6.2.4 Transcription

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Template & 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 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

Transcription of the template strand, downloadable AS & A Level Biology revision notes

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

Exam Tip

Note the use of sense and anti-sense strands in transcription has been replaced with non-transcribed and transcribed (or template) strands respectively.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 RNA, which has the base Uracil, replacing Thymine.

Post-Transcriptional Modification

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)