Subjects and lessons to explore.

Law

Concepts of Biology · Molecular Biology

The Structure of DNA

7 min read
8 key termsSourcesReport an error
Numeric values (helix diameter ~2 nm, ~10 base pairs per turn, ~3.4 nm per turn, hydrogen-bond counts) are commonly taught reference concepts; verify against current texts before citing in assessments.
Want it in plain words first? Jump to Eli explains — the same idea, no jargon.
On this page 9 sections
  1. In 30 seconds
  2. Why this matters
  3. The college version
  4. Eli explains
  5. Worked example
  6. Key takeaway
  7. Check yourself
  8. Study tools
  9. Sources & references

In 30 seconds

Deoxyribonucleic acid — DNA — is the molecule that stores and transmits the hereditary information of nearly all living things. This topic traces how scientists proved that DNA, not protein, is the genetic material, then builds the molecule's structure piece by piece: nucleotides (phosphate, sugar, and one of four nitrogenous bases), the pairing rules that hold the strands together (A with T, G with C), and the geometry modeled by James Watson and Francis Crick in 1953 using evidence from Rosalind Franklin, Maurice Wilkins, Erwin Chargaff, and others. Understanding this structure is the foundation for how DNA is copied, how it directs protein production, how mutations arise, and how biotechnology reads and edits genomes.

Why this matters

The structure of DNA is one of the great explanatory ideas in science: once you see the double helix, a dozen biological mysteries make sense. The base-pairing rules explain how genetic information is copied with astonishing fidelity (each strand templates the other), how mutations pass on, and how a molecule thinner than a wavelength of visible light encodes instructions for an organism. Practically, the structure underlies forensics, paternity testing, ancestry services, gene therapy, and PCR-based pathogen tests — all depend on base pairing. The history also teaches a lesson: the discovery was a team effort, and Rosalind Franklin's X-ray diffraction work was essential even though unrecognized at the time.

The college version

Core Concepts

Establishing DNA as the genetic material

Three experiments established DNA's role. In 1928, Frederick Griffith showed that something from heat-killed virulent (S) bacteria could transform harmless live rough (R) bacteria into killers — the "transforming principle." In 1944, Oswald Avery, Colin MacLeod, and Maclyn McCarty purified that principle and showed it was DNA: destroying protein or RNA did not stop transformation; destroying DNA did. In 1952, Alfred Hershey and Martha Chase settled it: they labeled bacteriophage T2 protein with ³⁵S and its DNA with ³²P, infected bacteria, and found only the phosphorus entered the cells — DNA, not protein, is the phage's genetic material.

The building blocks: nucleotides

DNA is a polymer of nucleotides, each made of three parts: a phosphate group, the five-carbon sugar deoxyribose, and a nitrogenous base. There are four bases in two families. The purines — adenine (A) and guanine (G) — have a double-ring structure. The pyrimidines — cytosine (C) and thymine (T) — have a single ring. Sugar and phosphate form the backbone, while the bases project inward. Each 's phosphate links to the next sugar, forming a chain with a direction — the 5′ end and the 3′ end — and enzymes read and copy DNA only in specific directions.

Chargaff's rules and base pairing

In the 1940s, Erwin Chargaff found two regularities across species: DNA amount varies between species but is constant within one, and — the famous rule — adenine equals thymine and guanine equals cytosine (A = T, G = C). Chargaff's data hinted bases pair up; model building supplied the structure: adenine pairs with thymine, guanine with cytosine — . A–T pairs are held by two hydrogen bonds, G–C pairs by three (so G–C-rich DNA is slightly more stable). Because pairing is specific, one strand's sequence determines the other's — the basis of replication and of every DNA-based technology.

The double helix: Watson, Crick, Franklin, and Wilkins

By the early 1950s, Rosalind Franklin had produced superb X-ray diffraction images of DNA fibers, most famously "Photo 51," whose X-shaped pattern — the signature of a helix — revealed key dimensions. Maurice Wilkins shared this data with James Watson and Francis Crick, who were building physical models in Cambridge. Using Franklin's measurements, Chargaff's ratios, and the knowledge that bases must face inward, Watson and Crick worked out the structure in 1953: two strands wound into a right-handed double helix, backbones outside, paired bases as rungs inside — a pairing that, as their paper noted, "immediately suggests a possible copying mechanism." Watson, Crick, and Wilkins shared the 1962 Nobel Prize; Franklin, who died in 1958, was not included, though her evidence was indispensable. Commonly taught reference dimensions: about 2 nanometers in diameter, roughly 10 base pairs per turn, about 3.4 nanometers per full turn.

DNA versus RNA

RNA (ribonucleic acid) differs from DNA in three ways: its sugar is ribose instead of deoxyribose (ribose has an –OH on the 2′ carbon; deoxyribose has only a hydrogen — the "deoxy"), it uses uracil (U) instead of thymine, and it is typically single-stranded. RNA's lower stability suits its role as a temporary messenger; DNA's stability suits its job as the long-term archive of the genome.

