28 - Translation I

Question 1

Protein synthesis is in what direction?

Answer 1

N-terminal to C-terminal

Question 2

What are active precursors of protein biosynthesis?

Answer 2

Aminoacyl-tRNAs

Question 3

What are the three broad stages of translation?

Answer 3

Initiation
Elongation
Termination

Question 4

What are the four broad steps of translation initiation in prokaryotes?

Answer 4

• mRNA, SSU (30S) of ribosome and fMet-tRNA form the 30S initiation complex
• Anticodon of fMet-tRNA binds to start signal codon (AUG) on mRNA
• 30S initiation complex and large (50S) ribosomal subunit form 70S initiation complex
• fMet-tRNA occupies the P (peptidyl) site on the LSU

Question 5

What are the five steps of translation elongation in prokaryotes?

Answer 5

• Next aminoacyl-tRNA (after Met) binds in A (aminoacyl) site on the LSU of the 70S ribosome
• Peptide bonds is formed by peptidyltransferase
• Discharged tRNA in P site is moved (translocated) into E (empty) site on LSU
• Peptidyl-tRNA is translocated from A site to P site (GTP hydrolyzed)
• Next aminoacyl-tRNA binds in A site and there is another round of elongation

Question 6

What are the three steps of translation termination in prokaryotes?

Answer 6

• Termination (release factor), a protein, interacts with the stop signal (termination codon: UAA, UAG, UGA) on mRNA in A site
• Aminoacyl-ester bond in peptidyl-tRNA in P site is hydrolyzed
• Completed polypeptide chain is released

Question 7

What is an important difference in the way mRNAs are recognized by the ribosome between prokaryotes and eukaryotes?

Answer 7

Eukaryotes: The 5’ cap on mRNA is recognized and the ribosome scans mRNA for first AUG after cap

Prokaryotes: 16S rRNA of SSU recognizes specific sequence (Shine-Dalgarno) upstream of AUG start codons

Question 8

Recognition of start codons is facilitated by the presence of ____ in prokaryotes?

Answer 8

Ribosome recognition motifs (Shine-Dalgarno, or S-D sequence)

Question 9

What is the enzyme that caps mRNAs at the 5’ end of mRNA in eukaryotes?

Answer 9

Guanylyltransferase associated with Pol II CTD, once the cap is complete, guanylyltransferase dissociates and the cap-binding complex (CBC) binds.

Question 10

What constitutes the 5’ cap on eukaryotic mRNA transcripts?

Answer 10

A methylated guanosine (7-methylguanosine linked to the last base with a 5’-triphosphate linkage (rather than phosphodiester bond)

Question 11

How are are poly(A) tails added to the 3’ ends of eukaryotic mRNAs (polyadenylation)?

Answer 11

• Pol II transcribes the poly(A) addition site (AAUAAA)
• Polyadenylation factor binds the poly(A) signal initiating mRNA cleavage
• PAP (polyadenylate polymerase) synthesizes the poly(A) tail and PABP (poly(A) binding protein protects the tail from degradation.

Question 12

What are three importantces of 5’ capping in eukaryotes? (3)

Answer 12

• Is essential for cell growth
• Serves to identify translation start sites
• Has important effects on mRNA maturation, translation and stability

Question 13

mRNA turnover is catalyzed by ___ in eukaryotes? (3)

Answer 13

• Sequential deadenylation
• Decapping
• Exonucleolytic degradation

Question 14

What What are three importantces of 3’ polyadenylation in eukaryotes? (3)

Answer 14

• Export from the site of production in the cell nucleus
• Translation into protein
• Stability

Question 15

How does the Shine-Dalgarno sequence in mRNA (prokaryotes) help it bind to ribosomes?

Answer 15

The purine-rich Shine-Dalgarno sequence, just upstream of the start (AUG) codon, interacts by complementary, antiparallel base pairing with a pyrimidine rich sequence (Anti S-D sequence very close to the 3’ end of the small ribosomal subunit)

Question 16

Each open reading frame in a polycistronic mRNA in prokaryotes contains what three cis elements?

Answer 16

• Start
• Stop
• Shine-Dalgarno sequence

Question 17

What is a polysome? Do these exist in both prokaryotes and eukaryotes?

Answer 17

Collection of ribosomes on a mRNA molecule that is actively being transcribed, this is coupling where transcription and translation occur at the same time. This is only seen in prokaryotes. As the mRNA grows, the number of ribosomes associated with it increases.

Polysomes exist in eukaryotes as well, except translation is not coupled to transcription

Question 18

How can mRNA be circularized during translation in eukaryotes?

Answer 18

Factors attached to the 5’ cap can interact with factors bound to the poly(A), brining both ends of the mRNA together during translation.

Question 19

What is the purpose of circularized mRNA in eukaryotes?

Answer 19

Mechanism of gene regulation and expression at the translational level

Question 20

Is AUG the first codon of mRNA?

Answer 20

Not necessarily, there are usually upstream sequences for the binding of ribosomes. AUG does however begin the open reading frame and coding of protein

Question 21

What is a fMet?

Answer 21

Methionine with a formic acid group