Final Exam: Genetic Code

Question 1

What is the correlation between frequency of amino acid use in proteins and the number of codons representing an amino acid?

Answer 1

The more common an amino acid is, the more codons that represent it

Question 2

What is the wobble hypothesis?

Answer 2

• The ability of a tRNA to recognize more than one codon by unusual pairing with the third base codon (non G-C, non A-T).
• There are 61 potential anticodons, but most cells only contain about 40 different types of tRNA
• Multiple codons that represent the same amino acid most often differ at the third-base position.
• Unusual binding allows some tRNAs to pair with more than one codon
• The A-site of the ribosome gives more flexibility in the 3rd base position (looser monitoring).

Question 3

True or false: Only two bases of tRNAs can be modified

Answer 3

False: all four bases in tRNAs can be modified

Question 4

Define post-transcriptional modification

Answer 4

All changes made to the nucleotides of DNA or RNA after their initial incorporation into the polynucleotide chain

Question 5

How does tRNA modification typically occur?

Answer 5

Alteration of primary bases (methylation, etc.)

Question 6

Why are modifications in the anticodon important in determining tRNA specificity? Provide an example.

Answer 6

Modifications in the anticodon affect the pattern of wobble pairing and therefore are important in determining tRNA specificity.

Example: Inosine can pair with any of U, C, or A. Inosine is used in place o adenine in tRNA because it can bind with U, C, and A to make Isoleucine. Inosine cannot bind with G meaning the codon AUG codes for Met while the codons AUU, AUC, and AUA all code for lle

Question 7

True or false: most changes in the genetic code in bacterial or eukaryotic nuclear genomes change one amino acid to another

Answer 7

False: most changes in the genetic code in bacterial or eukaryotic nuclear genomes assign an amino acid to a stop codon or change a codon so it no longer specifies an amino acid. Changing the meaning from one amino acid to another is rare

Question 8

How do aminoacyl-tRNA synthetases charge tRNA?

Answer 8

The synthetase recognizes all the tRNAs that get loaded with a particular amino acid.

Question 9

Define isoacceptor tRNA

Answer 9

-Same amino acid; chemically different species of tRNA which are acylated by the same amino acid

Question 10

What is the difference between the two classes of tRNA synthetase?

Answer 10

Class I tRNA synthetase contacts tRNA at the minor groove of the acceptor stem and at the anticodon.

Class II contacts tRNA at the major groove of the acceptor helix and at the anticodon loop

Question 11

What type of proof reading is used by tRNA synthetase?

Answer 11

Chemical proof reading that occurs in two stages: a proofreading mechanism in which the correction event occurs after the addition of an incorrect subunit to the polymeric chain; addition reaction is reversed.

kinetic proofreading: incorrectly matched substrates dissociate more rapidly before reactions can take place.

Question 12

Define suppressor

Answer 12

A second mutation that compensates for or alters the effects of a primary mutation

Question 13

Define nonsense suppressor

Answer 13

A gene encoding a mutant tRNA that is able to respond to one or more of the termination codons and insert an amino acid at that site

Question 14

Define missense suppressor

Answer 14

A suppressor that encodes a tRNA that has been mutated to recognize a different codon. By inserting a different amino acid at a mutant codon, the tRNA suppresses the effect of the original mutation.

Missense suppression occurs when the anticodon of tRNA is mutated so that it responds to the wrong codon.

Question 15

Define readthrough

Answer 15

RNA polymerase or the ribosome ignores a termination signal because the mutation of the template or the behavior of an accessory factor