The DNA Double Helix
In 1953, Watson and Crick unveiled a model that fit decades of clues, Rosalind Franklin's X-ray images, Chargaff's base ratios, into one elegant shape: two strands twisted into a double helix, their bases paired in the middle like rungs on a spiral ladder. That structure instantly suggested how DNA could be copied, and it remains the single most important molecule you will study. This lesson builds it from the bottom up.
Get oriented
Make a prediction, read the goals and the key terms, then clear the trap before you start.
Before You Read
warm-upDNA is often drawn as a twisted ladder. The "uprights" are made of one repeating backbone; the "rungs" are pairs of bases.
Before reading: if DNA stores information, where in this ladder do you think the actual "message" is written, in the uprights or the rungs? Explain your guess.
Learning Intentions
goalsKnow
- The three parts of a nucleotide
- The four bases and the base-pairing rules (A–T, C–G)
- That base pairs are held together by hydrogen bonds
Understand
- Why DNA is described as a double helix
- Why complementary base pairing is the key to copying DNA
- Why hydrogen bonds are strong together but easy to separate
Can Do
- Use the double helix model to describe DNA's structure
- Write the complementary strand for a given base sequence
- Label nucleotides, base pairs and hydrogen bonds on a diagram
Scan these before reading
vocabMisconceptions To Fix
watch outBuild the first idea
The core of the dot point, then two checks you can answer straight from the card.
Core Content
The Nucleotide Building Block
+5 XPPhosphate, sugar and base, repeated millions of times
DNA is a polymer built from repeating units called nucleotides, each made of three parts: a phosphate group, a deoxyribose sugar, and a nitrogenous base.
Text description
- Left to right: a phosphate group, joined to a deoxyribose sugar, joined to one nitrogenous base.
- The base is one of A, T, C or G.
- Many nucleotides join to form one DNA strand.
The four bases are adenine (A), thymine (T), cytosine (C) and guanine (G). Many nucleotides link through their sugars and phosphates to form a long sugar–phosphate backbone, with the bases pointing inward. A nucleotide has three parts, a phosphate group, a deoxyribose sugar and one of four nitrogenous bases (A, T, C, G). Joined sugars and phosphates form the backbone; the bases store the genetic information.
Pause, copy the three parts of a nucleotide into your book and label the diagram.
The three components of a DNA nucleotide are a nitrogenous base, a deoxyribose sugar and a _____ group.
Match each part of a nucleotide to what it does.
- Phosphate group
- Deoxyribose sugar
- Nitrogenous base
- The four bases
- Points inward and carries the genetic information
- Links to the next sugar to form the backbone
- Adenine, thymine, cytosine and guanine
- The other half of the backbone, and what the base attaches to
Take it further
The next move, with the diagram to work from and two more checks.
Complementary Base Pairing
+5 XPA pairs with T, C pairs with G, every time
We just built a single strand. That raises a question: how do two strands join into the double helix? This card answers it → through complementary base pairing between the bases.
The two DNA strands are held together by base pairs that follow a strict rule, adenine always pairs with thymine, and cytosine always pairs with guanine.
Text description
- Two sugar–phosphate backbones run down the left and right edges.
- A pairs with T, held by 2 hydrogen bonds (dashed lines).
- C pairs with G, held by 3 hydrogen bonds.
This is complementary base pairing. Because the pairing is fixed, if you know the sequence of one strand you can work out the other exactly, this is what makes DNA copying possible. Complementary base pairing: A always pairs with T (2 hydrogen bonds), C always pairs with G (3 hydrogen bonds). One strand's base sequence therefore specifies the other strand's sequence completely.
Pause, copy the base-pairing rules (A–T, C–G) and the hydrogen-bond counts into your book.
If one DNA strand reads A–C–G–T, what is the complementary strand?
Fill the gaps. Adenine always pairs with , held by hydrogen bonds. Cytosine always pairs with , held by hydrogen bonds.
Work it yourself
Put the idea on paper in your own book before you read on.
