Principles of Biochemistry Volume 3 - A. Lehninger 1985

Molecular Mechanisms of Genetic Information Transfer
DNA Replication and Transcription
Transcription can be observed

Oocytes of the South African clawed frog Xenopus contain A large number of copies of the genes encoding rRNA. These genes must be transcribed very rapidly to ensure The production of Ribosomes required for the rapid Cell Division and growth of early embryos following Fertilization. Electron Microscopy of such Cells reveals numerous thin, loop-like axial filaments about 20 nm in diameter, covered at regular intervals with Hair-like radial fibrils that gradually increase in size (Fig. 28-23). The continuous axial thread passing through these structures consists of DNA bound to protein. The lateral, hair-like fibrils are none other than growing RNA chains. The contiguous regions of DNA covered with RNA fibrils are repeating rRNA genes, all of which are actively transcribed. At any given time, a large number of RNA chains are transcribed from a single Gene, each synthesized by an RNA polymerase molecule moving from one end of the gene to the other. As the enzyme moves along the gene, the RNA chains elongate. The dark dots on the DNA at the Base of the RNA fibrils represent molecules of DNA-dependent RNA polymerase. These molecules are closely packed together, moving along the gene virtually back-to-back, so to speak. The size of the DNA region undergoing METABOLISM/31.html">Transcription is 2–3 µm, which roughly corresponds to the length of DNA required to encode the eukaryotic 45S rRNA precursor. The nontranscribed Regions of the axial filament represent spacer DNA.

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Fig. 28-22. Role of small nuclear RNA (snRNA) in intron splicing and exon ligation. The bases at the ends of the intron form complementary Base Pairs with specific regions of the snRNA. The joining of exons is coupled with the excision of the intron.

Fig. 28-23. A. Diagram illustrating the transcription process. Due to Secondary Structure formation, newly synthesized single-stranded RNA molecules appear much shorter than the DNA template from which they are transcribed. B. Electron micrograph of Xenopus ribosomal genes undergoing simultaneous transcription by a large number of RNA polymerase molecules.