Transcription

• Gene


  A sequence of nucleotide bases in a DNA molecule that codes for the production of a specific sequence of amino acids, that in turn make up a specific polypeptide (protein)
• Protein synthesis


  1. Transcription

  2. Translation
• Transcription


  DNA is transcribed and an mRNA molecule is produced
• Transcription


  • Occurs in the nucleus of the cell
  • Part of a DNA molecule unwinds
  • The exposed gene can then be transcribed
  • A complimentary copy of the code from the gene is made by building a single-stranded nucleic acid molecule known as mRNA (messenger RNA)
  • Free RNA nucleotides pair up (via hydrogen bonds) with their complementary (now exposed) bases on one strand (the template strand) of the 'unzipped' DNA molecule
  • The sugar-phosphate groups of these RNA nucleotides are then bonded together (by phosphodiester bonds) by the enzyme RNA polymerase to form the sugar-phosphate backbone of the mRNA molecule
  • When the gene has been transcribed (when the mRNA molecule is complete), the hydrogen bonds between the mRNA and DNA strands break and the double-stranded DNA molecule re-forms
  • The mRNA molecule then leaves the nucleus via a pore in the nuclear envelope
• Coding strand


  The other strand of the DNA molecule (not the template strand), the base sequence of this strand will be the same as the base sequence of the mRNA transcript, but with uracil replacing thymine
• Template strand


  The strand of the DNA molecule that is used to produce the mRNA molecule
• RNA polymerase moves along the template strand in the 3' to 5' direction
• The mRNA molecule grows in the 5' to 3' direction
• The mRNA molecule contains the exact same sequence of nucleotides as the DNA coding strand (although the mRNA will contain uracil instead of thymine)