Activity 1
Build the Complementary Strand
Pattern B, Worked application
In your book, write out this DNA strand: T–A–C–G–G–A–T–C. Underneath it, write the complementary strand using the base-pairing rules. Then label, for any one base pair, how many hydrogen bonds hold it together. Finish with one sentence stating why this pairing matters for copying DNA.
Finish the picture
The last piece of the dot point, then a true or false check across all three cards.
The Double Helix and Hydrogen Bonds
+5 XPWhy "weak" bonds are exactly what DNA needs
We just saw the pairing rule. That raises a question: why hold the strands with weak hydrogen bonds rather than strong ones? This card answers it → because they must be both stable and separable.
The two strands twist into a double helix held by hydrogen bonds, individually weak, but numerous enough to be stable, while still easy to "unzip" when the DNA needs to be read or copied.
Watson and Crick's model shows the two sugar–phosphate backbones running in opposite directions (antiparallel) and twisting around each other into a double helix, a spiral ladder, with paired bases as the rungs. Each base pair is held by hydrogen bonds: weak individually, but with millions along the molecule they together hold the strands firmly.
Crucially, because each hydrogen bond is weak, enzymes can unzip the two strands easily, separating the bases without breaking the strong backbone. This is exactly what allows DNA to be copied (next lesson) and read. DNA is a double helix: two antiparallel sugar–phosphate backbones twisted around each other, with base pairs as rungs held by hydrogen bonds. The bonds are weak individually (so strands can unzip for copying) but strong collectively (so the molecule stays stable).
Add the double-helix description and the "weak-but-many hydrogen bonds" point to your notes.
Adenine pairs with thymine and cytosine pairs with guanine.
Hydrogen bonds are weak individually, which lets the two DNA strands unzip for copying.
The genetic information of DNA is stored in the sugar–phosphate backbone.
See where it goes
Where this shows up outside the classroom, and why the dot point is worth knowing.
DNA in forensics
The complementary base-pairing rule you learned today is the engine behind DNA fingerprinting used in criminal investigations. By targeting regions of DNA that vary between people and using base pairing to copy and read them, forensic scientists can match a sample from a crime scene to an individual with extraordinary certainty. The same principle underlies paternity testing, ancestry kits, and the diagnosis of genetic disease, all of which rely on the fact that A only ever pairs with T, and C only ever with G.
Use the double helix model to describe the structure of DNA, including nucleotide composition, complementary base pairing and hydrogen bonding.
The question names the three things it wants — nucleotide composition, complementary base pairing and hydrogen bonding. Treat them as three headings; an answer that describes a helix without all three cannot reach full marks however vivid it is.
6 marks. Two for the nucleotide and the backbone, two for the pairing rule, two for the bonding and the overall shape. Build from the smallest unit upward.
DNA is a polymer built from repeating monomers called nucleotides. Each nucleotide has three parts: a phosphate group, a five-carbon deoxyribose sugar, and one of four nitrogenous bases — adenine, thymine, cytosine or guanine. The only difference between one nucleotide and another is which of the four bases it carries.
Nucleotides join into a strand through bonds between the phosphate group of one and the sugar of the next, producing a continuous sugar-phosphate backbone with the bases projecting inward from it. Two such strands lie alongside one another, running in opposite directions, so the molecule is antiparallel.
The two strands are held together by pairing between their inward-facing bases, and the pairing follows a strict rule: adenine always pairs with thymine, and cytosine always pairs with guanine. Because each base pairs with only one partner, the two strands are complementary, and the sequence of one determines the sequence of the other exactly.
Each base pair is held by hydrogen bonds: two between adenine and thymine, and three between cytosine and guanine. Individually these bonds are weak and can be broken without damaging either strand, but there are millions of them along the length of the molecule, so collectively they hold the double strand together securely.
The paired strands twist around a common axis to form a double helix, with the sugar-phosphate backbones on the outside, where they are exposed to water, and the bases stacked on the inside, shielded from it. The structure is often likened to a twisted ladder, the backbones forming the uprights and each base pair a rung.
