Chapter 18: The Genetic Code and Transcription

Concepts

• The genetic code exhibits a number of characteristics
• Early studies established the basic operational patterns of the code
• Studies by Niremberg, Matthaei, and others led to deciphering of the code
• The Coding Dictionary reveals the function of the 64 triplets
• The genetic code has been confirmed in studies of bacteriophage MS2
• The genetic code is nearly universal
• Different initiation points create overlapping genes
• Transcription synthesizes RNA on a DNA template
• Studies with bacteria and phages provided evidence for the existence of mRNA
• RNA polymerase directs RNA synthesis
• Transcription in eukaryotes differs from prokaryotic transcription in several ways
• The coding regions of eukaryotic genes are interrupted by intervening sequences
• Transcription has been visualized by electron microscopy

Transcription

Transcription of DNA into mRNA is the first step in the
process of turning genotype into phenotype. Subsequent
translation of mRNA into protein is the next step. The
expression of genetic information in prokaryotes and
eukaryotes shares many common features. Messenger RNA
is synthesized by RNA polymerases. In prokaryotes like E.
coli there is a single type of RNA polymerase, while
eukaryotes may have three or more forms of this enzyme.
Besides RNA polymerase, transcription requires additional
factors.

The synthesis of mRNA can be divided into four processes. 
Three of the four steps require additional proteins. 
Recognition of the specific nucleotide sequences that define
the promoter requires sigma factor. Initiation of mRNA
synthesis occurs at a specific start point and also requires
sigma factor. Elongation is carried out by the core enzyme of
RNA polymerase alone. Termination may require additional
protein factors such as rho.

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