The big idea: In transcription each free RNA nucleotide is held to the template by hydrogen bonds, and only the complementary base fits.
The points to remember
- RNA polymerase exposes the bases of the template strand.
- A free RNA nucleotide forms hydrogen bonds with the template base it matches.
- A on the template pairs with U on the RNA; T with A; C with G; G with C.
- Because each base has only one partner, the mRNA sequence is set by the template.
- Hydrogen bonds are weak, so the mRNA can be released and the DNA closes again.
Remember it as: Template A calls for U; hydrogen bonds hold, then let go.
Real example: the insulin gene. In the beta cells of the pancreas the insulin gene is transcribed over and over. Base pairing makes every insulin mRNA the same, so every insulin molecule is built the same way.
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From template to mRNA: Write the mRNA one base at a time from the template strand, using the pairing rule.
| Base on the DNA template | Base added to mRNA |
|---|---|
| A (adenine) | U (uracil) |
| T (thymine) | A (adenine) |
| C (cytosine) | G (guanine) |
| G (guanine) | C (cytosine) |
Worked example. Template T A C G G A T C gives mRNA A U G C C U A G.
Template or coding strand?: The mRNA is complementary to the template strand. It has the same sequence as the other (coding) strand, but with U in place of T.
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How it is asked: Writing the mRNA from a template strand, or the role of base pairing inside a transcription answer.
A DNA template strand reads C G T A A C. Deduce the mRNA transcribed from it, showing how you used base pairing.
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