Self Study- Translation & Protein Synthesis

Question 1

What makes up the Protein Synthesis Initiation Complex

Answer 1

1. mRNA- the coded message, read as codons (5’ to 3’)
2. tRNA- reads message and brings in “Activated” amino acids
3. ribosomes- made up of rRNA (catalytics center for synthesis and organization of proteins) and proteins, made of large and small subunits

Question 2

Codons

How many codons

Which don’t specify AA?

Which is start?

Degenerate? (Exceptions)

Universal?

Continuous?

Direction of translation (And hence, way to read codons)

Direction of protein synthesis (and hence, written)

Answer 2

• 64
• UAA, UAG, UGA are termination (Stop codons)
• AUG (Met) is start, Met is frequently removed during protein processing
• Amino acids (except Met, Trp) have more than 1 codon
• Codons are same in prokaryotes/eukaryotes
• No spacers between codons
• 5’ to 3’
• amino to carboxy terminus

Question 3

Codon (mRNA)-Anticodon (tRNA) pairing

What direction are codon and anti-codon read? Parallel or antiparallel?

Wobble effect? Which base can vary?

What does inosine do?

Answer 3

• both 5’ to 3’
• Antiparallel
• Ability of anticodon to pair with more than 1 codon (only 50 anticodons for 51 codons), normally last base in codon can vary for a given anti-codon
• Inosine (I) in anticodon permits less stringent base pairing (often at 5’ position), can bp anything except G

Question 4

Activation (aminoacylation) of tRNA

How does this occur? What are 2 reactions catalyzed?

Does this enzyme have proofreading activity?

Answer 4

• An enzyme called aminoacyl-tRNA synthetase links the AA to its specific tRNA (at least one exists for each AA). Reactions catalyzed:
  1. Activate AA w/ ATP forming aminoacyl group
  2. Transfer aminoacyl group to tRNA
• Yes, very important to avoid translation errors! Done by size, charges.

Question 5

Initiator tRNA

Which AA initiates protein biosynthesis in prokaryotes/eukaryotes?

How many tRNA are there for this in prokaryotes/eukaryotes?

Answer 5

• Met for both
• Prokaryotes have 2:
  1. Initiatior tRNA (fMet-tRNA^fMet with formyl group)
  2. Carries Met for normaly places in protein
• Only 1 in eukaryotes

Question 6

Formation of Protein Synthesis Initiation Complex

Shine-Dalgarno sequence

Kozak sequence

Where does first Met enter? 2nd AA?

Answer 6

• 5’ nontraslated leader in prokaryotes’ sequence of mRNA that allows mRNA to properly align on rRNA small subunit
• Same as above for eukaryotes
• P site, A site

Question 7

Elongation and Translocation

What enzyme begin process?

A site? P site? E Site?

How does this process stop?

Answer 7

• peptidyltransferase (in ribosome)- transfer initial Met and growing peptide from carrier tRNA to AA of aminoacyl-tRNA specific by next codon
• Aminoacyl site (new AA enters), Peptidyl site (protein is built), Exit site (mRNA exits ribosome)
• Stop codon enters A site (no amino-acyl tRNA binds these codons), peptide is released, ribosome subunits disassemble

Question 8

Antibiotics that interfere w/ protein synthesis

Streptomycin

Tetracyclines

puromycin

Erythromycin

Cycloheximide

Ricin