Key Concepts of Section 9.2

• The primary purpose of gene control in multicellular organisms is the execution of precise developmental programs so that the proper genes are expressed in the proper cells at the proper times during embryologic development and cellular differentiation.

• Transcriptional control is the primary means of regulating gene expression in eukaryotes, as it is in bacteria.

• In eukaryotic genomes, DNA transcription-control elements may be located many kilobases away from the promoter they regulate. Different control elements can control transcription of the same gene in different cell types.

• Eukaryotes contain three types of nuclear RNA polymerases. All three contain two large and three smaller core subunits with homology to the β′, β, α, and ω subunits of E. coli RNA polymerase, as well as several additional small subunits (see Figure 9-13).

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• RNA polymerase I synthesizes only pre-rRNA. RNA polymerase II synthesizes mRNAs, some of the small nuclear RNAs that participate in mRNA splicing, and micro- and small interfering RNAs (miRNAs and siRNAs) that regulate the translation and stability of mRNAs. RNA polymerase III synthesizes tRNAs, 5S rRNA, and several other small stable RNAs (see Table 9-2).

• The carboxy-terminal domain (CTD) in the largest subunit of RNA polymerase II becomes phosphorylated during transcription initiation and remains phosphorylated as the enzyme transcribes the DNA template.