DNA is a polymer of repeating monomers called nucleotides, each consisting of a phosphate group, a five-carbon deoxyribose sugar and one of four nitrogenous bases — adenine, thymine, cytosine or guanine — so that the only difference between nucleotides is which base they carry, and the information must therefore lie in the base sequence. Nucleotides join through bonds between the phosphate of one and the sugar of the next to form a continuous sugar-phosphate backbone with the bases projecting inward, and two such strands lie alongside one another running in opposite directions, making the molecule antiparallel. The strands are held together by complementary base pairing under a strict rule: adenine always pairs with thymine and cytosine always pairs with guanine, so the two strands are complementary and the sequence of one determines the sequence of the other exactly. Each pair is held by hydrogen bonds, two between adenine and thymine and three between cytosine and guanine; individually these are weak and can be broken without damaging either strand, but millions of them along the molecule hold the double strand together securely. The paired strands twist about a common axis into a double helix, the sugar-phosphate backbones on the outside where they are exposed to water and the bases stacked within, shielded from it — a twisted ladder whose uprights are the backbones and whose rungs are the base pairs.
Explain how complementary base pairing and hydrogen bonding make DNA suited to storing and copying genetic information.
This is a structure and function question in disguise. Explain means take each structural feature and show how it makes one of the two named jobs — storing and copying — possible.
4 marks, two features, two jobs. Deal with storage first, then copying, and name the feature responsible in every sentence.
The sugar-phosphate backbone is chemically identical along the whole molecule, so it cannot carry information; the bases can, because their order varies without limit. Since a DNA molecule may be millions of base pairs long and any base can occupy any position, an effectively unlimited number of sequences is possible, which is what allows the enormous quantity of information in a genome to be encoded.
The bases are turned inward, stacked in the core of the helix and shielded by the backbones from the surrounding water, which protects them from chemical reactions that would alter the stored sequence. The double-stranded arrangement also means the information is held twice, so damage to one strand can be recognised and repaired using the other as a reference.
Because adenine pairs only with thymine and cytosine only with guanine, each strand specifies its partner completely. When the strands are separated, each can act as a template on which free nucleotides assemble in the only order the pairing rule permits, so the two molecules produced are identical to the original. Copying therefore needs no separate record of the sequence: the molecule contains its own instructions for being copied.
The strands must be separated for copying, and hydrogen bonds make this possible: they are weak enough to be broken by an enzyme such as helicase using little energy, and because they hold the strands to one another rather than holding either strand together, breaking them separates the molecule without breaking the covalent bonds of either backbone. The same weak bonds then re-form to hold each new double strand together.
The sugar-phosphate backbone is chemically identical along the entire molecule and so cannot carry information, whereas the order of the bases can vary without limit; since a DNA molecule may be millions of base pairs long and any base can occupy any position, an effectively unlimited number of sequences is available, which is what allows a genome’s information to be encoded. That information is protected because the bases are turned inward and stacked within the helix, shielded by the backbones from the surrounding water and from reactions that would alter the sequence, and because the double-stranded arrangement holds the information twice, so damage to one strand can be repaired using the other as reference. Copying is made possible by the pairing rule: since adenine pairs only with thymine and cytosine only with guanine, each strand completely specifies its partner, so when the strands are separated each can template the assembly of free nucleotides in the only order the rule permits, and the two resulting molecules are identical to the original — the molecule carries its own instructions for being copied. Hydrogen bonds allow that separation without damage, being weak enough for an enzyme such as helicase to break with little energy, and holding the two strands to each other rather than holding either strand together, so breaking them parts the molecule while the covalent backbone of each strand remains intact; the same bonds then re-form to hold each new double strand.
Practise independently
Answer in your own words first. The model answers are for checking, not for copying.
Multiple Choice
+5 XPA fresh set drawn from this lesson's question bank, feedback shown immediately. +5 XP per correct · +25 XP all correct
Pick your answer, then rate your confidence, that tells the system what to drill next.
