Translation / Generation of Proteins Flashcards
1
Q
What is protein synthesis
A
- Synthesis of mRNA from DNA template
- Migration of mRNA from nucleus to cytoplasm (ribosome)
- tRNA brings amino acids complimentary to codons on mRNA
- Peptide bonds between AAs create polypeptide chains
2
Q
What is genetic code
A
- Set of rules that determines how a nucleotide sequence is converted into AA
- Complementary structure allows precise replication during cell division
- First codon establishes reading frame, 5’ to 3’
- Codon: Language of mRNA, groups of three mRNA nucleotides that code for a particular AA
- 3rd base is a ‘wobble’ base (altered without changing AA)
- Start / Stop: Translation begins at start codon (AUG) and ends at stop codon (UAA, UAG, UGA)
3
Q
How are AA’s loaded onto tRNAs
A
- Active site on aminoacyl tRNA synthetase binds ATP and AA, 2 P groups released
- AMP and AA bound to enzyme, ‘activated’ amino acid has high potential energy
- Activated AA is transferred from tRNA synthetase to tRNA specific to that AA, AMP leaves
- Aminoacyl tRNA now able to participate in translation
4
Q
What is the wobble hypothesis
A
- Non-standard base pairing at third position allows for a limited flexibility in base pairing
- Many AA are specified by one than one codon
- Codons of the same AA have same nucleotides at first and second positions
Examples: - CAA and CAG code for glutamine
- The anticodon GUU can base pair with both CAA and CAG
5
Q
What are ribosomes
A
- ‘Decoding’ units of cell
- Two subunits (30s and 50s - 70s)
- Contain binding sites for both tRNA and mRNA molecules
6
Q
Briefly list the steps involved in translation
A
- Initiation: mRNA binds ribosomal 30s
- Elongation: Peptide bone formation using ribozyme
- Termination: Releasing factor binds stop codon, catalyses hydrolysis of bond between tRNA in P side and peptide change
7
Q
What occurs during initiation of translation
A
- Ribosomes bind 5’ cap
- Move downstream mRNA until they reach first AUG (methionine)
- AUG is an untranslated region at beginning of mRNA, the 5’ untranslated region (5’ UTR)
8
Q
What occurs during elongation of translation
A
- The ribosome has 2 sites for tRNAs, called P and A
- Initial tRNA with attached AA is in the P site
- A new tRNA, corresponding to next codon on mRNA, binds to A site
- Ribosome catalyses a transfer of AA from P site onto the amino acid at the A site, forming a new peptide bond
- Ribosome moves down one codon, and now-empty tRNA at P site is displaced
- tRNA that has the growing peptide chain is moved from the A site to the P site
9
Q
What are poly-ribosomes
A
- Once elongation is underway additional ribosomes begin translation at the start site
- Observed in both prokaryotic and eukaryotic cells
- Increases amount of protein generated from transcript copies
10
Q
What occurs in termination of translation
A
- Three codons are called “stop codons”
- Code for no AA and all protein-coding regions end in a stop codon
- When the ribosome reaches a stop codon, there is no tRNA that binds to it
- Instead, proteins called “release factors” bind
- Note that the mRNA continues on past the stop codon
11
Q
What are peptide bonds and polypeptide chains
A
- All AA have an amino group (-NH2) group on one end, and an acid group (-COOH) on the other end
- When polypeptides are synthesised, the acid group of one AA is attached to amino group of the next AA, forming a peptide bond
- New polypeptide folds spontaneously into its active configuration and combines with other necessary subunits to form an active protein
12
Q
What compounds are able to inhibit protein synthesis
A
- Certain antibiotics / compounds interfere with protein synthesis
- Tetracyclines: Bind 30s and interfere with attachment of tRNA
- Aminoglycosides: Broad spectrum, toxic at high levels
- Cycloheximide: Interferes with elongation
- Ricin: Acts as an enzyme and causes cell death by cleaving 28A subunit of RNA
