RNA as a polymer at Higher Level: The same statement as SL. At HL you also need the direction in which the strand grows: each new nucleotide is joined to the 3′ end. That rests on exactly this reaction: condensation, sugar to phosphate.
Practise this as you read
- Name the joining reaction: condensation.
- Draw a short RNA strand with ribose, one strand, no thymine.
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The big idea: RNA is a long chain of nucleotides. Each one is added by a condensation reaction, which releases water and makes a covalent bond.
The points to remember
- A polymer is a long chain of small units called monomers.
- RNA is a polymer; its monomers are RNA nucleotides.
- Nucleotides are joined by condensation reactions.
- Each condensation releases one water molecule (H₂O).
- It makes a covalent bond from the sugar of one nucleotide to the phosphate of the next.
Remember it as: Condensation: join and lose water.
Do not swap the two reactions: Condensation joins monomers and releases water. Hydrolysis does the opposite: water is used to split the bond.
These chains can be very long:
Real example: the RNA at the heart of a ribosome: The largest RNA in a human ribosome is about 5 000 nucleotides long, every one added by a condensation reaction.
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The big idea: To draw an RNA polymer, draw nucleotides one under another and join each ribose to the next phosphate.
The points to remember
- RNA is a single strand: draw one chain only.
- The sugar is ribose; the bases are A, C, G and U, never T.
- Join each ribose to the phosphate of the next nucleotide.
- All the bases stick out on the same side of the strand.
Three things that lose marks: Drawing two strands (that is DNA). Putting thymine in RNA. Joining the bases to each other instead of joining sugar to phosphate.
How it is asked: A one-mark State on the reaction, and drawing: a single nucleotide or a short RNA strand.
State the type of chemical reaction that joins two RNA nucleotides together.
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And a drawing task in the same style:
Draw a labelled diagram of a short RNA polymer made of three nucleotides.
Model answer plan
See the answer broken into clear points, with guidance on how each point earns marks.