• A?gene?is 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)
• This process of protein synthesis occurs in?two stages:
  • Transcription?–?DNA?is transcribed and an?mRNA?molecule is produced
  • Translation?–?mRNA?(messenger RNA) is translated and an?amino acid sequence?is produced

   

Transcription

• This stage of protein synthesis occurs?in the nucleus?of the cell
• Part of a DNA molecule?unwinds?(the?hydrogen bonds?between the complementary base pairs?break)
• The exposed?gene?can then be transcribed (the gene from which a particular polypeptide will be produced)
• 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

The transcription stage of protein synthesis – DNA is transcribed and an mRNA molecule is produced

The coding strand and the template strand

• In the?transcription?stage of protein synthesis, free RNA nucleotides pair up with the exposed bases on the DNA molecule
• RNA nucleotides only pair with the bases on?one strand of the DNA molecule
  • This strand of the DNA molecule is known as the?template strand?(or the?transcribed strand) and it is used to produce the?mRNA molecule
  • The other strand is known as the?coding strand?(or the?non-template?or?non-transcribed strand)

   

• RNA polymerase?moves along the template strand in the?3' to 5' direction
  • This means that the?mRNA?molecule grows in the?5' to 3' direction

   

• Because the mRNA is formed by complementary pairing with the DNA template strand, the mRNA molecule contains the?exact same sequence of nucleotides?as the DNA?coding strand?(although the mRNA will contain?uracil?instead of?thymine)

The template strand of the DNA molecule is the one that is transcribed