GENETIC CODE

Question 1

How do multiple codons code for the same amino acids?i.e. how is the code degenerate?

Answer 1

• different aminoacyl tRNAs recognise different codons but specify same amino acid
• some aminoacyls can regonise different codons due to wobble base pairing

Question 2

What is wobble base pairing?

Answer 2

wobble base position nucleotide on tRNA can make noncanonical pairings

Question 3

What is a non-canonical pairing?

Answer 3

eg. g-u instead of g-c

Question 4

How were synthetic homopolymers used to prove the codon and non-overlapping nature of DNA?

Answer 4

poly U RNA for example added to a cell extract and all of the amino acids radioactively labelled

• proved that UUU was Phe
• only contained radioactive

Question 5

How were copolymers used to demonstrate codon table?

Answer 5

similar to homo just using ACACACA for example

Question 6

What is a triplet binding assay ?

Answer 6

• based on ribosome complexes
• mix a synthetic RNA triplet with the ribosome subunits and a known C14 labelled aminoacyl tRNA
• complex forms and trapped in filter

Question 7

Start codons

Answer 7

AUG

Question 8

Stop codons

Answer 8

UAA, UAG, UGA

Question 9

Silent mutations

Answer 9

no influence on amino acid produced

Question 10

Nonsense mutations

Answer 10

Coded for a premature stop codon

Question 11

Missense mutation

Answer 11

Codes for wrong amino acid and protein rendered non-functional

Question 12

Neutral mutation

Answer 12

amino acid changes but protein remains functional

Question 13

What is a second mutation that restores function known as?

Answer 13

A revertant

Question 14

True revertants __(a)___ original DNA sequence. Pseudorevertants __(b)___ the first mutation.

Answer 14

a) restore

b) suppress

Question 15

2 forms of pseudorevertant suppressors?

Answer 15

INTRAgenic & INTERgenic

Question 16

Describe how intragenic suppressors work?

Answer 16

• they suppress frameshift mutations by reverse frameshift

Question 17

Describe how intergenic suppressors work?

Answer 17

• they suppress nonsense mutations, by having the tRNA understand what constitutes a nonsense mutation and hold an amino acid instead of a release factor

Question 18

Why can intergenic suppressors be disadvantageous?

Answer 18

• can simply read through stop codons and cause extended polypeptides

Question 19

How do cells survive despite the development of elongated polypeptides?

Answer 19

• toleration will occur as long as there is enough of major normal tRNA^Tyr available
• stop codon UAG infrequently used