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MCAT

Unit 2: Lesson 1

Foundation 1: Biomolecules

Early experiments on the genetic code

Problem

Researchers in the 1960's designed an experiment to determine how proteins could be synthesized from DNA. The experimental set-up included E. coli cell extract containing all of the components needed for protein synthesis in a cell-free system. To create this, researchers included a synthetic RNA strand made up of only uracil, with DNase added to remove other DNA in the system. They then prepared test tubes that each contained a single Cstart superscript, 14, end superscript radioactively labeled amino acid and nineteen unlabeled amino acids.
When they ran the experiment, they found the results shown in Table 1.
Table 1. Radiolabeled protein production with addition of poly-uracil RNA
Cstart superscript, 14, end superscript amino acidPoly-uracilCounts/min/mg protein
Alanine+33
Alanine-20
Phenylalanine+38, comma, 300
Phenylalanine-68
Leucine+899
Leucine-276
This same experiment was repeated with poly-adenine and poly-cytosine RNA. This was the first of many steps in deciphering the codons of the genetic code. By 1966 researchers announced they had deciphered all of the codons for the twenty amino acids.
Adapted from: Nirenberg, M. W. & Matthaei, M. (1961). The Dependence of Cell-Free Protein Synthesis in E. coli Upon Naturally Occurring or Synthetic Polyribonucleotides. PNAS, 47, 1588-1602.
The following base sequence is given for the DNA coding strand: 5' ACTGTTACATTG 3'. Insertion of the base thymine (T) between the 8th and 9th base may affect how many amino acids?
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