Genetic code - Lecture 2

Question 1

How did they determine what codon specify what AA on a repeating base mRNA?

Answer 1

• Nirenberg and Khorana added artificial mRNA containing only one repeating base to a protein making cell
• The polypeptide produced contained a single amino acid and then researchers could determine what homopolypeptide AA were produced by what codon

Question 2

Why is there degeneracy in the genetic code?

Answer 2

Base pairing between the codon and the anti-codon at the 3rd base is flexible, this is known as the wobble position
This allows the 3rd base to not be complementary to the 3rd base of the anticodon but it will still bind so it will still encode the same AA

Question 3

What is the wobble hypothesis?

Answer 3

explains that stringent base pairing only occurs between the first two base pairs, so a single tRNA can respond to more than one codon

Question 4

What is inosine?

Answer 4

Another base that can bind at the wobble position

Question 5

Describe the Nirenberg and Leder experiment.

Answer 5

• short mRNA with known codons were synthesized and added to a ribosome/ tRNA (with already bounded AAs) mixture
• the tRNA molecules that had AA corresponding to the mRNA sequence bound it the mRNA, the tRNA to containing the correct AAs were passed through a filter

Question 6

How were mixed mRNAs analysed for which AA they produced?

Answer 6

the same way as homo mRNAs, using one repeating codon (ex:UCC) and different translation starts to determine 3 codon sequences for 3 AA