The Genetic Code Miss Richardson SBI4U

A physician at Oxford University, studied the disease alkoptonuria A build-up of homogentisic acid causes urine to turn black when exposed to air Garrod collected family and medical data Deduced that the condition was a recessive disorder caused by a defective enzyme in the metabolic pathway of phenylalanine Garrod’s work had shown a link between genes and proteins 1902 – Archibald Garrod

1941 - Beadle & Tatum Studied the bread mold Neurospora crassa Grew cultures on minimal mediums and exposed them to x-rays This produced some organisms with mutant genes They crossed the F1 generation with non-iradiated Neurospora 1941 - Beadle & Tatum

The F2 generation that was produced could only replicate with the addition of the amino acid arginine They had shown the arginine-dependent strains had lost use of the specific gene facilitating the production of this amino acid 1941 – Beadle & Tatum

One Gene-One Polypeptide Hypothesis Beadle & Tatum concluded that one gene specifies one enzyme They shared the Nobel prize for the “One gene – One enzyme hypothesis” The hypothesis has been revised to “One gene – One polypeptide” One Gene-One Polypeptide Hypothesis

Worked on identifying amino acids in insulin and noticed each insulin molecule had the same amino acid sequence Concluded each protein has a specific amino acid sequence Suggested that DNA too must have an ordered sequence 1953 – Frederick Sanger

Protein synthesis was known to occur in the cytoplasm, yet DNA is only found in the nucleus 1961 – Francois Jacob & Jacques Monod proposed that another nucleic acid, RNA was synthesized by DNA of genes messenger RNA (mRNA) would be a complement of the DNA sequence, providing the amino acid sequence required for the specific protein Messenger RNA

1964 - The mRNA Hypothesis Working with Sydney Brenner & Mathew Meselson, Jacob & Monod confirmed their hypothesis

1961 – Through work with E. coli and bacteriophages, Crick & Brenner demonstrated that the genetic code is read continuously in triplets, or codons Research groups worked for four years to determine the codons specific to each amino acid The Genetic Code

AUG = START codon UAA, UAG & UGA = STOP codons The Genetic Code

It is redundant – more than one codon can code for the same amino acid It is continuous – it reads codons without spaces or overlap It is nearly universal – all organisms build proteins with the same genetic code The Genetic Code