By George Johnson

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ZAMECNIK’S EXPERIMENT What sort of a molecule might the proposed adapter be? A good candidate was soon found. Paul Zamecnik, attempting to develop a cell-free system to carry out in vitro RNA synthesis in 1957, discovered that 14C ATP precursors indeed produced the expected newly-synthesized radioactive RNA (containing 14C adenine). To ensure that the new RNA was not in some manner mated with protein (it could be that the 14C ATP is broken down and metabolized, and the 14C carbons used in amino acid and subsequent protein synthesis), Zamecnik ran 14C leucine as a control.

Like a letter, the amino acid would be delivered according to the address, not the contents. CHAPEVILLE’S EXPERIMENT This key concept in the tRNA adapter hypothesis was subject to a direct test. 1). What they did was charge the UGU anticodon tRNA that normally carries cysteine (tRNAcys) with radioactive amino acid, using the appropriate activating enzyme, to obtain 14C-cysteinyl-tRNAcys. They then chemically modified the attached amino acid without removing it from the tRNA, and looked to see how the new tRNA performed in protein synthesis.

CHAPTER 10 OKAZAKI: DNA SYNTHESIS IS DISCONTINUOUS In 1968, Reiji Okazaki determined that DNA synthesis was not a smooth, continuous process. Rather, fragments of DNA were synthesized discretely, and assembled later on. THE PUZZLE IN THE DNA SYTHESIS MODEL There is a fundamental problem implicit in the Watson and Crick model of DNA structure. The model requires, and Kornberg’s work demonstrated, that the two DNA strands of a double helix are antiparallel, so that looking along one direction an investigator would see one strand going form 5´ to 3´, while the corresponding strand went from 3´ to 5´.

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