RNA Structure — mRNA, tRNA and rRNA

While DNA is the long-term store of genetic information, RNA is the molecule that actually gets things done in the cell. RNA molecules carry genetic information to the ribosome, deliver amino acids during protein synthesis, and form part of the structure of the ribosomes themselves. This lesson covers the OCR A-Level Biology A specification point 2.1.3 (c) — the structure of RNA — with particular reference to the three main classes: messenger RNA (mRNA), transfer RNA (tRNA) and ribosomal RNA (rRNA).

---

1. Key Differences Between RNA and DNA

Feature	DNA	RNA
Pentose sugar	Deoxyribose	Ribose
Bases	A, T, C, G	A, U, C, G (uracil replaces thymine)
Typical strand number	Double-stranded	Single-stranded (but may fold)
Typical length	Millions–billions of nucleotides	Tens–tens of thousands
Location	Mostly in the nucleus (also mitochondria and chloroplasts)	Nucleus + cytoplasm (made in nucleus, used in cytoplasm)
Stability	High (long-lived)	Low (short-lived, degraded by RNases)
Function	Long-term store of genetic information	Expression and translation of genetic information

Key Definition — RNA: A polymer of ribonucleotides containing ribose sugar, the bases A, U, C, G and a phosphate group; normally single-stranded and involved in protein synthesis.

The two chemical differences — ribose instead of deoxyribose, and uracil instead of thymine — are small, but they matter. The 2'-OH in ribose makes RNA more chemically reactive and less stable than DNA: a feature, not a bug, for a molecule whose job is to be made, used and destroyed quickly.

---

2. Messenger RNA (mRNA)

Messenger RNA is the single-stranded copy of a gene that carries the genetic information from the nucleus to the ribosomes in the cytoplasm, where it is translated into a polypeptide.

Key features:

• Single-stranded, linear polynucleotide
• Made in the nucleus by transcription (Lesson 6) using a gene as the template
• Length is typically a few hundred to a few thousand nucleotides — it corresponds to the length of the gene it encodes
• Codons: successive triplets of bases that each code for a specific amino acid (e.g., AUG = methionine, UAA = stop)
• Shorter-lived than DNA — degraded by the cell after it has been used a few times for translation

Role

mRNA acts as a working copy of the gene. Rather than transporting the precious DNA out of the nucleus (where it could be damaged), the cell makes disposable RNA copies of individual genes as they are needed. Each mRNA carries a set of codons that specifies a particular amino acid sequence in a polypeptide.

5' → 3' codon	AUG	UUU	GGC	UAA
Amino acid	Methionine (Met)	Phenylalanine (Phe)	Glycine (Gly)	STOP

In this example, the mRNA encodes a three-amino-acid polypeptide (Met-Phe-Gly) followed by a stop codon.

---

3. Transfer RNA (tRNA)

Transfer RNA is the adaptor molecule of protein synthesis. Each tRNA molecule carries a specific amino acid and matches it to the correct codon on the mRNA using its own anticodon.

Key features:

• A short RNA molecule, typically 75–95 nucleotides long
• A single RNA strand that folds back on itself, forming internal base pairs that give it a clover-leaf shape in 2D and an L-shape in 3D
• Contains an anticodon — three unpaired bases at one end that are complementary to a codon on mRNA
• Contains an amino acid attachment site (the CCA sequence at the 3' end) — this is where a specific amino acid is covalently attached
• Each amino acid has at least one specific tRNA (some have several, because the code is degenerate — see Lesson 5)

graph LR
  A[Amino acid] --> B["tRNA acceptor stem<br/>CCA at 3' end"]
  B --> C[Clover-leaf folded tRNA]
  C --> D[Anticodon loop X-Y-Z]
  D --> E["Pairs with mRNA codon<br/>at the ribosome"]

Exam Tip: A codon is on mRNA; an anticodon is on tRNA. They are complementary, so they can pair by hydrogen bonding at the ribosome.

Role

During translation (Lesson 7), tRNA molecules shuttle amino acids to the ribosome in an order determined by the codons on the mRNA. The enzyme aminoacyl-tRNA synthetase is responsible for attaching the correct amino acid to each tRNA, using ATP as the energy source.

