Cracking the genetic code

To crack the genetic code, scientists had to work out which of the 64 codons determined each amino acid. To do this, they made mRNA molecules with a known sequence of bases. This mRNA was added to a cell-free system that contained isolated ribosomes, radioactively labelled amino acids, and all the enzymes needed for polypeptide synthesis. The polypeptides that were synthesised were then analysed to determine their amino acid sequence.

The first synthetic mRNA molecule made was a chain of uracil bases and was called poly-U. The polypeptide chain synthesised from it contained only phenylalanine. It was therefore concluded that the codon UUU codes for phenylalanine.

The complete genetic code was confusionfinally deciphered in 1966.

Quick check:

1. What is the “central dogma” of biology?

2. What is the name given to the result of adding one or two nucleotide bases to a DNA sequence?

3. Describe the relationship between DNA, messenger RNA, and proteins.

4. Explain how frame-shift experiments support the triplet code hypothesis.

5. Discuss the main features of the genetic code.

6. Divide the text into an introduction, principal part and conclusion.

7. Express the main idea of each part.

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