6.1.46 · Nucleic Acids and Protein Synthesis

The Structure of RNA — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions

5 min read 10 parts Lesson 6 of 13 1 quiz

📚 Cambridge AS & A Level Biology 9700 Hub / Topical Revision

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Lesson summary

6.1.4 The Structure of RNA — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Describe the structure of RNA, compare it with DNA, and outline the structure and functions of mRNA, tRNA and rRNA.

Polynucleotide
Reactivity
Short life
Efficiency
Amino acid binding site
Catalytic activity

Part 1 of 8General structure of RNA

General structure of RNA

Polynucleotide

Polynucleotide nucleotides contain ribose, a phosphate group and one of four bases: adenine, guanine, cytosine and uracil.
- (replacing thymine)

Backbone

Backbone nucleotides join by phosphodiester bonds as in DNA.

Strands

RNA is usually single-stranded and much shorter than DNA,
but a strand can fold back on itself with complementary sections held by hydrogen bonds to give specific 3D shapes.

Origin

Origin it is made in the nucleus by transcription, with DNA as template and RNA polymerase, and most types move into the cytoplasm.

Part 2 of 8Comparing DNA and RNA

Feature	DNA	RNA
Sugar	Deoxyribose	Ribose
Bases	A, T, C, G	A, U, C, G
Strands	Two (double helix)	Usually one
Length	Very long (millions of base pairs)	Short (tens to thousands of bases)
Stability	Very stable	Less stable, short-lived
Location	Nucleus, mitochondria, chloroplasts	Nucleus, cytoplasm, ribosomes
Function	Stores genetic information	Transfers and uses information to make protein

Reactivity
- the extra OH on carbon 2′ makes RNA more reactive and more easily hydrolysed, which suits a temporary copy of a gene.

Part 3 of 8Messenger RNA (mRNA)

Messenger RNA (mRNA)

Structure

Structure a single linear strand that is a copy of one gene, variable in length.
- (in eukaryotes after introns are removed)

Function

Function it carries the genetic code from the DNA in the nucleus to the ribosomes, so DNA does not leave the nucleus.

Codons

Codons the sequence is read in groups of three bases, each coding for one amino acid.

Short life

Short life it is broken down rapidly after use, so the cell can control how much protein is made.

Efficiency

Efficiency several ribosomes can use one mRNA at once to make many copies of a polypeptide.

Part 4 of 8Transfer RNA (tRNA)

Transfer RNA (tRNA)

Structure

Structure a single strand of about 75 to 90 nucleotides folded by complementary base pairing into a clover-leaf, and further into an L-shape.
Hydrogen bonds between paired regions keep the shape.

Anticodon

Anticodon three bases on one loop complementary to a particular mRNA codon.

Amino acid binding site

Amino acid binding site at the 3′ end (always CCA), where a specific amino acid is attached by an aminoacyl tRNA synthetase using ATP.

Types

Types a different tRNA for each amino acid, at least 20 types in the cell.

Function

Function in translation, tRNA brings its amino acid to the ribosome,
- where the anticodon pairs with the codon, placing amino acids in the correct order.

Part 5 of 8Ribosomal RNA (rRNA)

Abundance

Abundance it is the most abundant type of RNA.

Ribosome

Ribosome it combines with proteins to form the two subunits, and is made in the nucleolus in eukaryotes.
- (eukaryotes 80S with 60S and 40S subunits; prokaryotes 70S)

Catalytic activity

Catalytic activity rRNA folds into complex shapes and forms the peptide bonds between amino acids in translation, so it is a ribozyme.

Binding sites

Binding sites the ribosome holds mRNA and tRNA in the correct positions.

Part 6 of 8The three types compared

Type	Shape	Location	Function
mRNA	Linear, variable length	Nucleus to cytoplasm	Carries the code for a polypeptide
tRNA	Clover-leaf folded to an L-shape, with anticodon	Cytoplasm	Brings a specific amino acid to the ribosome
rRNA	Globular, combined with protein	Ribosomes	Forms the ribosome and forms peptide bonds

Part 7 of 8Roles of RNA in evolution and viruses

RNA world

RNA may have been the first genetic material, since it can store information in its base sequence and act as a catalyst.
- (ribozymes such as rRNA)

Later change

DNA took over as the more stable store, and proteins as more versatile catalysts.

Viruses

Viruses in many viruses, including HIV, influenza and coronaviruses, RNA is the genetic material.

Reverse transcriptase

Reverse transcriptase some viruses use it to make DNA from RNA.

Part 8 of 8More from the specification

Key terms

Term	Meaning
RNA	Ribonucleic acid, a polynucleotide containing ribose and uracil.
mRNA	Messenger RNA: carries the code for a polypeptide from DNA to ribosomes.
tRNA	Transfer RNA: carries a specific amino acid to the ribosome.
rRNA	Ribosomal RNA: forms part of the ribosome.
Codon	Three bases on mRNA coding for one amino acid.
Anticodon	Three bases on tRNA complementary to a codon.
Ribose	Pentose sugar in RNA.
Transcription	Making of an mRNA copy of a gene using DNA as a template.

Avoid these mistakes

Saying RNA never has base pairing — tRNA and rRNA have regions of complementary pairing.

Writing that mRNA contains thymine — it has uracil.

Confusing codon (on mRNA) with anticodon (on tRNA).

Saying tRNA is a straight chain — it is folded.

Last updated: 15 October 2026