---

4. Ribosomal RNA (rRNA)

Ribosomal RNA is a structural and catalytic component of ribosomes — the molecular machines that carry out protein synthesis.

Key features:

• Produced in the nucleolus from rRNA genes in the DNA
• Combines with ribosomal proteins to form ribosomal subunits (the large and small subunits of a ribosome)
• rRNA molecules vary in size; eukaryotic ribosomes contain four different rRNAs in total
• Accounts for a large proportion of all the RNA in a cell by mass
• Very stable compared with mRNA

Role

rRNA does more than just hold the ribosome together — it also catalyses the formation of peptide bonds between amino acids. The ribosome is therefore a ribozyme: an RNA-based catalyst. (You do not need to go further than knowing that rRNA has a structural and catalytic role.)

---

5. Comparing mRNA, tRNA and rRNA

Feature	mRNA	tRNA	rRNA
Strand(s)	Single, linear	Single, folded (clover-leaf / L-shape)	Single, folded (complex)
Length	Variable — hundreds to thousands of nucleotides	~75–95 nucleotides	Hundreds–thousands of nucleotides
Function	Carries genetic code from DNA to ribosome	Carries amino acids; matches codons via anticodon	Structural and catalytic component of ribosome
Location	Nucleus → cytoplasm	Cytoplasm	Nucleolus → cytoplasm
Stability	Short-lived	Long-lived	Long-lived
Carries	Codons	Anticodon + amino acid	Protein synthesis machinery

---

6. Overview: The Three RNAs Working Together

graph LR
  A[DNA gene] -- transcription --> B[mRNA]
  B --> C["Ribosome<br/>rRNA + proteins"]
  D[tRNA + amino acid] --> C
  C -- translation --> E[Polypeptide]

All three types of RNA are essential to the flow of genetic information:

• mRNA carries the code.
• tRNA translates the code (into amino acids).
• rRNA provides the machinery in which translation occurs.

---

7. Common Exam Mistakes

• Writing "thymine" in RNA. RNA contains uracil (U), not thymine.
• Saying that RNA is always single-stranded. RNA is usually single-stranded but can fold back on itself — tRNA has extensive internal base pairing.
• Confusing codon and anticodon. Codon = mRNA, anticodon = tRNA.
• Calling rRNA "RNA in ribosomes". It is both structural and catalytic.
• Forgetting that RNA contains ribose, not deoxyribose.
• Saying the ribosome is made only of protein. It is made of protein + rRNA.

---

8. Exam-Style Questions

1. State two ways in which the structure of an RNA nucleotide differs from a DNA nucleotide. (2)
2. Compare and contrast the structures and functions of mRNA and tRNA. (6)
3. Explain why a gene with 900 base pairs would produce an mRNA molecule of 900 nucleotides (assuming no introns). (2)
4. Describe the role of rRNA in protein synthesis. (3)

Model answer for (1): "An RNA nucleotide contains the pentose ribose rather than deoxyribose, and may contain the base uracil rather than thymine."

---

Summary

• RNA is a polymer of ribonucleotides containing ribose and the bases A, U, C, G.
• RNA is usually single-stranded but can fold — for example, tRNA has a clover-leaf shape.
• mRNA carries a copy of the genetic code from the nucleus to the ribosome.
• tRNA delivers specific amino acids to the ribosome, matching codon to anticodon.
• rRNA forms the structure and catalytic core of the ribosome.
• All three work together in protein synthesis — transcription and translation.

---

A-Level Deep Dive

Spec mapping

Spec Mapping: This lesson is mapped to OCR H420 Module 2.1.3 — Nucleotides and nucleic acids, covering RNA structure and the named roles of mRNA, tRNA and rRNA (refer to the official OCR H420 specification document for exact wording).

RNA structure is the structural bridge between the information storage of DNA (Lesson 2) and its expression as protein (Lessons 6 and 7). It is also the structural foundation for understanding ribozymes, the RNA world hypothesis (a candidate Going-Further topic), and the modern messenger-RNA vaccine technology that came to prominence with the COVID-19 pandemic. Every later lesson in this course will draw on the molecular details introduced here.

