Gene Expression

Lesson

Part 1 — how a gene’s message is copied into RNA, decoded and turned into protein.

In this lesson you will

Describe transcription and the processing of hnRNA into mRNA
Define intron, exon, cistron, recon, muton, and monocistronic and polycistronic units
State the discovery and features of the genetic code
Explain initiation, elongation and termination of translation

Diagram

The transcription unit

A transcription unit has a promoter (where RNA polymerase binds), the structural gene and a terminator. Only the template strand (3′→5′) is copied; RNA is made 5′→3′ and matches the coding strand, with U in place of T.

Concept

Transcription in prokaryotes and eukaryotes

Transcription runs in three stages: initiation, elongation and termination.

Table comparing transcription in bacteria and eukaryotes
Feature	Bacteria	Eukaryotes
RNA polymerase	One enzyme makes all RNAs	Three: I (rRNAs), II (hnRNA, precursor of mRNA), III (tRNA, 5S rRNA, snRNAs)
Initiation	Sigma factor helps the enzyme bind the promoter	Polymerase with other factors binds the promoter
Termination	Rho factor	Terminator and other factors
Where	Cytoplasm; translation can start at once	Nucleus; RNA processed first

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