R8 The Genetic Code

Question 1

What is the maths that proves that there are 3 nucleotides for each amino acid ?

Answer 1

can’t be one otherwise there would only be 4 amino acids

can’t be 2 as there would only be 16 (4x4)

so must be 3 (4x4x4) making 64 possible amino acids, obviously the code is degenerative so there are only 20 amino acids

Question 2

What would happen if the genetic code was overlapping ?

Answer 2

A single base change would lead to a change in 3 amino acids

Question 3

What would culturing a cell in only Uracil show ?

Answer 3

That phenylalanine must be coded by UUU

the same for AAA is lysine, CCC is proline and GGG

Question 4

What would a synthetic mRNA strand of -ACACAC- show when translated ?

Answer 4

The alternating ACA and CAC lead to alternating threonine and histidine.

further modifications to CAACAA and AACAAC proved asparagine and glutamine

Question 5

What is a triplet binding assay?

Answer 5

The aminoacyl tRNA are labelled with carbon-14, this can be detected by radiation

This determined more amino acids

Question 6

In what direction are codons read?

Answer 6

5’ to 3’

Question 7

What is the use of the wobble in the anticodon ?

Answer 7

allows for flexibility when pairing so G can code for both C and U, allows for versatility

Question 8

What is the open reading frame (ORF)?

Answer 8

The code between the START and STOP codon

Question 9

What is a point mutations ?

Answer 9

A single base substitution

Question 10

What are the different types of mutations

Answer 10

• silent mutations – no effect on amino acid
• neutral mutation – amino acid changes but not protein function
• missense mutation – amino acid changes and protein is non-functional
• Nonsense mutation – codon becomes a stop codon, shorter protein forms (truncated)

Question 11

What is a frameshift mutation?

Answer 11

single base insertion or deletion
- Shifts reading frame to the right, changing all codons downstream – frameshift mutation

Question 12

How can a second mutation restore gene function?

Answer 12

• True revertant – original DNA sequence restored
• Pseudo-revertant – a second mutation supresses the first, suppressor mutations

o Intragenic suppressors – The mutation downstream would shift the frameshift back to the left, deletion can help this

o Intergenic suppressors – suppression of nonsense mutations, Nonsense mutation in one gene can be corrected by a second mutation in another gene. A mutation in a tRNA gene can form a suppressor tRNA – a tRNA that binds to a stop codon by codes for tyrosine (UAC/UAU/ UAG)
However, this can cause read-through in other genes resulting in an extended polypeptide
There is enough normal tRNAtyr available to combat the suppressors, suppressors compete with release factors for STOP codons in all mRNAa
tRNA suppressors are rare in organisms like E. coli

Question 13

Why does RNA have uracil, and DNA has thymine?

Answer 13

RNA is considered the evolutionary precursor molecule to DNA. T has higher resistance to photochemical mutation: better to store information in the long-term.

Question 14

How does tRNA deal with degeneracy in the genetic code?

Answer 14

Some tRNAs shows flexibility (wobble) at the 5’ anticodon, so that G can pair with U at the 3’. Different tRNAs recognise different codons but specify the same amino acid.

Question 15

What is a synonymous mutation?

Answer 15

Due to the degeneracy of the genetic code, the mutation leads to the same amino acid so is a silent mutation, this could be due to the variety of tRNA

Question 16

What can missense lead to ?

Answer 16

Doesn’t always lead to a non-functional protein and can lead to similar functions or a different aspect of a function

Question 17

What is the worst case scenerio for open frame shift?

Answer 17

can lead to no STOP codon being coded for, leading to an elongated protein or the transcription of another gene further downstream.