Components of the Transcription Machinery,Template,Activated Precursors,Divalent Metal Ion,RNA Polymerase,Holozymes

Components of the Transcription Machinery

	Components of the Transcription Machinery - The copying of a complementary messenger RNA strand on a DNA strand is called transcription. Transcription requires a template, activated precursors, a divalent metal ion, and RNA polymerase. 1. Template. The preferred template for transcription is double stranded DNA (dsDNA). Of the two strands, however, only one strand, called the "sense" strand, transcribes mRNA. Single stranded DNA (ssDNA) can also serve as a template.
Some viruses apparently replicate through a double stranded RNA intermediate. That mRNA is usually transcribed on a DNA template is shown by the fact that there is correspondence between the base ratios of DNA and RNA. 2. Activated precursors. Synthesis of the mRNA strand requires all four ribonucleoside triphosphates-ATP, GTP, UTP and CTP Adenine -ribose-triphosphate (A TP) Guanine-ribose-triphosphate (GTP) Uracil-ribose-triphosphate (UTP) Cytosine-ribose-triphosphate (CTP)
3. Divalent metal ions, Mg++ or Mn++ are effective in transcription. In vivo synthesis requires Mg++ 4. RNA polymerase. Transcription of RNA requires the enzyme RNA polymerase. Ribonucleoside triphosphates are joined together by RNA polymerase with elimination of pyrophosphates (PPi). The entire enzyme (holozyme) consists of a core enzyme and a sigma (δ) factor. The core enzyme consists of four polypeptide chains α, α, β, β’ in E. coli.