Transcription (CIE A Level Biology)

• 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 bonds 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

• Within eukaryotic genes, there are both coding and non-coding sequences of DNA
  • The coding sequences are called exons and these are the sequences that will eventually be translated into the amino acids that will form the final polypeptide
  • The non-coding sequences are called introns and are not translated (they do not code for any amino acids)

• When transcription of a gene occurs, both the exons and introns are transcribed
• This means the RNA molecule formed (known as the primary transcript) also contains exons and introns
• As the introns are not to be translated, they must be removed from the RNA molecule
• The exons are then all fused together to form a continuous RNA molecule called mature mRNA that is ready to be translated
• This process is sometimes called ‘splicing’ and is part of the process of post-transcriptional modification (referring to the modification of the RNA molecule after transcription but before translation occurs)

The RNA molecule produced from transcription of a gene contains introns that must be removed before translation can occur