Key Idea: Nucleic acids — DNA and RNA — are the molecules that store and carry genetic information. Both are long chains built from one repeating monomer, the nucleotide (a phosphate, a pentose sugar and a nitrogenous base). The information is held in the order of the bases along the chain. This topic appears most on Paper 1A (quick MCQs on DNA/RNA structure, base pairing and genomes) and on Paper 2 (drawing a nucleotide, distinguishing DNA from RNA, or working out base ratios).
🧬 Nucleotides & DNA vs RNA
DNA and RNA are both nucleic acids built from the same monomer — the nucleotide. One nucleotide is three pieces joined together: a phosphate group, a pentose sugar (five-carbon sugar) and a nitrogenous base. Nucleotides link sugar-to-phosphate into a long strand, and the bases stick out to the side as the message.
| Feature | DNA | RNA |
|---|---|---|
| Monomer | nucleotide | nucleotide |
| Pentose sugar | deoxyribose | ribose |
| Bases used | A, C, G, T | A, C, G, U |
| Different base | thymine (T) | uracil (U) |
| Number of strands | double (two) | single (one) |
DNA has Deoxyribose and Double strands. RNA swaps Thymine for uRacil and has one strand.
🪢 The double helix & base pairing
DNA is two strands twisted into a double helix. The strands run in opposite directions — they are antiparallel. The strands are held together where the bases meet in the middle, following strict complementary base pairing: A always pairs with T, and G always pairs with C.
| Base on one strand | Pairs with | Held by |
|---|---|---|
| Adenine (A) | Thymine (T) | 2 hydrogen bonds |
| Thymine (T) | Adenine (A) | 2 hydrogen bonds |
| Guanine (G) | Cytosine (C) | 3 hydrogen bonds |
| Cytosine (C) | Guanine (G) | 3 hydrogen bonds |
Because A only ever pairs with T, the amount of A equals the amount of T. In the same way, the amount of G equals the amount of C. Use this to fill in a missing base percentage.
📚 DNA as an information store & packaging
DNA stores information in the order (sequence) of its bases — A, T, C and G. With only four bases but a very long molecule, the number of possible base orders is almost limitless, so DNA can store a vast, stable amount of information. A human cell holds about two metres of DNA in a tiny nucleus, so the DNA must be packaged: it wraps around proteins called histones to condense into compact chromosomes.
- Base sequence
- The order of the bases (A, T, C, G) along a DNA strand — this order is the stored information.
- Histone
- A protein that DNA wraps around to package (condense) it inside eukaryotic cells (plants, animals, fungi).
- Chromosome
- A single long DNA molecule wound around histones and condensed into a compact, organised structure.
Histones are proteins that DNA wraps around to package/condense the very long molecule so it fits inside the cell.
🌍 Genomes & whole genome sequencing
A genome is all of the DNA of an organism — its complete set of genetic instructions. Genome size varies enormously between species, but a bigger genome does not mean a more complex organism. The genetic code is almost universal — the same bases code for the same amino acids in nearly every living thing — which is strong evidence that all life shares a common ancestor. Whole genome sequencing reads the full order of bases and is used in medicine, evolution studies and conservation.
| Idea | What it means | Why it matters |
|---|---|---|
| Genome | all the DNA of an organism | the organism's complete set of instructions |
| Genome size varies | size is not linked to complexity | an onion has a larger genome than a human |
| Universal code | same codons → same amino acids | evidence of a shared common ancestor |
| Genome sequencing | reading the full base order | diagnosis, evolutionary trees, conservation |
A gene is one instruction; a genome is the whole set of instructions in all of an organism's DNA.
✍️ Worked examples
IB-style question — draw a nucleotide
Draw and label the structure of a single DNA nucleotide. [3]
Model answer:
Draw the three parts joined in a row. A phosphate group joined to a pentose sugar (deoxyribose) joined to a nitrogenous base — for example a circle (phosphate), a pentagon (sugar) and a rectangle (base).
Label each part clearly: phosphate, (deoxyribose) sugar, base.
Show how they join — the phosphate attaches to the sugar, and the base attaches to the sugar, so the sugar sits in the middle linked to both.
Three labelled parts — phosphate, deoxyribose sugar and base — with the sugar central, linked to both the phosphate and the base.
IB-style question — Chargaff base ratios
In a sample of DNA, 28% of the bases are adenine. Calculate the percentage of cytosine. [2]
Model answer:
Use the pairing rule. A pairs with T, so thymine is also 28%. Together A + T make 28 + 28 = 56% of the bases.
The rest is G + C. That leaves 100 − 56 = 44% shared between guanine and cytosine. G pairs with C, so they are equal: 44 ÷ 2 = 22% cytosine.
Cytosine = 22% (since %A = %T = 28%, so G + C = 44%, split equally between G and C).
IB-style question — applications of genome sequencing
Discuss the present and possible future applications of whole genome sequencing. [3]
Model answer:
Give a present-day use. Sequencing a patient's genome can diagnose inherited diseases and tailor treatment to the individual (personalised medicine).
Give a second, different use. Comparing genomes lets biologists work out how species are related and build accurate evolutionary (phylogenetic) trees.
Add a future possibility and a balancing point. Cheap sequencing could one day screen newborns for disease risk — but this raises concerns about privacy and how genetic data is used.
Present uses include disease diagnosis and evolutionary studies; future uses such as newborn screening are promising but raise privacy and ethical concerns.
✅ Quick self-check
Tap each card to check yourself.
What are the three parts of a nucleotide? A phosphate group, a pentose (five-carbon) sugar and a nitrogenous base, joined together.
How do DNA and RNA differ? DNA has deoxyribose, thymine and two strands; RNA has ribose, uracil instead of thymine, and one strand.
Which bases pair, and how are they held? A pairs with T by 2 hydrogen bonds; G pairs with C by 3 hydrogen bonds. The strands run antiparallel.
What does Chargaff's rule let you do? Since %A = %T and %G = %C, you can calculate any missing base percentage from the others.
What do histones do? Histones are proteins that DNA wraps around to package and condense the long molecule into chromosomes.
Does a larger genome mean a more complex organism? No — genome size is not linked to complexity. Some simple organisms have larger genomes than humans.
Exam Tips
- A nucleotide = phosphate + pentose sugar + base. Naming only two parts loses marks.
- DNA vs RNA: deoxyribose/ribose, thymine/uracil, double/single strand — give all three contrasts.
- A–T is held by 2 hydrogen bonds, G–C by 3. Bases pair, sugar-phosphate backbones do not.
- Antiparallel means the two strands run in opposite directions — state this, don't just say 'two strands'.
- For base-ratio questions: %A = %T and %G = %C, and all four add up to 100%.
- The information in DNA is the base SEQUENCE (the order), not the amount of DNA.
- Genome size is NOT a measure of complexity — never claim a bigger genome means a more advanced organism.