Cracking the Genetic Code

Rafael Rimoldi & Yossi Pinkus

Cracking the genetic code

George Gamow: genetic code was triplet and overlapping ACGT: ACG, CGT, CGT Proposed the “Gamow’s Diamond Hypothesis”

4 nitrogen bases / 20 amino acids If code was double: 4x4 = 16 possibilities

If code was triplet: 4x4x4 = 64 possibilities

Francis Crick & Sidney Brenner’s experiment

Cracking the genetic code

Overlapping vs. Non-overlapping

1957. Brenner theory: overlapping code would be impossible

Dipeptide example : ATCG-> ATC and TCG. 4x4x4x4 = 256 possibilities

Theoretically more than 256 sequences could not be possible

There are 20 different amino acids, so: 20x20 = 400 possibilities

1961. Analysis of mutation-altered proteins

If genetic code was overlapped change of one base could alter as many as three amino acids.

ATCGTA – ATC, TCG, CGT, GTA

ATAGTA – ATA, TAG, AGT, GTA

A single base change has altered only one amino acid instead of three.

Polynucleotide Phosphorylase

Used the enzyme to synthesise single strand RNA molecules, to identify codons and their matching amino acid

No DNA needed for synthesis Enzyme degrades RNA in E.coli Nirenberg & Matthaei

experimentPoly U

What else was needed to crack the Code?

Har Gobind Khorana used biochemical methods to produce RNA with known bases

• Nirenberg and Leder experiment- Short RNA sequences in free cell system to crack code for non repeating unit codons e.g. (UUC, CUG)

References

Kresge N, Simoni R, Hill R. Severo Ochoa’s Contributions to the Citric Acid Cycle. JBC. 2005; 280(11) p. 140

Fredholm L. How the Code was Cracked. Nobelprize.org [online]. 2004 [cited 17 March 2015] available from http://www.nobelprize.org/educational/medicine/gene-code/history.html

Hayes B. The Invention of the Genetic Code. American Scientist. 1998; 86(1) p. 8

Yanofsky C. Establishing the Triplet Nature of the Genetic Code. Cell. 2007; 3(5):815-818. doi:10.1016/j.cell.2007.02.029

Griffiths AJF, Miller JH, Suzuki DT, et al. An Introduction to Genetic Analysis. 7th edition. New York: W. H. Freeman; 2000. Genetic code.  [cited 16 March 2015] available from http://www.ncbi.nlm.nih.gov/books/NBK21950/