All cellular types of RNA are transcribed from DNA

inefficient for a cell to constantly transcribe all of its genes. Furthermore, much of the DNA does not code for a functional product, and transcription of such sequences would be pointless. Transcription is, in fact, a highly selective process—individual genes are transcribed only as their products are needed. But this selectivity imposes a fundamental problem on the cell—the problem of how to recognize individual genes and transcribe them at the proper time and place.

Like replication, transcription requires three major components:

1. a DNA template;

2. the raw materials (substrates) needed to build a new RNA molecule; and

3. the transcription apparatus, consisting of the proteins necessary to catalyze the synthesis of RNA.

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