Lecture 10 - Translation

Question 1

a-thalasemia

Answer 1

deletion of one base causes a frameshift, resulting in a read-through of the correct stop codon and usage of a new downstream stop codon. The longer protein, called a Wayne a-hemaglobin, has a higher rate of breakdown and contributes to the pathology of a-thalassemia

Question 2

Crick’s Wobble hypothesis

Answer 2

The “wobble” allows some tRNAs to recognize more than one codon. Some anticodons contain the nucleotide inosinate. Inosinate can hydrogen bond with either A, U, or C.

Question 3

5 stages of protein synthesis:

Answer 3

1. Activation of amino acids or “charging” of tRNA
2. Initiation of translation
3. Elongation of peptide chain
4. Termination of synthesis and release from ribosome
5. Folding and post-translational processing of peptide

Question 4

Stage 1: Activation of amino acids or “charging” of tRNA

Answer 4

aminoacyl-tRNA synthetases attach the correct amino acid to their tRNAs

Question 5

Stage 2: Initiation of translation

Answer 5

• Although methionine has one codon, AUG, all organisms have two tRNAs for methionine. One is used exclusively for the initiating methionine, the other for internal methionine residues.
• Bacteria initiate translation with N-formylmethionine-tRNA (fMet).
• Eukaryotes initiate cytosolic translation with a specialized initiating tRNA for methionine.
• Eukaryotes initiate translation in mitochondria and chloroplasts with fMet.
• In prokaryotes, the Shine-Dalgarno sequence binds to ribosome lining up AUG to P site to start translation.
• In eukaryotes, 5’ and 3’ end of mRNA are linked, mRNA is scanned until the first AUG within Kozak consensus, this binds to 40S subunit

Question 6

Stage 3: Elongation of peptide chain

Answer 6

• The second aminoacyl-tRNA enters the A site
• Peptidyl transferase catalyzes the bond between the two aa’s while they are still bound to the tRNAs
• Both tRNAs shift position

Question 7

Stage 4: Termination of synthesis and release from ribosome

Answer 7

• RF (release factor) recognizes and binds to the stop codon in the A site
• Peptidyl transferase transfers the growing polypeptide to a water molecule
• Other RFs come in and cause the whole mechanism to break apart

Question 8

Streptomycin

Answer 8

Processes Affected: Initiation, elongation

Site of Action: Prokaryotes: 30S subunit

Question 9

Neomycins

Answer 9

Processes Affected: Translation

Site of Action: Prokaryotes: multiple sites

Question 10

Tetracyclines

Answer 10

Processes Affected: Aminoacyl-tRNA binding

Site of Action: 30S or 40S subunits

Question 11

Puromycin

Answer 11

Processes Affected: Peptide transfer

Site of Action: 70S or 80S ribosomes

Question 12

Erythromycin

Answer 12

Processes Affected: Translocation

Site of Action: Prokaryotes: 50S subunit

Question 13

Fusidic acid

Answer 13

Processes Affected: Translocation

Site of Action: Prokaryotes: EF-G

Question 14

Cycloheximide

Answer 14

Processes Affected: Elongation

Site of Action: Eukaryotes: 80S ribosomes

Question 15

Ricin

Answer 15

Processes Affected: Multiple

Site of Action: Eukaryotes: 60S subunit