Short Answer, 12 marks
+5 XPUnderstandBand 4(4 marks) 1. Use the double helix model to describe the structure of DNA. Refer to the composition of a nucleotide, complementary base pairing and hydrogen bonding.
1 mark: nucleotide = phosphate + sugar + base · 1 mark: double helix / antiparallel backbones described · 1 mark: complementary pairing (A–T, C–G) · 1 mark: hydrogen bonds hold pairs / allow unzipping
ApplyBand 4(3 marks) 2. One strand of a DNA molecule reads T–A–C–G–G–A. Write the complementary strand, state how many hydrogen bonds hold this six-base-pair section together, and explain how you worked the number out.
1 mark: correct complementary strand · 1 mark: correct total number of hydrogen bonds · 1 mark: shows the working, counting 2 per A–T and 3 per C–G
AnalyseBand 5–6(5 marks) 3. Use the double helix model to explain why the hydrogen bonds between base pairs are individually weak, and why this is an advantage rather than a weakness for the DNA molecule.
1 mark: describes the double helix and where the hydrogen bonds sit · 1 mark: states that they are individually weak · 1 mark: explains that many bonds together give stability · 2 marks: explains that weak bonds allow unzipping without breaking the backbone, and links this to copying or reading
Show all answers
Multiple choice
MC answers and full explanations are shown inline as you complete each question. Use the retry button to attempt a fresh set from the lesson bank.
Short Answer Model Responses
Q1 (4 marks): DNA is a polymer of nucleotides, each consisting of a phosphate group, a deoxyribose sugar and one of four nitrogenous bases (adenine, thymine, cytosine or guanine). The sugars and phosphates link to form two sugar–phosphate backbones that run in opposite directions (antiparallel) and twist around each other into a double helix, like a spiral ladder. The bases face inward and pair by complementary base pairing: adenine always pairs with thymine, and cytosine always pairs with guanine. Each pair is held by hydrogen bonds (two between A and T, three between C and G). These bonds are weak individually but numerous, making the molecule stable while still allowing the strands to "unzip" easily for the DNA to be copied or read.
Q2 (3 marks): The complementary strand is A–T–G–C–C–T [1]. The section is held by 14 hydrogen bonds [1]. The pairs are T–A, A–T, C–G, G–C, G–C and A–T. That is four A–T pairs at 2 hydrogen bonds each, giving 8, and two C–G pairs at 3 hydrogen bonds each, giving 6, so 8 plus 6 is 14 [1].
Q3 (5 marks): The double helix is two sugar–phosphate backbones running in opposite directions and twisting around each other, with base pairs forming the rungs between them [1]. The hydrogen bonds sit between the paired bases, two holding each A–T pair and three holding each C–G pair, and each one is weak [1]. Because a DNA molecule contains millions of base pairs, the many weak bonds acting together hold the two strands firmly, so the molecule is stable [1]. If each bond were strong the strands could not be separated without damage [1]. Because each is weak, an enzyme can unzip the two strands by breaking hydrogen bonds only, leaving the strong sugar–phosphate backbones intact, which is exactly what allows the molecule to be copied and read, so the weakness is the design feature that makes DNA useable [1].
Retrieve what stuck
Work the spaced-review set first. It mixes this lesson with earlier ones on purpose.
Check what actually stuck
A short retrieval set drawn from this lesson and the ones before it, so the earlier work keeps coming back rather than fading.
Lock it in
bossYou can now describe DNA's structure from the nucleotide up to the double helix, including base pairing and hydrogen bonding. Next lesson uses that structure to model exactly how DNA copies itself before a cell divides.
↩ Back to focus-area hubCompare it with where you started
Read your opening prediction again, then mark the lesson complete.
How did your thinking change?
At the start you guessed where DNA's message is stored. The answer is the rungs, the sequence of bases (A, T, C, G), not the uprights, which are an identical, repeating sugar–phosphate backbone. Four base "letters", read in order along a strand, spell out all the instructions a cell needs. The backbone simply holds them in place.
If you guessed the backbone, revisit Card 1: the backbone is the same all along the molecule, so it cannot carry varying information.