8 - Genetic Code

Question 1

How did they find out which codon codes for which amino acid (1)?

Answer 1

• synthetic RNA was made by rUDP, rGDP, rADP and rCDP
• polynucleotide phosphorylase was used to make the chain
• E. coli cellfree extract (everything needed for translation)
• peptide chain was then made
• this was repeated with different known sequences of synthetic RNA 20 times with the addition of each amino acid each time

Question 2

How did they find out which codon codes for which amino acid (2)?

Answer 2

• synthetic RNA triplets incubates with ribosome sub units and aminoacyl tRNA with C14 label attached
• if correct triplet and amino acid - ribosome complex formed
• this is trapped on filer
• radiation would be detected
• this was done for all triplets and all amino acids

Question 3

What does it mean that the genetic code is degenerate?

Answer 3

an amino acid may be coded for by 1-6 codons

Question 4

What direction do you read an anticodon?

Answer 4

right to left

Question 5

What is aminoacyl tRNA synthase?

Answer 5

an enzyme which attached the correct tRNA to the correct amino acid

Question 6

What allows flexibility in base pairing of the anti-codon to the codon?

Answer 6

• wobble of the 5’ anticodon base

Question 7

What codon codes for start and which amino acid is this?

Answer 7

• AUG - methionine

Question 8

What codons code for stop?

Answer 8

• UGA
• UAA
• UAG

Question 9

What is an open reading frame?

Answer 9

start and stop codon in the same frame

Question 10

How many possible codons are there?

Answer 10

64

Question 11

how many codons code for amino acids?

Answer 11

61

Question 12

Name 2 different single base substitution outcomes

Answer 12

synonymous - same amino acid - most will be silent

non-synonymous - missense (different amino acid) or non-sense (premature stop codon)

Question 13

Name 2 different types of mutations reverting back to normal

Answer 13

• true revert - back to original DNA sequence
• pseudo-revert - second mutation suppresses the first - supressor mutation

Question 14

What are the 2 different types of suppressor mutations?

Answer 14

• intragenic suppressors - suppression of frameshift - happens within the same gene
• intergeneric suppressors - suppress a nonsense mutation - happens outwith the gene - modify the gene that codes for tRNAs, so they code for different amino acids

Question 15

How do cells containing suppressor mutations survive?

Answer 15

• all tRNA genes are duplicated in the genome
• a mutation in one is ok if there is enough of the other
• some proteins may be extended due to suppressor tRNAs binding to stop codons
• stop codon infrequently used in E. coli

Question 16

How do you translate a DNA sequence into an mRNA sequence?

Answer 16

• take the coding strand exactly as it is
• swap the Ts for Us

Question 17

What determines which strand is the template trend or the coding strand?

Answer 17

• the position of the promoter
• there is nothing intrinsically different in the two strands

Question 18

Why does RNA have uracil instead of thymine?

Answer 18

• T has a higher resistance to photochemical mutation, better to store long term info
• RNA evolved forst

Question 19

How do tRNAs deal with the degeneracy of the genetic code?

Answer 19

• some tRNAs have a wobble base at the 5’ of the anticodon
• different tRNAs recognise different codons but specify the same amino acids

Question 20

describe the structure of tRNA

Answer 20

• ss with extensive base pairing
• 2D - clover leaf
• 3D - L

Question 21

What are the 2 different types of missense mutations?

Answer 21

• conservative - similar amino acid
• non-conservative - quite different amino acid

Question 22

What are the 3 classifications of mutations on fitness?

Answer 22

• deleterious
• neutral
• beneficial