Background on Transcription

Transcription is a process where information in DNA is assembled into RNA using complementarity similar to that used in making double-stranded DNA. Mechanistically, transcription is similar to DNA replication (see here), particularly in the use of nucleoside triphosphate substrates and the template-directed growth of nucleic acid chains in a 5' to 3' direction.

Two major differences between transcription and replication are as follows:

> 1. With few known exceptions, only one DNA template strand is transcribed

> 2. Only a small fraction of the entire genetic potential of an organism is converted to RNA in one cell.

In a differentiated eukaryotic cell, very little of the total DNA is transcribed. Even in single-celled organisms, in which virtually all of the DNA sequences can be transcribed, far fewer than half of all genes may be transcribed at any time.

The mechanisms used to select particular genes and template strands for transcription operate largely at the levels of initiation and termination of transcription, through the actions of proteins that contact DNA in a highly site-specific manner (Figure 26.1).

Three major types of RNA are found in cells--ribosomal RNA (rRNA), transfer RNA (tRNA), and messenger RNA (mRNA). They function in ribosome structure/function, translating the genetic code, and carrying the message to be translated, respectively. mRNA is a small percentage of total cellular RNA (1% to 3% in bacteria).

Bacterial genes are organized in clusters under common regulation. These clusters are called operons and are controlled by regulatory elements (Figure 26.2).

RNA polymerase is an enzyme that makes RNA using DNA as a template. RNA polymerase uses the nucleoside triphosphates, ATP, GTP, CTP, and UTP (uridine triphosphate) to make RNA. The nucleoside bases adenine, guanine, cytosine and uracil pair with the bases thymine, cytosine, guanine, and adenine, respectively, in DNA to make RNA. Like DNA polymerase (see here), RNA polymerases catalyze polymerization of nucleotides only in the 5' to 3' direction. Unlike DNA polymerases, however, RNA polymerases do not require a primer to initiate synthesis.

Like DNA replication (see here) and protein synthesis (see here), transcription occurs in three distinct phases-initiation, elongation, and termination (see here and here). Initiation and termination signals in the DNA sequence punctuate the genetic message by directing RNA polymerase to specific genes and by specifying where transcription will start, where it will stop, and which strand will be transcribed. The signals involve instructions encoded in DNA base sequences mediated by interactions between DNA and proteins.

See also: Eukaryotic Transcription (from Chapter 28) RNA Polymerases, Initiation and Elongation, Promoter Organization, Factor-Independent Termination of Transcription, Factor-Dependent Termination of Transcription Transcription Regulation in Phage, trp Operon Regulation

INTERNET LINKS:

> 1. The RNA World

> 2. Transcription

> 3. Eukaryotic Transcription