10: Transcription and RNA Processing
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- 10.1: Introduction
- Transcription, the synthesis of RNA based on a DNA template, is the central step of the Central Dogma proposed by Crick in 1958. The basic steps of transcription are the same as for replication: initiation, elongation and termination. Differences between transcription in prokaryotes and eukaryotes are in the details.
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- 10.2: Overview of Transcription
- All cells make three main kinds of RNA: ribosomal RNA (rRNA), transfer RNA (tRNA) and messenger RNA (mRNA). rRNA is a structural as well as enzymatic component of ribosomes, the protein-synthesizing machine in the cell. Quantitatively, rRNAs are by far the most abundant RNAs in the cell and mRNAs, the least. Three rRNAs and about 50 ribosomal proteins make up the two subunits of a bacterial ribosome, as illustrated below.
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- 10.3: Details of Transcription
- Some proteins bind DNA to regulate transcription, inducing or silencing transcription of a gene. We will discuss their role in the regulation of gene expression later. Other proteins interact with DNA simply to allow transcription. These include one or more that, along with RNA polymerase itself, that must bind to the gene promoter to initiate transcription.
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- 10.5: Ribosomal RNA Processing in Eukaryotic Nuclei
- In most eukaryotes, a large rRNA gene in most eukaryotes transcribes a 45S precursor transcript containing (from shortest to longest) 5.8S rRNA, 18SrRNA and 28S rRNA. The ‘S’ stands for Svedberg, the biochemist who developed the sedimentation velocity ultracentrifugation technique to separate molecules like RNA by size. The higher the S value, the larger the molecule and therefore the faster it moves through the viscous sugar gradient during centrifugation. RNA Polymerase I transcribes 45S precu
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- 10.6: tRNA Processing in Eukaryotic Nuclei
- RNA polymerase III also transcribes tRNA genes from internal promoters, but unlike the 5S rRNA genes, tRNA genes tend to cluster in the genome. The tRNA folds into several hairpin loops based on internal H-bond formation between complementary bases in the molecule. The 3’-terminal A residue of this (and every) tRNA will bind to an amino acid specific for the tRNA.
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- 10.7: RNA and Ribosome Export from the Nucleus
- The synthesis and processing of rRNAs are coincident with the assembly of the ribosomal subunits, as shown below. The 45S pre-rRNAs initially bind to ribosomal proteins in the nucleolus (that big nuclear body!) to initiate assembly and then and serve as a scaffold for the continued addition of ribosomal proteins to both the small and large ribosomal subunits.
Thumbnail: Simplified diagram of mRNA synthesis and processing. (CC BY 3.0 - unported ; Kelvinsong ).