Week 3 (Translation) Flashcards
Why is the genetic code said to be like a written language?
1 base= 1 letter
1 codon= 1 word
1 gene= 1 sentence
Why is a codon 3 bases long?
1 base= 4 possibilities
2 bases = 16 possibilities
3 bases = 64 possibilities
10 bases= 1,000,000 possibilities
What is the start codon?
AUG
What are stop codons?
UAA
UAG
UGA
What is the consensus sequence of the ribosome binding site?
AGGAGGU
Why start transcription ahead of the genes
To leave space for the ribosome to bind before they get to the start codon
What is the direction used to describe polypeptides?
From the amino terminus, NH2 (corresponds to the start of the gene) to the carboxyl end, COOH (corresponds to the end of the gene)
Explain the 10 stages of translation (E.Coli)
- The small ribosomal subunit binds to the ribosome binding site (consensus sequence: 5’-AGGAGGU-3’)
- The small subunit moves along the mRNA to the start codon (5’-AUG-3’) which encodes methionine
- The first tRNA binds to the P site (tRNAfmet- formyl
methionine that is modified to protect the amino group) requires IF2 (initiation factor 2) - The large subunit binds (requires hydrolysis of GTP to GDP + Pi) forming E, P and A sites within the ribosome
- The second tRNA binds to the A site- requires EFTu (elongation factor)
- A petite bond is formed by peptidyl transferase (part of the ribosome)
- The uncharged tRNA is freed from the ribosome by tRNA deacylase via the E site
- Translocation occurs (requires hydrolysis of GTP to GDP + Pi) require EFG
- Stages 5 to 8 repeat in a continuous cycle until
- A termination codon is encountered (no tRNA exists)
RF1 or RF2 (releasefactor) binds A site and cleaves the polypeptide from the tRNA in the P site. RF3 cooperates
What are the 3 translation signal?
- Start codon
- Ribosome binding site
- Stop codon
What is the C-terminal end of an amino acid also known as?
The carboxyl end
What is the N terminal of amino acids also known as?
The amine terminus
A peptide chain grows by the addition of amino acids. To which end are consecutive amino acids added?
To the C terminal end
Why is the formation of a peptide bond energetically favourable?
Because the growing C terminus has been activated by the covalent attachment of a tRNA molecule
Why do amino acids carry energy for the addition for the next amino acid rather than its own?
Addison disrupts the high energy covalent linkage (amino acid-tRNA) , but immediately replaces it with an identical linkage on the most recently added amino acid (peptide bond)
What is the function of the small subunit of a ribosome?
Provides a framework on which tRNAs are accurately matched to the codons of mRNA
What is the function of the large subunit of the ribosome?
Catalysed the formation of the peptide bonds that link the amino acids together in a polypeptide chain
What happens when the ribosomes are not synthesising proteins?
The two subunits separate
What does the A, P and E sites stand for ?
A: aminoacyl (tRNA)
P: peptidyl (tRNA)
E: Exit
What helps the ribosome maintain the correct reading frame on the mRNA?
The A and P sites are close enough together for their two tRNA molecules to be forced to form base pairs with adjacent codons on the mRNA molecule
How does the entire ribosome positioned to start the next cycle?
- Translocation of the large subunit followed by that of the small subunit causing the entire ribosome to move 3 nucleotides along the mRNA
- This resets the ribosome with a fully empty A site ready for the next aminoacyl- tRNA molecule to bind
What enzyme catalysed the formation of a peptide bond?
Peptidyl transferase
Why do both bacterial and eukaryotic cells have transcription elongation factors?
They make translation especially efficient and accurate by proofreading the codon-anticodon match. In this way am incorrectly paired tRNAs are selectively rejected
They hydrolyse GTP to GDP*
What are the transcription (elongation) factors in bacteria called?
EF-TU and EF-G
What are the transcription (elongation) factors in eukaryotes called?
EF 1 and EF2
