R8 The Genetic Code Flashcards
What is the maths that proves that there are 3 nucleotides for each amino acid ?
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
What would happen if the genetic code was overlapping ?
A single base change would lead to a change in 3 amino acids
What would culturing a cell in only Uracil show ?
That phenylalanine must be coded by UUU
the same for AAA is lysine, CCC is proline and GGG
What would a synthetic mRNA strand of -ACACAC- show when translated ?
The alternating ACA and CAC lead to alternating threonine and histidine.
further modifications to CAACAA and AACAAC proved asparagine and glutamine
What is a triplet binding assay?
The aminoacyl tRNA are labelled with carbon-14, this can be detected by radiation
This determined more amino acids
In what direction are codons read?
5’ to 3’
What is the use of the wobble in the anticodon ?
allows for flexibility when pairing so G can code for both C and U, allows for versatility
What is the open reading frame (ORF)?
The code between the START and STOP codon
What is a point mutations ?
A single base substitution
What are the different types of mutations
- 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)
What is a frameshift mutation?
single base insertion or deletion
- Shifts reading frame to the right, changing all codons downstream – frameshift mutation
How can a second mutation restore gene function?
- 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
Why does RNA have uracil, and DNA has thymine?
RNA is considered the evolutionary precursor molecule to DNA. T has higher resistance to photochemical mutation: better to store information in the long-term.
How does tRNA deal with degeneracy in the genetic code?
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.
What is a synonymous mutation?
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
