Describe messenger RNA structure
Messenger RNA is a single-stranded ribonucleotide molecule with ribose, uracil and a base sequence that carries codons.
Section 6.1 · Structure of nucleic acids and replication of DNA
AS Level · Cambridge International Biology 9700, 2028–2030. These lessons explain DNA, RNA and protein synthesis. Written practice is self-checked; it does not replace teacher review or practical assessment.
Before you begin
- Recognise that DNA stores genetic information and that proteins are made from amino acids.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which base is present in RNA instead of thymine?
What you will learn
- Describe mRNA as a single-stranded RNA molecule
- Distinguish RNA nucleotides from DNA nucleotides
- Relate the mRNA sequence to codons used in protein synthesis
RNA has a different sugar and base
RNA is a polynucleotide that contains ribose and the bases A, U, C and G. It is usually single-stranded. Uracil replaces thymine in RNA. Adjacent nucleotides are joined by phosphodiester bonds.
mRNA is messenger RNA. Its sequence carries genetic information from a gene to a ribosome in codons, each a triplet of bases. The sequence of mRNA is related to the DNA template by complementary base pairing.
Structure supports its role
mRNA can leave the nucleus in eukaryotic cells and associate with a ribosome, where its codons are read during translation. It is not a double helix like DNA and does not use deoxyribose or thymine.
A codon is a triplet on mRNA. A tRNA anticodon pairs with a complementary mRNA codon during translation; do not call a DNA triplet an mRNA codon unless the question defines it that way.
Pause and explain
What does an mRNA codon consist of?
Worked example
A strand contains ribose and the bases A, U, C and G. Identify the molecule type and explain one role.
Show the worked solution
- Ribose and uracil identify RNA.
- mRNA carries codons to a ribosome.
Answer This is RNA. If it is mRNA, its sequence carries codons to a ribosome, where the information is used during translation.
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
State three structural features that distinguish RNA from DNA in the syllabus model.
Give me a hint
Compare strand number, sugar and one base.
Compare my reasoning
- RNA is usually single-stranded and contains ribose.
- RNA contains uracil rather than thymine.
RNA is usually single-stranded, contains ribose and uses uracil instead of thymine.
Look for these in your work
- Three distinctions given
- Base replacement correct
Explain how the structure of mRNA relates to its role in protein synthesis.
Give me a hint
Use codons and the ribosome.
Compare my reasoning
- The base sequence is read as codons.
- mRNA associates with a ribosome during translation.
The sequence of mRNA is read as codons at a ribosome during translation, providing the information used to assemble a polypeptide.
Look for these in your work
- Codons included
- Ribosome role linked
A student writes an mRNA sequence using T and calls each tRNA triplet a codon. Correct both statements.
Give me a hint
Use the RNA base set and distinguish codon from anticodon.
Compare my reasoning
- RNA uses U instead of T.
- mRNA triplets are codons; complementary tRNA triplets are anticodons.
An mRNA sequence uses uracil (U), not thymine (T). A triplet on mRNA is a codon; the complementary triplet on tRNA is an anticodon.
Look for these in your work
- RNA base corrected
- Codon and anticodon distinguished
Common mistakes
- Using thymine in an RNA sequence
- Describing mRNA as a double-stranded DNA molecule
- Confusing an mRNA codon with a tRNA anticodon
What can you explain now?
Which base is present in RNA instead of thymine?
Compare with the explanation
Uracil
RNA contains uracil.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
How do ribose, uracil, strand number and codons distinguish mRNA from DNA and explain its role?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.