L11: The Genetic Code Flashcards
How many possible triplet codons exist for a 4-letter code?
4x4x4=64
how many amino acids are there?
20
how many stop codons?
3
what does it mean for the genetic code to be degenerate
- 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
define synonyms
two different codons specifying the same amino acid
what has the genetic code evolve to minimize?
deleterious effects of mutation via codon positions
codon position evolution - first position mutation
mutations often change amino acids to one with similar properties
codon position evolution - second position specifies what?
- pyrimidines (T, C, U) tend to specify hydrophobic amino acids
- purines (A, G) tend to specify hydrophilic amino acids
codon position evolution - second position mutation
- results in transition mutation
- unlikely to change amino acid property
codon position evolution - third position mutation
any mutation is silent
number of tRNAs vs amino acids
- one tRNA for each 61 amino acid codon
- but there are 30-50 tRNA (depending on the species)
one tRNA for each 61 amino acids - how is this possible when theres 30-50 tRNAs
- the wobble hypothesis
- proposed by Francis Crick
what is the wobble hypothesis
- 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)
wobble hypothesis - how is the third position in the codon flexible?
- 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
wobble hypothesis - how does the 5’ position of the anticodon wobble?
- 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
wobble hypothesis - what is inosine (I)
- happens with a deamination mutation in adenine (A)
- I can basepair with A, U, or C
cracking the genetic code - Francis Crick and Sydney Brenner
- worked on bacteriophage T4 gene
called rII - they determined triplet nature of genetic code
Francis Crick and Sydney Brenner - what is the evidence explaining that DNA consists of a triplet code (codons)
- 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
cracking the genetic code - Heinrich Matthaei and Marshall Nirenberg
- found the meaning of each codon
Heinrich Matthaei and Marshall Nirenberg - what did they do to find the meaning of each codon
- made and translated polynucleotide synthetic RNAs
- made repetition of 2- and 3-nucleotide sequences
- sum of all data facilitated the establishment of codon table
what are the rules of the genetic code?
- 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)
mutations in the code - what are the classes?
- point mutation
- frameshift mutation
- suppressor mutation
mutation in the code - what is a suppressor mutation?
a second mutation that reverses the harmful effects of a first mutation
mutation in the code: suppressor mutation - what are the two types?
- reverse (back) mutation
- intragenic suppressor
mutation in the code: suppressor mutation - what are reverse (back) mutations?
changes an altered nucleotide back to its original identity
mutation in the code: suppressor mutation - what are intragenic suppressors
a suppressor mutation in the same gene as the first lesion but at a different location
mutation in the code: intragenic suppressors - how can these mutation restore a mutation?
- single base deletion results in the suppression of an earlier single base insertion
- the first mutation alters protein folding and the second suppressor mutation restores the proper structure
explain the advantages of the universality of the genetic code
- reveals evolutionary relationships
- genetic engineering
universality - reveals evolutionary relationships
establish phylogenies by comparing protein-coding sequences between organisms
universality - genetic engineering
human proteins can be expressed in a large quantity in bacteria
