RNA Translation Flashcards
mRNA
- what sequences not translated
- difference between eukaryotic & prokaryotic mRNA
- leader & trailer sequences (30-100bp) are not translated
- Eukaryotes each mRNA = single polypeptide chain
- Prokaryotes each mRNA = single polypeptide chain OR polygenic (multiple chains)
- protein coding regions in polygenic mRNA usu. separated by spacer region and have their own Ribosomal binding site
The Genetic Code
- no. of codons & how many used for a.a.
- start & stop codons
- 3 features of the code
- synonymous codons
- out of 64 codons, 61 used for the 20 amino acids
- AUG = initiation codon (also for methionine)
- UAA, UAG, UGA = stop codons (for termination of syn)
Code is;
- unambiguous (each codon = 1 a.a. ONLY)
- degenerate
- Is universal (some exceptions)
Synonymous codons = diff. codons of one a.a.
Ribosomes
- what is it
- composition
-what does it do
- Are large ribonucleic particles that contain RNA & proteins and dissociate into large and small subunits
- subunits characterised by rate of sedimentation when spun in ultracentrifuge = Svedberg (s) coefficients
- 2/3 - RNA
- 1/3 - protein
- Binds to mRNA & tRNA during translation
- has to perform several jobs w/ precision & speed - is complex
Ribosomes
- location of binding sites
- 3 sites
- Decoding centre
- Peptidyltransferase centre
- binding site for mRNA completely w/in small subunit
- 3 binding sites for tRNA
- A site (for aminoacyl): binds incoming aminoacyle-tRNA whose anticodon matches the codon w/in A site
- P site (for peptidyl): tRNA in P site binds the growing peptide chain
- E site (for exit): contains deacylated tRNA that is ready to be released from ribosome
Decoding centre: in 30S subunit - ensures along tRNAs carrying matching anticodon accepted into A site
Peptidyltranserase centre: in 50S subunit - is site where peptide bond formation is catalysed
-this is the function of large subunit - to catalyze peptide-bond formation
tRNA
- what it does
- shape
- anticodon loop
- where aa bonded to
- enzyme responsible
- Dual properties of an adapter that recognizes both aa and codon
- ss molecule in cloverleaf shape w/ 3 ss loops
- shape v. important for tRNA function - Anticodon loop contains the anticodon - complementary to codon carried by tRNA
- orientated 3’ to 5’ (needs to complement mRNA being 5’ to 3’) - 3’ adenosine covalently linked to an a.a. by aminoacyl-tRNA synthetase (are 20 types in cell - 1 for each aa)
tRNA - wobble rule
- certain charged tRNA species can bring their specific a.a. to any one of several codons
- first base of anticodon (wobble position) can have unusual pairing w/ third base of codon
- occurs when 3rd nucleotide of an anticodon can form either of 2 alignments (either w/ normal complementary pair, or a different nucleotide)
- SEE WOBBLE RULES FOR THOSE ALLOWED
- first base of anticodon (wobble position) can have unusual pairing w/ third base of codon
Wobble rules
1st base (anti) bases recognised 3rd position (codon)
C G only
A U only
U A or G
G C or U
I A, U or C
Aminoacyl tRNA synthetase
- what it is/assists in
- how many
- attaches right amino acid at 3’ end of tRNA
- 3’ end sequence & anticodon region provide specificity to tRNA
- 20 different synthetases for each a.a.
- charged tRNA recognise codons by anticodons; the attached a.a. does not play any role in codon recognition
-base at end of 3’ is always adenine
Mechanism of Translation (3 points)
- Codons of mRNA read 5’ to 3’
- A.A. added to growing chain that begins at N terminus and proceeds to C terminus
- Divided into 3 stages (initiation, Elongation & Termination)
Initiation (Prokaryotes)
- Main task
- UTR & Shine-Dalgarno sequence
- Initiation factors (3)
- what they do
-Final step for initiation complex to be formed
- Main task = place 1st aminoacyl-tRNA in P site of ribosome & est. correct reading frame
- 5’UTR (Untranslated region) adjacent to AUG is critical for ribsome binding in prokaryotes
- Initiation codons preceded by special sequence called SHINE-DALGARNO SEQUENCE (pairs w/ 3’ end of an rRNA)
- pairing correctly positions initiator codon in P site where initiator tRNA will bind - 3 proteins required for correct initiation (IF1, IF2 & IF3) = INITIATION FACTORS
- IF3: keeps 30S dissociated from 50S subunit (binds before mRNA does)
- IF1 & 2: ensure that only initiator tRNA enters P site
-All IFs dissociate, GTP hydrolyzed to GDP and large subunit joins to create 70S initiation complex
Initiation of Translation in Eukaryotes
- Major differences: what is there and what is not
- Initiation complex - what it is made of and what it does
- No Shine-Dalgarno sequences present
- Secondary structure must be removed to expose initiation codon - done by eukaryotic initiation factors (eIF4a, b & g)
- Poly(A) tail plays a major role in initiation of translation
- proteins that attach to 3’ end of tail interact w/ cap binding protein & 40S subunit & initiator tRNA to form initiator complex
- complex moves 5’ to 3’ direction & unwinds base-paired regions
- proteins that attach to 3’ end of tail interact w/ cap binding protein & 40S subunit & initiator tRNA to form initiator complex
- initiation complex joined by 60S after AUG codon properly aligned
- initiation factors dissociate from ribosome before elongation occurs
Elongation
- 3 steps
- Elongation factors associated
-a.a. brought to ribosomes by tRNAs
Takes place in 3 steps;
1. charged a.a. binds to A site. EF-T & EF-Tu involved
2. Peptide bond is formed (in P site)
3. Ribosome moves forward by one codon - EF-G involved
-deacylated tRNA moved into E site and recycled
*energy required provided by GTP
Termination
- Overview on what happens
- What release factors each recognize/do (3)
- RRF
- Stop codons recognised by the release factors
- binding of release factor at A site terminates translation - RF-1 recognizes UAG & UAA
- RF-2 recognizes UAA & UGA
- RF-3 binds to ribosomes and forms complex w/ GTP
- RF terminates protein synthesis by releasing protein chain
- RRF (Ribosomal recycling factor) releases last tRNA, EF-G releases RRF, causing ribosome to dissociate
