L11: The Genetic Code

Question 1

How many possible triplet codons exist for a 4-letter code?

Answer 1

4x4x4=64

Question 2

how many amino acids are there?

Answer 2

20

Question 3

how many stop codons?

Answer 3

3

Question 4

what does it mean for the genetic code to be degenerate

Answer 4

• there are too many possible codons (64) compared to the number of amino acids (20)
• so there are multiple codons for the same amino acid

Question 5

define synonyms

Answer 5

two different codons specifying the same amino acid

Question 6

what has the genetic code evolve to minimize?

Answer 6

deleterious effects of mutation via codon positions

Question 7

codon position evolution - first position mutation

Answer 7

mutations often change amino acids to one with similar properties

Question 8

codon position evolution - second position specifies what?

Answer 8

• pyrimidines (T, C, U) tend to specify hydrophobic amino acids
• purines (A, G) tend to specify hydrophilic amino acids

Question 9

codon position evolution - second position mutation

Answer 9

• results in transition mutation
• unlikely to change amino acid property

Question 10

codon position evolution - third position mutation

Answer 10

any mutation is silent

Question 11

number of tRNAs vs amino acids

Answer 11

• one tRNA for each 61 amino acid codon
• but there are 30-50 tRNA (depending on the species)

Question 12

one tRNA for each 61 amino acids - how is this possible when theres 30-50 tRNAs

Answer 12

• the wobble hypothesis
• proposed by Francis Crick

Question 13

what is the wobble hypothesis

Answer 13

• a tRNA molecule can recognize multiple codons for the same amino acid
• this is because of the flexible base pairing at the third position of the codon (mRNA) and the first position of the anticodon (tRNA)

Question 14

wobble hypothesis - how is the third position in the codon flexible?

Answer 14

• base at 5’ end of anticodon can interact with multiple bases at 3 ‘ end of codon
• 5’ position of anticodon can wobble
• other positions within anticodon are spatially constrained

Question 15

wobble hypothesis - how does the 5’ position of the anticodon wobble?

Answer 15

• it is positioned at the free end of a base-stacking sequence
• where there are non-covalent interactions between adjacent nitrogenous bases and bases stack on top of each other

Question 16

wobble hypothesis - what is inosine (I)

Answer 16

• happens with a deamination mutation in adenine (A)
• I can basepair with A, U, or C

Question 17

cracking the genetic code - Francis Crick and Sydney Brenner

Answer 17

• worked on bacteriophage T4 gene
  called rII
• they determined triplet nature of genetic code

Question 18

Francis Crick and Sydney Brenner - what is the evidence explaining that DNA consists of a triplet code (codons)

Answer 18

• rII gene was unable to tolerate insertions of 1 or 2 basepairs
• three insertions close together: functional product
• single detection: can rescue a single insertion mutant

Question 19

cracking the genetic code - Heinrich Matthaei and Marshall Nirenberg

Answer 19

• found the meaning of each codon

Question 20

Heinrich Matthaei and Marshall Nirenberg - what did they do to find the meaning of each codon

Answer 20

• made and translated polynucleotide synthetic RNAs
• made repetition of 2- and 3-nucleotide sequences
• sum of all data facilitated the establishment of codon table

Question 21

what are the rules of the genetic code?

Answer 21

1. Codons are read in the 5´ to 3´ direction
   2) Codon sequence is contiguous (nonoverlapping and without gaps)
   3) Message is translated in one fixed reading frame (established by start codon)

Question 22

mutations in the code - what are the classes?

Answer 22

• point mutation
• frameshift mutation
• suppressor mutation

Question 23

mutation in the code - what is a suppressor mutation?

Answer 23

a second mutation that reverses the harmful effects of a first mutation

Question 24

mutation in the code: suppressor mutation - what are the two types?

Answer 24

1. reverse (back) mutation
2. intragenic suppressor

Question 25

mutation in the code: suppressor mutation - what are reverse (back) mutations?

Answer 25

changes an altered nucleotide back to its original identity

Question 26

mutation in the code: suppressor mutation - what are intragenic suppressors

Answer 26

a suppressor mutation in the same gene as the first lesion but at a different location

Question 27

mutation in the code: intragenic suppressors - how can these mutation restore a mutation?

Answer 27

1. single base deletion results in the suppression of an earlier single base insertion 2. the first mutation alters protein folding and the second suppressor mutation restores the proper structure

Question 28

explain the advantages of the universality of the genetic code

Answer 28

- reveals evolutionary relationships - genetic engineering

Question 29

universality - reveals evolutionary relationships

Answer 29

establish phylogenies by comparing protein-coding sequences between organisms

Question 30

universality - genetic engineering

Answer 30

human proteins can be expressed in a large quantity in bacteria