The big idea: RNA polymerase builds mRNA 5' to 3', and ribosomes read mRNA 5' to 3'.
The points to remember
- 5' to 3' transcription: RNA polymerase adds each new nucleotide to the 3' end of the mRNA.
- So the template strand is read 3' to 5': the mRNA is antiparallel to it.
- The 5' end of the mRNA is made first; the 3' end is made last.
- 5' to 3' translation: the ribosome starts near the 5' end and moves towards the 3' end.
- So the first codons read are the ones nearest the 5' end, starting with AUG.
Remember it as: Made five to three, read five to three.
Real example: bacteria. In bacteria such as E. coli there is no nucleus, so ribosomes attach to the 5' end of an mRNA while its 3' end is still being transcribed. Because both processes run 5' to 3', they can follow one another along the same molecule.
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Reading the ends: Every strand has a 5' end and a 3' end. Ask which end is made first, and which end is read first.
| Transcription | Translation | |
|---|---|---|
| Enzyme or machine | RNA polymerase | Ribosome |
| Direction | mRNA built 5' to 3' | mRNA read 5' to 3' |
| Template or strand | Template DNA read 3' to 5' | mRNA moved along from its 5' end |
| Where new parts join | Nucleotides added to the 3' end | Amino acids added to the growing chain |
Do not mix the strands: The template is read 3' to 5', but the mRNA is built 5' to 3'. Say which strand you mean.
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How it is asked: A short Explain on what 5' to 3' means, or a 1A item on which end of the mRNA is made or read first.
IB-style questionExplain[3 marks]
Explain what is meant by 5' to 3' transcription and 5' to 3' translation.
Model answer plan
See the answer broken into clear points, with guidance on how each point earns marks.