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14

The Nature of Living Things

14.8.1

Transcription

The essence of transcription is that RNA polymerases (RNAp, a large molecule

with upper M Subscript r Baseline tilde 500 000Mr ∼500 000) bind to certain initiation sites (sequences of DNA to which

their affinity is superior) and synthesize RNA complementary to the DNA, ⁴⁹ tak-

ing RNA monomers (nucleotide pyrophosphates) from the surrounding cytoplasm.

These enzymes catalyse the formation of a covalent bond between the nucleotide

part of the monomer and the extant uncompleted RNA strand, and they release the

pyrophosphate part into the cytoplasm as a free molecule. Presumably appropriate

hydrogen bonds are formed to the DNA, RNA, and incoming nucleotide pyrophos-

phate, such that if the incoming nucleotide is correctly base-paired with the DNA

template, it is held in the correct conformation for making a covalent bond to the

extant RNA. The catalysis is reversible but is normally driven in the direction of

RNA extension by a constant supply of monomers and the continual removal of the

pyrophosphate.

Inition and termination of RNA synthesis are encoded within the DNA sequence.

The RNAp is therefore similar in its action to the DNA polymerase in DNA replica-

tion.

The RNA folds up as it is synthesized (cf. Fig. 15.4), but extant structure may

have to be disassembled as synthesis proceeds in order to achieve the final structure

of the complete sequence. ⁵⁰

14.8.2

Regulation of Transcription

The key factor in transcriptional regulation is the affinity of RNAp for DNA. The

prerequisite for RNA production is the binding of RNAp in the initiation zone of the

DNA. The binding affinity is inter alia influenced by the following ⁵¹:

1. The binding of molecules to the RNAp;

2. The binding of molecules to the DNA initiation zone.

It is convenient to consider separately transcriptional regulation in prokaryotes and

eukaryotes.

49 The transformation is given bydown arrow StartLayout 1st Row upper A upper G upper C upper T 2nd Row upper U upper C upper G upper A EndLayout↓^{A G C T}

U C G A ^.

50 See Fernández (1989a), also Fernández (1989b).

51 Suppression of transcription is not perfect. There appears to be a basal rate of transcription of

some genes even in tissues in which they are not required. See Chelly et al. (1989) and Sarkar and

Sommer (1989).