Base pairing at Higher Level: The same statement as SL. At HL the same pairing rule runs through the direction of transcription and the start at the promoter.
Practise this as you read
- Explain the role of hydrogen bonding in transcription.
- Deduce an mRNA sequence from a DNA template strand.
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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: speed and accuracy. Human RNA polymerase adds about 20 to 50 nucleotides a second, yet nearly every base it adds is correct, because each template base can hydrogen bond properly with only one RNA base.
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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.
How it is asked: Writing the mRNA from a template strand, or the role of base pairing inside a transcription answer.
IB-style questionExplain[2 marks]
Explain the role of hydrogen bonding in transcription.
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