Nucleic acids
Every cell carries a molecular instruction manual, and that manual is written in a four-letter chemical alphabet. Nucleic acids — DNA and RNA — are polymers built from repeating units called nucleotides, each one a nitrogenous base bolted to a sugar and a phosphate group. DNA's two strands wind around each other in a double helix, held together rung by rung through a strict pairing rule: adenine always finds thymine, guanine always finds cytosine. That single pairing rule is why DNA can be copied faithfully every time a cell divides, and why knowing the amount of one base tells you the amount of all four. This article builds the nucleotide from its three parts, classifies the five nitrogenous bases as purines or pyrimidines, walks through the Watson-Crick double helix, and contrasts DNA with RNA on the handful of points your syllabus actually examines — the sugar, the swapped pyrimidine, and strandedness — while quantifying the base-pairing rule with a fully worked calculation.
Nucleic acids are polymers of nucleotides (base plus sugar plus phosphate) in which DNA's two antiparallel strands are held together by complementary hydrogen-bonded base pairing, A with T and G with C, so that the percentage of each base is fixed once one is known.
The core idea
The nucleotide: three parts, one unit. A nucleic acid is built by linking many nucleotides end to end. Each nucleotide has exactly three components: a nitrogenous base (purine or pyrimidine), a five-carbon pentose sugar (deoxyribose in DNA, ribose in RNA), and a phosphate group. Strip away the phosphate and a base joined only to its sugar is called a nucleoside; add the phosphate back and it becomes a nucleotide. Successive nucleotides link through phosphodiester bonds, where the phosphate of one nucleotide bridges to the sugar of the next, building an alternating sugar-phosphate backbone with the bases projecting sideways off it.
Purines and pyrimidines. The five nitrogenous bases split cleanly into two structural families by ring count. Purines are fused double-ring bases: adenine (A) and guanine (G), present in both DNA and RNA. Pyrimidines are single-ring bases: cytosine (C), present in both; thymine (T), found only in DNA; and uracil (U), found only in RNA, where it takes thymine's place. A useful memory aid is "pure As Gold" — the two purines are Adenine and Guanine.
The double helix. DNA is not one strand but two, wound around a common axis in a right-handed double helix, running antiparallel to each other (one strand's backbone runs in the opposite chemical direction to its partner's). The two strands are held together by complementary base pairing: adenine hydrogen-bonds to thymine across two hydrogen bonds, and guanine hydrogen-bonds to cytosine across three hydrogen bonds. Because a purine always pairs with a pyrimidine of a fixed identity, the two strands carry exactly the same genetic information in complementary form — read one strand and you already know the other. The sugar-phosphate backbones form the twisted outer rails of the helix; the paired bases form the internal rungs, giving the ladder-like structure that copies itself accurately whenever a cell divides.
RNA. Where DNA is a stable double helix, RNA is generally single-stranded, though it can fold back on itself locally into short paired hairpin loops. Its detailed chemical structure sits outside the syllabus; what matters here is the comparison with DNA below. The examinable contrast reduces to three structural differences: DNA carries deoxyribose sugar, thymine as its pyrimidine, and a double-stranded helix devoted to long-term information storage; RNA carries ribose sugar, uracil in place of thymine, and is generally single-stranded, active in protein synthesis and gene expression (as messenger, ribosomal, and transfer RNA). Because it lacks the 2'-OH's exposure to certain reactions and pairs with a full complementary partner strand, DNA is also the chemically more stable of the two.
The goal is to show, from the single structural fact that adenine pairs only with thymine and guanine pairs only with cytosine across the two strands of a DNA double helix, that knowing the percentage of just one base fixes the percentages of all four other bases, without measuring the other three directly. This is Chargaff's rule, and it follows from counting, not from any new chemistry.
Step 1. Since every adenine on one strand is matched to exactly one thymine on the partner strand (and vice versa, because pairing is one-to-one across the whole molecule), the total count of adenine bases in the double helix must equal the total count of thymine bases. Expressed as percentages of all bases present, .
Step 2. By the identical argument applied to the other pair, every guanine is matched to exactly one cytosine, so .
Step 3. The four bases account for the whole molecule — there are no other bases to place — so their percentages must sum to the whole: .
Step 4. Substitute Step 1 and Step 2 into Step 3. Writing as and as gives , so . This means that once either (hence ) or (hence ) is known, the remaining pair is fixed by subtraction from — a single measurement pins down the entire base composition of a double-stranded DNA sample. Note: this rule depends entirely on there being two complementary strands. It has no analogue in single-stranded RNA, which carries no obligatory partner strand to enforce equal counts.
Worked example
Question. A double-stranded DNA segment contains 30% adenine (as a percentage of total bases). Find the percentage of each of the other three bases.
Step 1. By the pairing rule from the derivation above, , so with , immediately .
Step 2. Also from the rule, , and from Step 4 of the derivation, , so .
Step 3. Then . Check the total: , confirming consistency.
Final answer: A = 30%, T = 30%, G = 20%, C = 20%. This calculation is valid only for double-stranded DNA; it would not apply to single-stranded RNA, which has no fixed partner strand enforcing equal base counts.
Competitive stretch
If instead 22% of the bases were guanine, then and each — the same logic, just starting from the other pair.
Common traps
- Assuming both DNA and RNA contain all four of A, T, G, C — RNA has no thymine; it has uracil instead. Only adenine, guanine, and cytosine occur in both.
- Reversing which base is a purine and which is a pyrimidine — adenine and guanine are the double-ring purines; cytosine, thymine, and uracil are the single-ring pyrimidines.
- Forgetting the pairing is A-T (2 hydrogen bonds) and G-C (3 hydrogen bonds), not any other combination — this pairing is what makes percentage-composition questions solvable from a single given value.
- Trying to apply Chargaff's rule to RNA, or to single-stranded DNA — the equal-percentage relationship only holds when a full complementary partner strand is present to enforce one-to-one pairing.
Ask the tutor: want to practise more base-percentage calculations or see how complementary pairing enables accurate DNA replication step by step?
Why does RNA use uracil instead of thymine?
Both bases pair correctly with adenine, so functionally either would work in principle. Chemically, uracil is simply thymine without an extra methyl group. RNA's syllabus role here is just the comparative point: wherever DNA would use thymine, RNA uses uracil, and this swap is one of the three examinable differences between the two nucleic acids alongside the sugar and the strand count.
Can Chargaff's rule be used on RNA at all?
Not in general. Chargaff's rule follows entirely from RNA-independent structural logic: two complementary strands wound together, with every purine on one strand matched to a fixed pyrimidine partner on the other. RNA is generally single-stranded, so there is no obligatory partner strand to force equal base counts, and %A need not equal %U, nor %G equal %C, in an RNA sample.
Is a nucleoside the same thing as a nucleotide?
No, and this distinction is a common exam trap. A nucleoside is just a base joined to its sugar, nothing more. A nucleotide is a nucleoside plus a phosphate group attached to the sugar. Since phosphodiester bonds link the phosphate of one nucleotide to the sugar of the next, phosphate is essential for building the polymer chain, so nucleic acids are made of nucleotides, not bare nucleosides.