Alternative genetic polymers

Another important technique was based on the observation that synthetic trinucleotides induced the binding to ribosomes of tRNA molecules that were "charged" with their specific amino acids 38/39 For example, the trinucleotides UpUpU and ApApA stimulated the binding to ribosomes of 14C-labeled phenylalanyl-tRNA and lysyl-tRNA, respectively. The corresponding dinucleotides had no effect, an observation that not only verified the two codons but also provided direct evidence for the triplet nature of the genetic code. Another powerful approach was the use of artificial RNA polymers, synthesized by combined chemical and enzymatic approaches.40 For example, the polynucleotide CUCUCUCUCU led to the synthesis by ribosomes of a regular alternating polypeptide of leucine and serine. 

The structure of the DNA polymer is similar to that of RNA, except there are no hydroxyl groups on the 2 carbon atoms of the ribose rings. The alternating deoxyribose rings and phosphates act as the backbone, while the bases attached to the deoxyribose units carry the genetic information. The sequence of nucleotides is called the primary structure of the DNA strand. 

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