• RNA is produced by transcription from the DNA template by RNA polymerase
    • initiation - requires a promoter sequence in the DNA that the polymerase binds to, but no primer (unlike DNA polymerase)
    • elongation - polymerizes ATP, GTP, CTP and UTP into a polynucleotide using only one strand of the DNA as a template (asymmetric transcription)
      • synthesis is 5' to 3', going 3' to 5' on the DNA template strand (antiparallel just like the DNA polymerase)

  • termination - specific DNA sequences called terminators signal the end of the transcript
  • Try these questions about transcription: question1 question2 question3 from the Biology Project
  • Two types of RNA produced by transcription
    • unstable mRNA that is a carrier of genetic information and codes for proteins
    • stable RNA that has a specific function of its own
      • rRNA and tRNA - protein synthesis
      • snRNA - splicing
      • telomerase RNA - telomere synthesis
      • editing RNA's
  • A gene codes for a functional RNA molecule
    • Either a mRNA or an RNA with a direct function
  • There are some difference's between prokaryotic and eukaryotic transcription
    • Different promoter and termination sequences
    • Eukaryotes have three specialized RNA polymerases instead of just one
    • Eukaryotic mRNAs are heavily processed
  • Eukaryotic mRNA is heavily processed, unlike prokaryotic mRNA
    • the initial transcript is called hnRNA (heterogenous RNA) because of the large range of sizes. This hnRNA is found only in the nucleus
    • the hnRNa has sequences removed from each end and then a 5' cap (7methyl guanasine added 5' to 5') and a poly A tail (a string of about 300 As added to the 3' end of the RNA) are added
    • introns are then spliced out - various stretches of RNA sequence are cut out of the transcript and then the remaining exons are spliced back together, the introns are presumably degraded
    • The finished mRNA is exported to the cytoplasm

This document is copyright of Jeff Bell
Last Update: Thursday, March 29, 2001