Transcription (Cambridge International A-Level Biology)

Template & Non-Template Strands

Spec point — 6.2.4 · state that the strand of a DNA molecule that is used in transcription is called the transcribed or template strand and that the other strand is called the non-transcribed strand

• 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 is the strand that is transcribed to form the mRNA molecule

  • This strand of the DNA molecule is called the template strand or the transcribed strand

• The other strand of the DNA molecule is not transcribed

  • This strand is called the non-template strand or the non-transcribed strand

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

Exam Hint The mRNA codons have the same base sequence as the non-transcribed strand, and the tRNA anticodons have the same base sequence as the transcribed strand (except that Thymine in the DNA strands is replaced by the base Uracil in mRNA and tRNA).

Post-Transcriptional Modification

Spec point — 6.2.5 · explain that, in eukaryotes, the RNA molecule formed following transcription (primary transcript) is modified by the removal of non-coding sequences (introns) and the joining together of coding sequences (exons) to form mRNA

• Within eukaryotic genes, there are both coding and non-coding sequences of DNA

  • The coding sequences are called exons

    • These sequences will eventually be translated into the amino acids that will form the final polypeptide

  • The non-coding sequences are called introns

    • These sequences 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 called RNA splicing (or gene 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