How It Works / Step-by-Step Process

  1. Identify the genetic material: the experiments narrow the candidate to DNA.
  2. Assemble the monomer: a nucleotide joins a phosphate, deoxyribose, and one of four bases (A, G, C, T).
  3. Build the chain: nucleotides link phosphate-to-sugar into a strand with 5′ and 3′ ends and a base sequence — the genetic information.
  4. Pair the strands: a second strand aligns antiparallel, obeying A–T and G–C, so the sequences are complementary.
  5. Wind the helix: the paired strands coil into the double helix, backbones outside, bases inside.
  6. Verify the model: check predictions against Chargaff's ratios and Franklin's X-ray measurements.

Common Confusions

Common ConfusionCorrect Understanding
In DNA, A pairs with G and T pairs with C.A pairs with T (two hydrogen bonds) and G pairs with C (three).
DNA is a protein.DNA is a nucleic acid — a polymer of nucleotides, not amino acids.
The bases form the backbone of the helix.The sugar-phosphate backbones are outside; the bases are inside, forming the paired rungs.
The two strands run in the same direction.They are antiparallel: one runs 5′→3′, the other 3′→5′.
RNA uses thymine like DNA.RNA uses uracil (U) in place of thymine, pairing U with A.
Watson and Crick discovered the structure from their own experiments.They built the model using others' evidence — Franklin's X-ray diffraction and Chargaff's ratios.
Eli, the EliExplains learning guide

Eli explains

The same idea, in plain words

Explain it like I’m 10

DNA is like a twisted ladder (a spiral staircase). The rails are sugar and phosphate; the steps are pairs of letters: A always holds hands with T, G with C. Because every letter has only one partner, if you can read one side of the ladder you can always figure out the other — that's how cells copy it.

Worked example

A forensic science class is given a DNA sequence from one strand of a crime-scene sample: 5′–A G G T C C A T–3′. Applying the pairing rules and remembering the strands are antiparallel, the students write the partner strand running 3′ to 5′: T C C A G G T A. To explain why G–C pairs are harder to pull apart, they note the three hydrogen bonds versus two. The same reasoning that filled in the complementary strand is, in essence, how DNA polymerases copy a genome and how PCR amplifies DNA — just the pairing rules, no lab equipment.

Key takeaways

  • Evidence chain: Griffith (1928) → Avery, MacLeod & McCarty (1944) → Hershey & Chase (1952).
  • A nucleotide = phosphate + deoxyribose + nitrogenous base; purines (A, G) are double-ring, pyrimidines (C, T) single-ring.
  • Base pairing: A–T (two hydrogen bonds), G–C (three); strands are complementary and antiparallel (5′→3′ and 3′→5′).
  • Double helix: backbones outside, bases inside; ~2 nm diameter, ~10 base pairs per turn (commonly taught references).
  • Franklin's X-ray diffraction (Photo 51) supplied the helical evidence; Watson and Crick built the model.
  • DNA vs RNA: ribose vs deoxyribose, uracil vs thymine, single strand vs double strand.

Check yourself

5 review questions from the chapter. Try each one, then open the answer.

  1. Briefly explain how the Hershey–Chase experiment distinguished DNA from protein as the genetic material.

    Show answer

    Hershey and Chase labeled phage protein with ³⁵S and phage DNA with ³²P; after infection, only the phosphorus was inside the bacteria, showing DNA — not protein — carries the genetic instructions.

  2. Name the three components of a DNA nucleotide and the two families of nitrogenous bases.

    Show answer

    A phosphate group, deoxyribose sugar, and a nitrogenous base. The bases are purines (A, G — double ring) and pyrimidines (C, T — single ring).

  3. State and explain how complementary base pairing accounts for them.

    Show answer

    In any DNA sample, A = T and G = C, because A always pairs with T and G with C in the double helix.

  4. What does it mean for the two DNA strands to be antiparallel, and why does 5′/3′ direction matter?

    Show answer

    Antiparallel means the strands run in opposite directions (5′→3′ and 3′→5′); direction matters because polymerases synthesize and read strands only in specific directions.

  5. List three differences between DNA and RNA.

    Show answer

    RNA uses ribose instead of deoxyribose, uracil (U) instead of thymine (T), and is usually single-stranded.

Keep learning

Ready to build on this? Continue to the next lesson.

Study tools & related lessonsKey vocabulary · Related

Key vocabulary

nucleotide
DNA building block: phosphate + deoxyribose + nitrogenous base
deoxyribose
The five-carbon sugar in DNA, lacking an –OH group on the 2′ carbon
purine / pyrimidine
Double-ring bases (A, G) / single-ring bases (C, T)
complementary base pairing
Specific pairing of A with T and G with C
antiparallel
The two DNA strands run in opposite 5′→3′ directions
double helix
The two-stranded, twisted-ladder shape of DNA
Chargaff's rules
In any DNA sample, the amount of A equals T and G equals C
hydrogen bond
Weak bond holding paired bases together (A–T two, G–C three)

Sources & references

  1. openstax.org — Concepts Biology

This lesson was adapted from the open educational references above; their licenses and attributions are preserved. See Copyright & Licensing.

Educational content only. It is not medical, legal or professional advice. Found an error? Tell us.