![]() The discrete bands represent particular classes of RNA. Sedimentation pattern produced by high-speed centrifugation of RNA extracted from the precursors of rabbit red blood cells. Whether it is the polymerase or the DNA that does the spinning in vivo remains to be determined. In vitro, at least, when RNA polymerase is immobilized, it spins the DNA molecule around and around as it moves along the molecule. A report in the 4 January 2001 issue of Nature shows that RNA polymerase actually tracks around the double helix of DNA. In all cases, however, RNA polymerase transcribes the DNA strand in its 3′ → 5′ direction. ![]() Note that at any place in a DNA molecule, either strand may be serving as the template that is, some genes "run" one way, some the other (and in a few remarkable cases, the same segment of double helix contains genetic information on both strands!). ![]() When transcription is complete, the transcript is released from the polymerase and, shortly thereafter, the polymerase is released from the DNA.As each nucleoside triphosphate is brought in to add to the 3′ end of the growing strand, the two terminal phosphates are removed.Synthesis of the RNA proceeds in the 5′ → 3′ direction.(Described by Zenkin et al., in the 28 July 2006 issue of Science.) When this occurs in bacteria (and perhaps in all organisms), the enzyme backs up, removes the incorrect nucleotide (and the one preceding it) and tries again. Occasionally RNA polymerase will select and insert an incorrect, mismatched, ribonucleotide. However, each A on the DNA guides the insertion of the pyrimidine uracil ( U, from uridine triphosphate, UTP). Thus for each C encountered on the DNA strand, a G is inserted in the RNA for each G, a C and for each T, an A. Each ribonucleotide is inserted into the growing RNA strand following the rules of base pairing.As the RNA polymerase travels along the DNA strand, it assembles ribonucleotides (supplied as triphosphates, e.g., ATP) into a strand of RNA.The same complex replaces the nucleosomes after the DNA has been transcribed and Pol II has moved on. A complex of proteins is responsible for this. In eukaryotes, this requires - at least for protein-encoding genes - that the nucleosomes in front of the advancing RNA polymerase ( Pol II) be removed.The RNA polymerase proceeds to "read" one strand moving in its 3′ → 5′ direction.Working together, they open the DNA double helix.An enzyme, an RNA polymerase, binds to the complex of transcription factors.Some 50 different protein transcription factors bind to promoter sites, usually on the 5′ side of the gene to be transcribed.This page examines the first step: Gene Transcription: DNA → RNAĭNA serves as the template for the synthesis of RNA much as it does for its own replication. Taken together, they make up the "central dogma" of biology: DNA → RNA → protein. The majority of genes are expressed as the proteins they encode.
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