If cordycepin tri-phosphate is added to a cell-free transcription reaction, the nucleotide is added onto the growing rna chain but no more nucleotides can then be added. the added cordycepin is always found at the 3' end of an rna. examine the structure of cordycepin and explain why it ends transcription? a) it prevents the association of rna polymerase and sigma. b) it cannot be recognized by rna polymerase. c) it lacks a 3'/rm oh.

c) It lacks a 3' OH.

Explanation:

Transcription occurs in 5' to 3' direction by RNA polymerase. The enzyme RNA polymerase adds new ribonucleotide to the free 3'-OH. Here, the 3' hydroxyl group (OH) serves as a nucleophile and attacks the alpha phosphate of the incoming ribonucleoside triphosphate. The incoming ribonucleotide is added by phosphodiester bond and pyrophosphate is released.

When cordycepin triphosphate is added to the cell-free transcription reaction, the cordycepin triphosphate binds itself to the 3'-OH of RNA. The 3'-OH is not available for nucleophilic attack and no more ribonucleotides are added to this RNA.

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