Nirenberg and Matthaei as well as Khorana were able to synthesize RNA molecules

Question

Nirenberg and Matthaei as well as Khorana were able to synthesize RNA molecules with repeating sets of nucleotides of various lengths and then analyze the amino acid sequence of the proteins that were encoded by these molecules. So for instance, RNA molecules of repeating single nucleotides such as U to give UUUUU… or A to give AAAAA…. These give rise to single repeating amino acids. Repeating dinucleotides such as UC to give UCUCUCUC…These give rise to repeating units of 2 amino acids (Ser, Leu). Imagine that you carried out this experiment and made 9 RNA types composed of the following repeated sequences:

G… GU… GUA… GUAC… GUACC… GUACCG… GUACCGA… GUACCGAG… GUACCGAGU…

How many amino acids do you expect to make up a repeating unit in the polypeptides produced from these RNAs? How could this experiment be used to support the hypothesis that the genetic code uses a triplet code?

Explanation / Answer

Poly G = GGGGGG = Poly Glycine

Poly GU = GUGUGU = Poly Val-Cys dinucleotide repeat

Poly GUA = GUA = Poly Val

Poly GUAC = GUA CGU ACG UAC = Poly Val-Arg-Thr-Tyr tetranucleotide repeat

Poly GUACC = GUA CCG UAC CGU ACC = Poly Val-Pro-Tyr-Arg-Thr pentanucleotide repeat

Poly GUACCG = GUA CCG= Poly Val-Pro dinucleotide repeat

Poly GUACCGA = GUA CCG AGU ACC GAG UAC CGA = Poly Val-Pro-Ser-Thr-Glu-Tyr-Arg heptanucleotide repeat

Poly GUACCGAG = GUA CCG AGG UAC CGA GGU ACC GAG = Poly Val-Pro-Arg-Tyr-Arg-Gly-Thr-Glu octanucleotide repeat

Poly GUACCGAGU = GUA CCG AGU= Poly Val-Pro-Ser tri nucleotide repeat

As we can clearly observe from the above results that genetic code is triplet.

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