module 8 Flashcards
Are all tRNA molecules the same?
- tRNA molecules are relatively the same in all organisms
- similar structure
- amino acid attachment site is the same for all tRNA molecules
- sequence is always 5’-CCA-3’ on the acceptor arm
What is the difference between degeneracy and synonymous?
- degenerates: for amino acids that are encoded by multiple codons
- synonymous: codons that code for the same amino acid
What is the difference between partial and complete degeneracy?
- partial: changing the 3rd codon position from a purine to a purine
- complete: changing 3rd position to any nucleotide while coding for the same amino acid
What are some ways to deal with code degeneracy?
- isoaccepting tRNA: same amino acid but uses different anticodons
- wobble effect: the same tRNA molecule can bind to multiple codons due to the wobble effect
how many tRNAs are there?
- 20 tRNA, but only 20 amino acids
- different tRNAs accept same amino acid
What is a non-watson and base pair? When do these occur?
- occurs with the wobble effect, where a tRNA can bind to different codons due to a change in the base pair of the 3rd position
- wobbles form normal position to a non - Watson and crick base pair (as in changes to a base pair that wouldn’t normally bind)
What is insonene?
- intermediate in the metabolism of purine
- essential for translation of genetic code In wobble base pairs
What is a reading frame?
- protein coding region of mRNA
- initiated by AUG (methionine)
What are the four rules of RFs?
- all codons read in 5’ to 3’ direction
- no overlap
- no gaps
- message initiated by AUG
What are the kinds of RFs? How many possible reading frames are there in a DNA molecule?
- first, second, third
- 6 total possible reading frames in a double stranded DNA molecule
What do point mutations do? What is the result?
- point mutations alter a single nucleotide
- can result in nonsense, missense, silent mutations
- frame shift (in not groups of 3) or non frameshift (in groups of 3) from insertions and deletions
Where does translation occur? On what end are amino acids attached? What is it called when multiple ribosomes translate mRNA at once?
- translation occurs in the ribosome
- adds amino acids on the 3’ carboxyl (C-terminus) end of the growing polypeptide chain
- multiple ribosomes = polyribosome
Can a polyribosome occur in both prokaryotes and eukaryotes?
YES!
what does it mean for a prokaryotic mRNA to be polyscystronic?
- one promoter region initiates the translation of multiple proteins!
describe the prokaryotic ribosome?
- ribonucleoprotien
- 3 RNA molecules and 2 sub units
- 50S + 30S = 70S
Describe protein synthesis in the prokaryotic ribosome. What is the rate of synthesis?
- translation occurs in the cavity between subunits
- the small subunit holds mRNA and then exits through a tunnel in the back of the large subunit
- synthesizes 20 amino acids/second
How are amino acids added chemically
the amino acid on the tRNA makes contact with the catalytic region of the larger subunit that synthesizes peptide bonds
- then, arm of the tRNA acceptor arm positioned such that polypeptide can exit through the back of the ribosome
True or false: growing polypeptide is always attached to a tRNA?
True
how might an antibiotic target a ribosome?
- by binding to tRNA or blocking the exit tunnel
what are the four steps of translation
- tRNA charging, initiation, elongation, termination and peptide release
what energy does tRNA charging require ?
ATP
What is the process by which an amino acid is attached to tRNA?
- the carboxyl group of the amino acid binds with the 3’ OH group of the adenine nucleotide of the acceptor arm on tRNA (5-CCA-3)
how are amino acids binded to tRNAs called?
- aminoacyl - tRNA
What ensures that the correct amino acid is bonded to tRNA? how many are there?
- aminoacyl synthetase is an enzyme which bonds them: 20 different types: recognizes one Amin acid and attached to to the correct set of tRNAs
(one for each amino acid)
What are the requirements for initiation of translation? What is IF-3’s job?
- mRNA, small and large ribosome subunits, initiation factors, initiation tRNA, GTP
- IF-3 binds to the small subunit, preventing the large subunit from binding and allowing small subunit to bind to mRNA
what occurs after the IF-3 has bonded to small subunit?
16S rRNA complementary bae pairs to the shine dalgarno sequence
- then UAC base pairs with start codon : tRNA charged with Met
What are the roles of initiator proteins in translation?
- IF-3 binds to small subunit to prevent large subunit from binding: IF-3 keeps the subunits separate
- IF1 and IF2 direct the initiator TRNA to the correct site
= all together forms the 30S initiation complex
What forms of energy are required for translation ?
ATP for charging, GTP for initiation and elongation
What occurs after the 30-S initiation complex?
IF proteins dissociated and the large subunit binds
forms the 70S initiation complex (fully formed ribosome)
Describe the key steps of elongation
1) aa enters at the A site
2) peptide bond forms
3) ribosome translocation
4) tRNA leaves at the E site
describe amino acid delivery to the A site
- incoming aa tuna binds to the A site
- elongation factors guides incoming aa-TRNA t correct site and anticodon binds with mRNA codon
Describe. peptide bond formation in translation
- peptidyl transferase caratylses peptide bond formation
- enzyme activity occurs in the large ribosomal subunit
- peptide bond forms between amino acids of the tRNAs as the P site and A site
- dipeptide forms on the tRNA at the A site
describe ribosomal translocation
- translocates in the 5’ to 3’ direction
- uses elongation factors to move
- tRNA now at E site, can exit
- A site tRNA now at P site, A site open
Describe termination in translation
- termination occurs when ribosome translocates to a stop codon
- Release factor proteins bind to A site and trigger release of polypeptide from P site tRNA
Describe the difference between prokaryotic translation and eukaryotic
prokaryotic: 30S + 50S = 70S
- has shine dalgarno sequence
- polycystronic: multiple ORFS
- eukaryotic: 60S + 40S = 80S
- monocystronic: one ORF
