Decoding the Gene

The Genetic Code is contained in a three-letter sequence called a codon.

• A codon consists of three consecutive nucleotides, which specify a single amino acid.

• RNA codons are modeled after the DNA codons.

• Given a sequence of DNA with the letters:

• The corresponding mRNA sequence would be:

This sequence represents three codons:

What amino acids are represented by these three codons? (You may use a codon table or codon wheel.)

UCG

Serine

CAC

Histidine

GGU

Glycine

• Therefore the protein coded by the DNA gene segment

would have the amino acids:

Serine, Histidine & Glycine.

Notice AUG specifies methionine, or the “Start” codon & there are three “Stop” codons to specify the end of a protein.

How did they figure this out?

George Gamow postulated that a three-letter code must be employed to encode the 20 standard amino acids used by living cells.

The fact that codons consist of three DNA bases was first demonstrated in the Crick, Brenner et al. experiment.

Crick Brenner

Marshall Nirenberg and Heinrich J. Matthaei Experiment

1) They set up twenty test tubes, each with a different (radioactively -labeled) amino acid.

2) E.coli bacterial cytoplasm was added to each test tube. (Ribosomes etc. from bacteria)

3) Artificially synthesized mRNA with the codon UUU only, was place in each test tube.

4) The then analyzed each test tube to find out which one contains amino acids bonded in a chain.

Which amino acids was linked into chain?

UUU

Phenylalanine

They looked for the protein produced with the radiation. It was indeed made up of just phenylalanine.

Therefore Nirenberg and Matthaei knew the codon UUU represented the amino acid phenylalanine.

vial of poly U used in theexperiment

• They did a similar experiment with AAA which represents Lysine. They then published their results.

• Nirenberg assembled a team of scientists to crack the rest of the genetic code.