Breaking the Genetic Code

When it had been firmly established that the genetic code consists of codons that are three nucleotides in length, the next step was to determine which groups of three nucleotides specify which amino acids. This task required the development of a cell-free system for protein synthesis (< Figure 15.8), which would make it possible to study the translation of a known mRNA.

Logically, the easiest way to break the code would have been to determine the base sequence of a piece of RNA, add it to a cell-free protein-synthesizing system, and allow it to direct the synthesis of a protein. The amino acid sequence

Prepairing a cell-free synthesizing system a

Grow bacteria in culture and isolate by centrifugation.

^ Grind the cells to release the cellular contents, including RNA, DNA, ribosomes, enzymes, and other components needed for translation.

^ Grind the cells to release the cellular contents, including RNA, DNA, ribosomes, enzymes, and other components needed for translation.

Deoxyribonuclease -

^ Add deoxyribonuclease. This enzyme destroys all the cellular DNA, and no more mRNA is produced. Protein synthesis stops.

mRNA of known sequence

Labeled amino acids ^J

^ Restart translation by adding mRNA of known sequence and labeled amino acids to the tube, and incubate the solution at 37°C.

^ The protein produced by the system can be precipitated by adding trichloro-acetic acid.

^ Restart translation by adding mRNA of known sequence and labeled amino acids to the tube, and incubate the solution at 37°C.

^ The protein produced by the system can be precipitated by adding trichloro-acetic acid.

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