The big idea: Transcription reads a DNA strand but does not change it. The same template can be copied into mRNA again and again.
The points to remember
- Only one strand of a gene is used as the template.
- RNA nucleotides pair with it by hydrogen bonds; no DNA bases are added or removed.
- After transcription the mRNA leaves and the DNA strands pair up again.
- So the template's base sequence is unchanged, ready to be transcribed again.
Not the same as replication: Transcription makes RNA and leaves the DNA as it was. It is not a way of copying DNA.
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Templates that must last a lifetime: In body cells that never divide, the same DNA must keep working for the whole life of the cell.
Why it matters
- A cell that does not divide cannot replace its DNA with a new copy.
- Every protein it makes comes from transcribing the same genes.
- So these DNA sequences must be conserved (kept unchanged) throughout the cell's life.
- A change to a template would appear in every mRNA and protein made from it.
Remember it as: Read it, don't rewrite it.
Real example: heart muscle. Fewer than 1% of the muscle cells in an adult human heart are replaced each year, so most of them work for a lifetime. Their genes are transcribed again and again from the same DNA.
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How it is asked: An Explain on why DNA must stay unchanged in cells that do not divide, or a 1A item on what transcription does to the template.
IB-style questionExplain[2 marks]
Explain why the DNA templates in a non-dividing cell must be conserved throughout the life of the cell.
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See the answer broken into clear points, with guidance on how each point earns marks.