Transcription (AQA A Level Biology)

• 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)
  • Catalysed by helicase, like in DNA replication
• This exposes the gene to be transcribed (the gene from which a particular polypeptide will be produced)
• A complementary copy of the code from the gene is made by building a single-stranded nucleic acid molecule known as mRNA (messenger RNA)
• Free activated 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 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

Template and non-template strands

• In the transcription stage of protein synthesis, the section of the DNA molecule where the gene is located (the gene coding for a particular polypeptide) unwinds – the hydrogen bonds between the complementary base pairs break, causing the two DNA strands to ‘unzip’
• Free activated RNA nucleotides then pair up with the exposed bases on the DNA molecule but only with those bases on one strand of the DNA molecule
• This strand of the DNA molecule is called the template strand or the transcribed strand
• This is the strand that is transcribed to form the mRNA molecule (RNA polymerase binds the RNA nucleotides together to create the sugar-phosphate backbone of the mRNA molecule)
  • This mRNA molecule will then be translated into an amino acid chain

• The strand of the DNA molecule that is not transcribed is called the non-template strand or the non-transcribed strand

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