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0 10 20 30 40 50 60 70 80 90 100 Percentage of C in poly(AC)

4 15.10 Nirenberg and Matthaei's use of random copolymers provided information about the genetic code. The theoretical percentage of codons (vertical axis) is plotted against various percentages of cytosine in random AC copolymers. Notice that the distribution of histidine approximates the theoretical percentage of a codon with two Cs and one A, whereas the distribution of asparagine approximates the percentage expected of a codon with two As and one (Credit to come-allowed 1 line).

two Cs and one A should be more common than an amino acid encoded by two As and one C. By comparing the percentages of amino acids in proteins produced by random copolymers with the theoretical frequencies expected for the codons (< Figure 15.10), information about the base composition of the codons was derived. Findings from these experiments revealed nothing, however, about the codon base sequence; histidine was clearly encoded by a codon with two Cs and one A (see Figure 15.10), but whether that codon was ACC, CAC, or CCA was unknown. There were other problems with this method: the theoretical calculations depended on the random incorporation of bases, which did not always occur, and, because the genetic code is redundant, sometimes several different codons specify the same amino acid.

To overcome the limitations of random copolymers, Nirenberg and Philip Leder developed another technique in 1964 that used ribosome-bound tRNAs. They found that a very short sequence of mRNA—even one consisting of a single codon—would bind to a ribosome. The codon on the short mRNA would then base pair with the matching anticodon on a tRNA that carries the amino acid specified by the codon (< Figure 15.11). The ribosome-bound mRNA was mixed with tRNAs and amino acids, and this mixture was passed through a nitrocellulose filter. The tRNAs paired with the ribosome-bound mRNA stuck to the filter, whereas unbound tRNAs passed through. The advan-

Question: With the use of tRNAs, what other matches between codon and amino acid could be determined?

ffi Very short mRNAs with known codons were synthesized..

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