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High-Yield Terms

  • Transcription: describes the process by which the genetic information in a gene is converted into polyribonucleotides, the RNAs

  • Promoter: sequences in the gene that promote the ability of RNA polymerases to recognize the nucleotide at which initiation begins; must reside close to the gene and in a specific orientation for activity

  • Enhancer: sequences that act in cis by binding proteins that result in enhancement of transcription; can be located long distance from the gene or even on different chromosomes, also do not require orientation for activity

  • Splicing: the process whereby the introns are removed from heteronuclear RNA (hnRNA)

  • RNA editing: a novel enzymatic mechanism for the modification of nucleotide sequences of RNA, resulting in altered coding capacity

  • Small noncoding RNAs: includes the U small nuclear RNAs (snRNAs) of the splicing machinery, the microRNAs (miRNAs) derived from noncoding genes that are involved in processes of expression control, and the small interfering RNAs (siRNAs) responsible for the process of RNA-mediated interference (RNAi) in gene expression

  • ribozyme: an enzyme activity solely associated with an RNA molecule, such as the peptidyltransferase activity of the large ribosomal subunit

Classes of RNA and RNA Polymerases

Transcription is the mechanism by which a template strand of DNA is utilized by specific RNA polymerases to generate 1 of the 4 distinct classifications of RNA (Table 36-1).

TABLE 36-1:Classes of Eukaryotic RNA

In the eukaryotic cells, there are 3 distinct classes of RNA polymerase: RNA polymerase (pol) I, II, and III. Each polymerase is responsible for the synthesis of a different class of RNA. RNA pol I is responsible for rRNA synthesis (excluding the 5S rRNA). There are 4 major rRNAs in eukaryotic cells designated by their sedimentation size. The 28S, 5S, and 5.8S rRNAs are associated with the large ribosomal subunit and the 18S rRNA is associated with the small ribosomal subunit. The rRNAs are synthesized as long precursors termed preribosomal RNAs and the 45S preribosomal RNA serves as the precursor for the 18S, 28S, and 5.8S rRNAs. RNA pol II synthesizes the mRNAs and some of the small nuclear RNAs (snRNAs) and some of the miRNAs. RNA pol III synthesizes the tRNAs, the 5S rRNA, some snRNAs, and some miRNAs.

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