An RNA molecule is a relatively short polynucleotide chain.

The idea

RNA is a single, comparatively short polynucleotide: ribose, phosphate and the bases adenine, uracil, cytosine and guanine, joined by phosphodiester bonds. Three types matter at A-level. Messenger RNA is a linear copy of one gene, made in the nucleus by transcription. It is long enough to carry the codons for a whole polypeptide but tiny beside a chromosome, and its lifetime varies: some mRNAs are degraded quickly, others persist, and degradation is regulated. Transfer RNA is a small folded molecule, held by hydrogen bonds between complementary bases within the same strand, with an anticodon of three exposed bases at one end and an amino acid binding site at the other. Ribosomal RNA combines with protein to form the ribosome.

What earns the marks

• RNA is a single polynucleotide strand, whereas DNA is double-stranded.
• RNA contains ribose, whereas DNA contains deoxyribose.
• RNA contains uracil, whereas DNA contains thymine.
• RNA is much shorter, with far fewer nucleotides, than a DNA molecule.
• Transfer RNA is a folded molecule with an anticodon and an amino acid binding site, unlike the DNA double helix.
• Messenger RNA is a copy of one gene that leaves the nucleus through a nuclear pore for translation at a ribosome.

Common mistakes and how to put them right

• Writing T and U instead of thymine and uracil when the question asks for names.

  Put it right: Write the full names of the bases.

• Saying RNA is double-stranded because transfer RNA contains internal base pairing.

  Put it right: RNA is single-stranded but can fold via internal hydrogen bonds.

• Listing features of DNA and RNA in separate sentences so no comparison is actually made, which loses every comparison mark.

  Put it right: Write paired statements that directly contrast the two.

• Describing messenger RNA as a clover leaf, or transfer RNA as linear.

  Put it right: Match each RNA type to its correct shape.

• Giving deoxyribose as the sugar in RNA.

  Put it right: RNA contains ribose.

• Claiming mRNA is always destroyed after a few rounds of translation.

  Put it right: mRNA lifetimes vary and degradation is regulated.

Exam tips

• In a table of differences, complete both columns for every row; an empty cell scores nothing even if the other cell is right.
• The list rule applies, so give exactly the number of differences asked for and no more.
• Name the type of RNA you mean; 'RNA is short' is too vague when transfer, messenger and ribosomal RNA differ in structure.