Scientists and paradigms

The discovery that RNA carries information from DNA to the ribosome unfolded across two decades. Severo Ochoa (1955) discovered polynucleotide phosphorylase, the first enzyme capable of synthesising RNA in vitro, which allowed Marshall Nirenberg and Heinrich Matthaei (1961) to manufacture synthetic poly-U RNA and crack the first codon (UUU → phenylalanine; see Lesson 5).

François Jacob, Sydney Brenner and Matthew Meselson (1961) proposed the messenger-RNA hypothesis: that an unstable intermediate ferries genetic information from DNA in the nucleus to the cytoplasmic ribosome. The school of thought to take into the exam — paraphrase, never invent quotation — is that "DNA writes the message, RNA carries it, and the ribosome reads it".

Carl Woese (1967, 1977) recognised that ribosomal RNA sequences are so highly conserved across all of life that they can be used to construct a universal phylogenetic tree — and used 16S rRNA to discover the third domain of life, the Archaea. The rRNA you meet in this lesson is therefore not just a structural component of the ribosome; it is the molecular fossil that lets biologists trace the deepest branches of evolution.

Thomas Cech and Sidney Altman (1980s, Nobel Prize 1989) demonstrated that some RNA molecules — ribozymes — are themselves catalytically active. The peptidyl transferase activity at the core of the ribosome is, remarkably, an RNA-catalysed reaction. rRNA is therefore not merely structural scaffolding but the catalytic core of protein synthesis.

Synoptic links

This lesson connects forward to:

1. ocr-alevel-biology-nucleic-acids-enzymes — Transcription (Lesson 6): mRNA is the product of transcription. The 5'→3' polarity and complementary base-pairing rules you meet here are how RNA polymerase reads the DNA template.
2. ocr-alevel-biology-nucleic-acids-enzymes — Translation (Lesson 7): tRNA delivers amino acids to the ribosome; rRNA forms the catalytic peptidyl transferase. Both depend on the secondary and tertiary structure of single-stranded RNA folding back on itself.
3. ocr-alevel-biology-genetics-inheritance — Gene expression: alternative splicing of pre-mRNA, mRNA stability and half-life, microRNA regulation. All require single-stranded RNA structural plasticity.
4. ocr-alevel-biology-cell-structure: the nucleolus is the rRNA-production factory; ribosomes (rRNA + protein) translate mRNA.
5. ocr-alevel-biology-biological-molecules: the 2'-OH of ribose is the chemical reason RNA is less stable than DNA — RNA can undergo intramolecular self-cleavage via the 2'-OH attacking the adjacent phosphodiester bond. This chemistry underlies the lability of mRNA.

Specimen question modelled on the OCR H420 paper format

Question (6 marks): Compare and contrast the structure of a DNA molecule and an mRNA molecule.

Mark scheme decomposition (AO breakdown):

Mark	AO	Awarded for
1	AO1	Both polymers of nucleotides joined by phosphodiester bonds
2	AO1	DNA double-stranded helical; mRNA single-stranded (linear)
3	AO1	DNA contains deoxyribose; mRNA contains ribose
4	AO1	DNA contains thymine; mRNA contains uracil
5	AO1	DNA much longer than mRNA
6	AO2	DNA stable for long-term storage; mRNA short-lived for regulated expression

Split: AO1 = 5, AO2 = 1.

Grade C model answer (~125 words)

DNA and mRNA are both polymers of nucleotides joined by phosphodiester bonds. They both have phosphate, sugar and bases. The sugar in DNA is deoxyribose but in mRNA it is ribose. DNA has thymine but mRNA has uracil instead. DNA is two strands twisted into a double helix but mRNA is just one strand. DNA is very long and has thousands or millions of bases. mRNA is much shorter because it only carries a copy of one gene. DNA stays in the nucleus and is stable but mRNA can leave the nucleus and is broken down quickly.

Examiner commentary: M1 (phosphodiester), M1 (DNA double / mRNA single), M1 (deoxyribose vs ribose), M1 (T vs U), M1 (length). ~5/6. The candidate does not explicitly contrast longevity / regulatory implications for the AO2 mark.