Translation

Question 1

Translation to the code

Answer 1

Template strand - one of the two DNA strands provides the template for transcription
* During translation, the mRNA codons are read in the 5 to 3-direction
* Translation is initiated at a start codon and terminated at a stop codon
* These codons set the reading frame

Question 2

Translation machinery

Answer 2

• Translation of mRNA into protein is achieved via tRNA
• tRNA molecules – each carries a specific amino acid on one end and an anti-codon on the other
• Anti-codon pairs with a complementary codon on mRNA

Question 3

Structure and function of tRNA

Answer 3

• consists of a single RNA strand ~80 nucleotides long
• Flattened into one plane to reveal its base pairing, looks like a cloverleaf
• Amino acid site 3’
• Anticodon site

Question 4

Structure and function of tRNA

Answer 4

• Because of the hydrogen bonds, tRNA twists and folds to form a three-dimensional molecule
• tRNA is roughly L-shaped

Question 5

Accurate translation requires:

Answer 5

1. the tRNA anticodon is matched to the correct amino acid
2. the tRNA anticodons paired correctly to the mRNA

Question 6

The first of these is achieved by

Answer 6

the enzyme aminoacyl-tRNA synthetase

Question 7

There are 20 different tRNA synthetases

Answer 7

one for each amino acid - catalyze the attachment of a tRNA to its amino acid

Question 8

tRNAs and the Wobble Hypothesis

Answer 8

• 20 amino acids; 61/64 mRNA codons; 31 tRNAs
• 3rd base of the tRNA anticodon does not have to pair with a complementary codon base BUT can partially bond with any of several related bases i.e. it can wobble in its position during translation

Question 9

Base pair interactions

Answer 9

• Crick determined the following base-pair interactions:
• tRNA anticodons – Inosine at the third position

Question 10

tRNA-mRNA pairing

Answer 10

There are 61 codons but 45 tRNAs

Question 11

• Flexible pairing

Answer 11

at the 3rd base of an codon (‘wobble’) allows some
tRNAs to bind to more than one codon; e.g., UCU can bind to AGA or AGG – because U can bind to A or G
Contributes to further redundancy in the genetic code

Question 12

Ribosome structure

Answer 12

• A ribosome comprises two subunits, each made of proteins and ribosomal RNA
• It has three sites:
  o The A site holds the tRNA that carries the next amino acid to be added
  o The P site holds the tRNA that carries the growing polypeptide chain
  o The E site is the exit site, where discharged tRNAs leave the ribosome