Principles of Biochemistry Volume 3 - A. Lehninger 1985

Molecular mechanisms of genetic information transfer
DNA replication and transcription
Genes are transcribed to form RNA

Let us now turn to the next major stage in the transmission of Genetic information, namely the METABOLISM/31.html">Transcription of genetic information contained in DNA into the form of RNA. In this process, an enzyme system catalyzes the synthesis of an RNA strand whose nucleotide sequence is complementary to that of one of the DNA strands. Transcription must proceed with high fidelity, as The Cell requires Proteins with a normal, genetically determined Amino Acid Sequence. Transcription yields three distinct classes of RNA. First is Messenger RNA (mRNA), which travels to the Ribosomes and directs the synthesis of one or more Polypeptides whose amino acid sequence was encoded by a Gene or a group of genes on the chromosome. Approximately 90–95% of the E. coli chromosome encodes messenger RNAs. The remainder of the chromosome encodes transfer and Ribosomal RNAs, and also includes regulatory sequences, leaders, spacers, and tail sequences.

There is a fundamental difference between Replication and transcription. During replication, the entire chromosome is copied to produce daughter DNAs identical to the parental DNA. In contrast, transcription does not necessarily involve copying the entire cellular DNA. Instead, typically only specific genes or groups of genes are transcribed. Thus, the Transcription of DNA is a selective process that must be guided by specific regulatory sequences indicating the start and end points of the DNA segments to be transcribed.