Topic 7.4 Translation

Question 1

7.4.1 Explain that each tRNA molecule is recognized by a tRNA-activating enzyme that binds a specific amino acid to the tRNA, using ATP for energy.

Answer 1

1. start codon (AUG) is on 5’ ends of all mRNAs; each codon (except 3 stop codons) attaches to a particular tRNA
2. tRNA has 5’ and 3’ end (like all other nucleic acid strands)
   
   • 3’ end of tRNA is free, has a base sequence CCA to which amino acids attach
3. hydrogen bonds form in 4 areas b/c of complementary bases in single stranded tRNA → causes tRNA to fold and become 3D structure
   
   • molecule has 2D appearance of clover leaf when flattened
     
     • 1 loop of “clover leaf” contains exposed anticodon; anticodon is unique to each type of tRNA (pairs with a specific codon of mRNA)
4. due to action of tRNA activating enzyme, each of the 20 amino acids bind to appropriate tRNA molecule
   
   • active site of tRNA activating enzyme allows a fit only between a specific amino acid & a specific tRNA
   • 64 anticodons (4³) (some degenerate i.e. multiple codons code for 1 amino acid) = 20 amino acids
5. attachment of amino acid & tRNA requires energy from ATP
6. finally, structure is called an activated amino acid; tRNA can now deliver the amino acid to a ribosome to produce a polypeptide chain

Question 2

7.4.2 Outline the structure of ribosomes, including protein and RNA composition, large and small subunits, three tRNA binding sites and mRNA binding sites.

Answer 2

• ribosomes made of many distinct proteins and ribosomal RNA (rRNA) molecules (generally small, account for ~2/3 ribosome mass)
• consist of 2 subunits:
  
  • small subunit contains an mRNA binding site
  • large subunit contains 3 tRNA binding sites - an aminacyl (A) site, a peptidyl (P) site and an exit (E) site
    
    • A site: holds tRNA carrying the next amino acid to be added to the polypeptide chain
    • P site: holds tRNA carrying the growing polypeptide chain
    • E site: site from which tRNA that has lost its amino acid is discharged
  • tRNA moves sequentially through 3 binding sites: A site → P site → E site
• Space between 2 subunits = where mRNA strand is decoded to produce a polypeptide
  
  • polypeptide assembled in cavity between 2 subunits
  • area generally free of proteins ∴ binding of mRNA and tRNA is carried out by rRNA
  • growing polypeptide chain exits ribosome through tunnel in large subunit core

Question 3

7.4.3 State that translation consists of _________, __________, _____________ and ___________.

Answer 3

Translation consists of initiation, elongation, translocation and termination.

Initiation: involves assembly of an active ribosomal complex - activated amino acid attached to tRNA attaches to mRNA start codon

Elongation: new amino acids brought to ribosome according to the codon sequence; rRNA forms peptide bonds between amino acids

Translocation: amino acids translocated to a growing polypeptide chain

• growing polypeptide chain attaches to tRNA at A site (which also contributes amino acid to chain)
• tRNA at P site transfers polypeptide chain to tRNA at A site; moves to E site and is released

Termination: at certain ‘stop’ codons, translation is ended & polypeptide is released

• protein called a release factor fills A site, carrying water - not amino acid → catalyses hydrolysis of bond linking tRNA in P site with polypeptide chain
• releases polypeptide from ribosome
• ribosome separates from mRNA and splits into its 2 subunits

Question 4

7.4.4 State that translation occurs in a __ → __ direction.

Answer 4

Translation occurs in a 5’ → 3’ direction

Question 5

7.4.5 Draw and label a diagram showing the structure of a peptide bond between two amino acids.

Answer 5

peptide bond forms when water is given off – condensation reaction; reaction between carboxylic acid group (COOH) & amino group (NH₂)

N C C N C C

Question 6

7.4.6 Explain the process of translation, including ribosomes, polysomes, start codons and stop codons.

Answer 6

1. tRNA containing matching anticodon to start codon binds to P site of small subunit of the ribosome
2. small subunit binds to the 5’ end of the mRNA & moves in a 5’→3’ direction until it reaches the start codon
3. large subunit then binds to the smaller one
4. the next tRNA with a matching codon to the next mRNA codon binds to the A site
5. amino acids on the 2 tRNA molecules then form a peptide bond (done by rRNA)
6. larger subunit moves forward over smaller one
7. smaller subunit rejoins larger one
   
   • this moves the ribosome 3 nucleotides along the mRNA, and moves 1^st tRNA to E site for release
8. second tRNA now at P site; a new tRNA with matching codon to next mRNA codon can now bind to A site
   
   • as process repeats, polypeptide is elongated
9. once ribosome reaches ‘stop’ codon on mRNA, translation ends and polypeptide is released
10. many ribosomes can translate a single mRNA at the same time; these groups of ribosomes are called polysomes

Question 7

7.4.7 State that free ribosomes synthesize proteins for use primarily _______ the cell, and that bound ribosomes synthesize proteins primarily for _________ or for _________.

Answer 7

Free ribosomes synthesize proteins for use primarily within the cell. Bound ribosomes synthesize proteins primarily for secretion or for lysosomes.