Cracking the code Flashcards
Marshall w. Nirenberg
Decided he could create a system to test which codons relate to which amino acids
Nirenberg’s first experiment
Polynucleotide phosphorylase is used to make a long chain of RNA (AAA/UUU).
These were put in a cell-free extract to see if the extract could synthesise a polypeptide out of the RNA.
DNAase enzyme added to break down mRNA already present.
20 test tubes each contain the cell free extract, a long synthetic RNA, a mixture of amino acids (one amino acid labelled).
A geiger counter was used to count the labelled vamino acid present. If radioactive then the protein must consist of that amino acid.
Codon UUU
Phe
Co-polymers
made up of a combination of nucleotides
You do not know which order the nulceotides are in and you have no control over the order of the bases. Only can control the proportions of two bases
Kharana
Synthesise of bases to an order
Nirenberg & working out the order of bases
Trinucleotides promote the binding of amino acid-tRNAs to ribosomes
All 64 codons were identified using Ninenberg’s and Kharana’s work.
Get codon. Stick to ribosome. Identify which tRNA and amino acid joins. If radioactively labelled you can detect which amino acid it is.
Ninerberg’s experiments: step by step
Very short mRNAs with known codons were synthesised.
Added to a mixture of ribosomes and tRNAs attached to amino acids.
Codon bound to ribosome will specify a tRNA & its amino acid to bind. Unbound tRNAs remain in solution, collected when solution passes through a filtering system (nitrocellulose filter). tRNA that is bound to a mRNA will stick to the filter, free tRNAs pass through.
Assay filter to determine which amino acid was bound. Check for radioactivity to determine amino acid,
Degeneracy
Several different codons all specify one amino acid
Wobble
Pairing at the first base is relaxed so there is improper pairing between tRNA and mRNA.
Genetic code was therefore tracked through a process of elimination
