein sorting
When proteins doesn’t reside in the cytosol
What can sorting signal be
- short peptides
- 3 dimensional domains (secondary/tertiary structures)
- other molecules
Modes of transport
- Gated transport (nuclear import)
- Trans-membrane transport
- Vesicular transport
Process of nuclear import
- Importins bind the nuclear localisation signals (NLS) on the cargo protein to the FG-nucleoporins
- Transient interactions with FG-nucleoporins allows movement of protein into the nucleus through repeated binding and dissociation steps
- Importin receptors disengage from cargo and the NLS is not cleaved off
How do importins know when to let go of cargo
- Importin binds to a GTPase switch called Ran
- Ran can either bind to GTP or GDP
- Depending which one it binds to there is a different conformational change
- Ran GDP conformation only exists in the cytosol
- Ran GTP conformation only exist in the nucleus
what determines asymmetric distribution of Ran-GTP and Ran-GDP
Ran specific GEF and GAP
GEF- exchanges GDP to GTP
GAP- hydrolysis of GTP to GDP
Why is the NLS not cleaved after import
- NLS maybe a functional domain
- It is needed again
- Re-import in need after every mitosis
what does ran-GTP do and what does it bind to
Binds to importins and displaces the imported protein
What does ran GDP do
it is generated and ran falls of the importin
Why does the cell cycle need breaks
to provide an opportunity to repair
What is genome instability
cell cycle runs at full speed, where there is little time to proof read
Hereditary retinoblastoma
- A child receive one normal and one defective Rb allele
- somatic mutation inactivates the normal allele
- Typically both eyes are affected
sporadic retinoblastoma
- A child recieves 2 normal alleles
- There are 2 separate mutations which inactivate each allele
- Typically 1 eye affected
Role of Rb gene
- it is a tumour suppressor gene and regulates the restriction point
- slows down entry into s phase by inhibiting cyclin/CDK complexes
Role of p53
- another tumour suppressor protein which decides if the cell cycle stops, repairs and restarts. or kills damaged cells
- p53 can arrest cells in G1 by activating genes that inhibit cyclin/CDK complexes
Apoptosis stages
cell shrinks, nucleus condenses, cells blebs, releasing apoptotic bodies which are then phagocytosed
Intracellular constituents are not released into the extracellular environment
what is Necrosis
cells that die through tissue damage and swell, intracellular constituents are released into the environment
What are the effectors for apoptosis
caspases which cleaves proteins of the nuclear lamina and cytoskeleton.
Caspases are kept inactive by trophic signals
Triggers for apoptosis
external: lack of tropic signals, stress, recognition of virally infected cells
integral: recognition of irrpairable DNA
Developmental: highly regulated non random cell death
role of the quiescent phase (G0)
- cells in G0 have the opportunity to differentiate into different tissue
- Other differentiated cells can be stimulated to re-enter the cell cycle and replicate, this requires mitogenic signals
- once cells pass the restriction point they are committed to the cell cycle
How does G0 protect cancer
- cells in G0 divide slowly so radiotherapy doesnt work
- cells dont replicate there DNA so chemotherapy doesnt work, which depends on the incorporation of toxic nucleotides
How do mitogenic signals work
- EFG binds to EGFR
- Activated EGFR recruits adaptor molecule which recruits Ras
- mRas recruits Raf to the membrane and a signal is passed via intermediates to MAPK
- MAPK stimulates early response genes c-FOS and c-JUN
- c-FOS and c-JUN are transcription factors that induces expression of delayed response genes, inducing G1 cyclins and CDKs
What can go wrong with mitogenic signals
- a mutation may cause something to be permanently activated causing unregulated proliferation
- viral subversion which produces molecules that mimics c-FOS and c-JUN
How do proteins enter the excretory pathway
- N-terminal signal sequence targets the surface of the ER
- Leads to the docking of a ribosome nascent chain complex onto the ER membrane
- The signal sequence is removed once the protein is in the ER
Role of ER
- lipid synthesis
- Protein translocation (proteins are N-glycosylated)
- Quality control
Role of the golgi
- protein and lipid modification
- Protein packaging and sorting
how do transport vesicles form
- recruit proteins to the site where the membranes will form
- bend the membrane to form vesicle
- cut the vesicle free
- send vesicle to target location
- fusion
As all organelles in the ER are interconnected what does this mean
Once in the ER proteins dont need to cross anymore membranes to be secreted
when are sorting signals need once in the ER
Needed when enzymes are heading to intracellular destinations. None are needed if they are travelling to the plasma membrane
Targeting hydrolytic enzymes to lysosomes
- Lysosomal enzymes have a M6P (target signal)
- M6P binds to M6P receptor which recruits clatherin
- clatherin bends the membrane
- dynamin cuts the budding vesicle
- vesicle loses the clathrin coat and fuses with the late endosome and release M6P
- late endosome fuses with the lysosome, delivering cargo enzymes
- receptors are recycled to TGN
How do lysosomal proteins acquire M6P
Lysosomal hydrolases have a 3D signal patch which puts a phosphate on mannose to end up with M6P
Role of GAGs
GAGs form hydrated gels and provide turgor, support and a medium for cell migration. They are negatively charged attracting cations causing large amounts of water to be sucked into the matrix
cytoskeleton components
- microtubules
- actin
- intermediate filaments
- associated proteins
cell - cell junctions
- cell adhesion molecules (CAMS) are part of the cadherin family
- cadherins of the same type interact weakly, but many act
- selective recognition ensures cadherins of the same type interact with each other, so they can be segregated and sorted
- cause coordinated contractions
cell-extracellular junctions
focal adhesion - integrins allow the cytoskeleton to grip to ECM molecules
in cell-extracellular junctions what must integrin do so cells can migrate
integrins must be able to switch from an active state to an inactive state. it does this by changing its confomation at both ends
what is anchorage dependence
when integrins activate intracellular signalling pathways to control a cells behaviour
Tight junctions
maintain cell polarity by blocking the mixing of apical and basal membrane proteins
-use claudins and occludins to interlock cells preventing lateral diffusion
Gap junctions
use connexins from adjacent plasma membranes to create channels between cells
what are the determinants of fate
- induction: where signal from one group influence the developmental fate of another, from morphogens
- Asymmetrical cell division: significant molecules are differently distributed
cortical rotation
results in asymmetry of mRNA
Gatrulation
lays down germ layers and body axis
body axis
- anterior end
- posterior end
- dorsovental axis
- mediolateral axis
2 types of smooth muscle
visceral - several fibres 1 ANS motor neuron synapse
multiunit - each fibre has one ANS motor neuron
smooth muscle are short fibres, non overlapping and tapered
skeletal muscles
long, fast contractions, striated (overlapping)
cardiac
branched, many gap junctions, connected to neighbouring fibres
2 types of neuron tissues
neuron
neuroglia: dont generate impulses, supporting role to neurons
