Cell Structure and Function Flashcards
Transcription: Initiation
TBP binds to TATA box found at the start of promoter sequence.
Transcription factors bind to promoter.
RNA polymerase II binds to promoter and begins transcription
Transcription: Elongation and Termination
Transcribes from 3’ to 5’ on the template strand to give a mRNA strand from 5’ to 3’.
10 bases exposed at any given time.
Transcription ends at the polyadenylation signal (AAUAAA). Enzymes detach RNA polymerase and mRNA.
Transcription: Processing
Guanine cap added to 5’ end.
‘Tail’ of 50-250 adenines added to the 3’ end.
Introns edited out by spliceosomes, which look for donor and acceptor sequences at the start and end of an intron.
Translation: Initiation
mRNA binds to small subunit of ribosome that already has tRNA with methionine bound. Subunit scans mRNA for AUG codon for tRNA to bind to. Large subunit binds using energy from a GTP.
Translation: Elongation
tRNA with complementary anticodon to the next codon binds to aminoacyl site. Enzymes catalyse peptide bond between new amino acid and peptide chain.
Original tRNA moves into E site to be ejected and new tRNA moves into p-site.
Ejected tRNAs reloaded with necessary amino acid by aminoacyl-tRNA-synthetase.
Translation: Termination
Release factor binds to a stop codon to stimulate hydrolysis of bond between tRNA and last amino acid. 2GTPs are hydrolysed to separate the two ribosome subunits.
Function of the SRP
SRPs recognise sequences of amino acids near the N-terminus called signal polypeptides and pause transcription. Ribosomes stimulated to move to RER, where the SRP detaches and translation recommences, releasing the polypeptide into the RER.
Polypeptide chain modified by chaperone proteins in RER for secretion.
Protein Synthesis Errors: Transcription
Absence of RNA Polymerase II: no transcription.
Spliceosomes non-functional: Introns not removed and also translated. Incorrect primary structure.
mRNA not proofread: Incorrect codons leading to incorrect primary structure.
Protein Synthesis Errors: Translation
Correct translation factor absent: Translation cannot begin
Signal Polypeptide not recognised by SRP: No translocation to RER, so no chaperone protein to fold the polypeptide-incorrect tertiary structures.
Protein Synthesis Errors: Sorting
Incorrect molecular tags on vesicles or proteins: Protein/vesicle delivered to wrong target structure.
Similarly, the SRP error can prevent secretory proteins from being secreted because they are not placed into vesicles.
Protein Synthesis Errors: Modification
Failure to/over phosphorylating: Protein is inactive/too active.
Cell Signalling: Reception
Ligand binds to cell surface protein receptor which undergoes conformational change.
Conformational change causes receptor to bind to G-protein and a GTP displaces the GDP currently on the G-protein.
Activated G-protein detaches and diffuses across the membrane until it reaches and binds to an integral protein enzyme, which also undergoes a conformational change and activates it to begin transduction.
GTP bound to G-protein is hydrolysed back to GDP and Pi and the G-protein dissociates from the enzyme.
Cell Signaling: Transduction (+ cAMP Example)
Membrane bound enzyme activated by G-protein begins to synthesise secondary messengers. eg: adenylyl cyclase which synthesises the secondary messenger cyclic AMP from ATP.
cAMP phosphorylates protein kinase A and activates it, which will go on to phosphorylate more protein kinases. Afterwards, protein phosphotase will dephosphorylate the kinases to deactivate them.
cAMP is degraded by phosphodiesterase to prevent constant stimulation of kinases.
Cell Signalling: Transduction (+Ca2+ example)
Phospholipase C is the integral membrane enzyme and hydrolyses PIP2 to IP3 and DAG. IP3 is the first secondary messenger and binds to Ca2+ channels on the RER to trigger Ca2+ release. Ca2+ is the second secondary messenger.
Reason for the Cascade
Every kinase-activation acts as a control for the activity of the protein. If a kinase at a level is over-activated, the dephosphrylating mechanism in the next level will reduce the effect by inactivating all the excess kinases.
