Lecture 15 Nature of the Genetic Code

Question 1

How many amino acids are there

Answer 1

20

Question 2

What is meant by the degeneracy of the genetic code.

Answer 2

The code is described as degenerate because there are more triplet codons than there are amino acids. In other words, for every amino acid, there are multiple codons.
- Eg glutamic acid is encoded by both GAA and GAG.

Question 3

How many codons are possible

Answer 3

4 bases^3 = 64 possible triplet codes.

Question 4

Why is the genetic code described as degenerate (model answer)

Answer 4

• Because there are 61 codons for just 20 amino acids

- This means every amino acid has multiple different codons

Question 5

Of the 64 mRNA codons, how many specify amino acids

Answer 5

61

Question 6

How many mRNA codons specify stop codons

Answer 6

3

Question 7

The following statement is true or false

’ There are more than 1 transfer RNA species for certain amino acids’

Answer 7

True, due to the redundancy of the genetic code.

Question 8

The genetic code is non overlapping… True or False

Answer 8

True. Each base is part of one codon at most.

Question 9

How was the genetic code broken/ cracked?

Answer 9

• Through a series of experiments

- By using synthetic messenger molecules.

Question 10

Define a stop codon and provide three examples

Answer 10

• Stop codons do not specify any amino acids

- Examples include UAA, UGA and UAG.

Question 11

What does the Open Reading Frame refer to, in molecular genetics

Answer 11

An open reading frame (ORF) is the part of a reading frame that has the ability to be translated. An ORF is a continuous stretch of codons that begins with a start codon (AUG) and ends with a stop codon.

Question 12

Explain the role of transfer RNA in translation

Answer 12

• Each transfer RNA serves as an adaptor that can bind an amino acid at one end and bind the mRNA codon
• The anticodon interacts with codons.
• The amino acid acceptor arm is where the amino acid binds.

Question 13

Some transfer tRNA molecules can recognize more than one mRNA codon. Explain how such phenomenon occurs.

Answer 13

-Leucine is encoded by AUG, AUC, AUU. To get around this, each transfer RNA molecule can recognize different codons due to its ability to form wobble base pairs.

Question 14

Explain the concept of Wobble base pairing

Answer 14

The first two bases of a codon must correctly pair with the anti-codon but the last base of the codon can “wobble” from its normal position to form unusual base pairs with the codon.
- Here G can pair with U

Question 15

G can base pair with U. Discuss.

Answer 15

G U wobble base pairing can occur.

Question 16

Why does wobble base pairing exist

Answer 16

This is the ability of transfer RNA to cope with the degeneracy of the genetic code. Eg, the fact that an amino acid has mutiple codons. So how does a transfer RNA charged with an amino acid bind to these slightly different codons. Well, it has flexibility, as long as the first 2 bases are the same, the third base can be different.

Question 17

Define wobble base pair

Answer 17

A wobble base pair occurs in an RNA molecule and is the formation of a base pairing that does not follow Watson-Crick base pairing rules (AT CG).

Question 18

Give an example of wobble base pairs

Answer 18

Guanine - Uracil pairing

Question 19

List the 4 predominant wobble base pairs

Answer 19

• GU
• I U
• I A
• I C

where I is hypoxanthanine.

Question 20

What is meant by the Wobble position

Answer 20

This is the 3rd nucleotide base in an mRNA codon.

Question 21

Some transfer RNA molecules (charged with a specific amino acid) can recognize more than one mRNA codon. What is the name given to the theory that accounts for such phenomeon

Answer 21

The Wobble Hypothesis

Question 22

What is the wobble phenomeonon

Answer 22

There is a relaxed set of base pairing rules at the third codon position, allowing a small number of transfer RNA molecueles to recognize multiple codons.

Question 23

What is the most common start codon

Answer 23

AUG, which codes for methionine

Question 24

Finish this sentance. Almost every protein chain begins with ….

Answer 24

AUG. The start codon Methionine.