Introduction to DNA Structure

Cover image. The double helix in the middle, its three forms on the left, and the same molecule packed onto protein spools into a chromosome on the right. This lesson measures the helix and shows how it fits inside a cell.

Level: GCSE foundations, A-level, and first-year university extension. You need the building blocks first — the nucleotide, the five bases and Chargaff's rules — which are taught in the introduction to nucleic acids. If atoms and chemical bonds feel shaky, the chemical basis of life is the place to start. Everything else is built from scratch here.

Picture a ladder, then twist it. That is the whole of DNA's shape, and it is also the whole of its trick: the rungs of the ladder are the letters of the genetic code, and the twist is what lets two metres of the stuff fit inside a nucleus you cannot see. This lesson is about that shape in full — not the sketch you met in the foundation lesson, but the measurements, the grooves, the three forms the helix can take, the way it is packed, and what happens when you heat it. The types of RNA get their own lesson next, types of RNA, so RNA is only mentioned here where the comparison helps.

The 1953 model — and the evidence behind it

The double helix was proposed by James Watson and Francis Crick in a paper of barely more than one page, published in Nature on 25 April 1953. It is the most famous paper in biology, and also one of the shortest. The same issue carried two supporting papers: one by Maurice Wilkins, Alexander Stokes and Herbert Wilson, and one by Rosalind Franklin and Raymond Gosling. All three appeared together, and all three were needed — but the story of who contributed what is not the tidy version that used to be told.

Three pieces of evidence made the model possible, and each one is worth knowing by name:

• Chargaff's rules (Erwin Chargaff, 1950). In double-stranded DNA, the amount of adenine always equals the amount of thymine, and guanine always equals cytosine. A = T and G = C, in every species tested. That is a fact about amounts, not about pairing — the pairing is the explanation for the fact, and the two must not be mixed up.
• Photo 51 (Rosalind Franklin, 1952). Franklin's X-ray diffraction image of DNA showed a clear cross — the signature of a helix — and gave the dimensions the model had to fit. Wilkins showed the image to Watson without Franklin's knowledge. She died in 1958, four years before the Nobel Prize went to Watson, Crick and Wilkins; Nobel Prizes are not awarded after death, but the plain fact is that the model could not have been built without her data.
• The model-building itself. Watson and Crick did not discover the helix by experiment. They built a physical model, trying shapes until one fitted Chargaff's numbers and Franklin's measurements. The shape that fitted was two strands, twisted, with the bases paired inside. Their paper ends with the most famous understatement in science: "It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material."

That last sentence is the point of the whole lesson. The structure is the function: because the two strands are complementary, each one is a template for the other, and copying the molecule is just a matter of separating the strands and building a new partner for each.

The helix, measured

A drawing of DNA without numbers is decoration. The numbers are what exams test, and there are only four of them to learn:

Measurement	Value (B-DNA)	What it means
Diameter	About 2 nm	The width of the helix, constant all the way along
Rise per base pair	0.34 nm	How far each base pair sits above the one below it
Base pairs per turn	10	The number of rungs in one complete twist
Pitch (length of one turn)	3.4 nm	10 base pairs × 0.34 nm

Two warnings before you memorise the table. First, the 10 base pairs per turn is Watson and Crick's model. In solution the helix twists a little more loosely, at about 10.5 base pairs per turn — if a question says "in solution", the answer is about 10.5, not 10. Second, these are the values for B-DNA, the form found in cells most of the time. Two other forms exist, and they have different numbers; they get their own section below.

Figure 1. The DNA double helix: two antiparallel sugar-phosphate backbones outside, base pairs inside. Diameter about 2 nm; rise 0.34 nm per base pair; one full turn 3.4 nm, holding 10 base pairs. A–T pairs have 2 hydrogen bonds, G–C pairs have 3. Two small slips in the drawing: the key at the bottom shows the green square without its "A = Adenine" label, and the wide major groove is not labelled — it is the wide gap between the backbones, opposite the narrow minor groove that is labelled.

Antiparallel strands — and how to write the other one

The two strands run in opposite directions. One runs 5′ to 3′ downwards; the other runs 3′ to 5′ downwards. That is what antiparallel means, and it is the single most examined word in this topic, because it is the reason students write the complementary strand the wrong way round.

The rule for writing the complement has two steps, and skipping the second is the classic error. Step one: pair each base — A with T, T with A, G with C, C with G. Step two: write the answer 5′ to 3′, which means reading it from right to left of the original strand. The pair you get by step one alone is the complement running the wrong way.

Worked example 1 — writing the complementary strand (practice figures)

Practice figures, not real data. A strand reads 5′-ATGCGT-3′. Write its complement, 5′ to 3′.

Step 1 — pair each base: A pairs with T, T with A, G with C, C with G, G with C, T with A, giving TACGCA running the other way. Step 2 — write it 5′ to 3′: read TACGCA backwards. Answer: 5′-ACGCAT-3′. The answer that most students give first — TACGCA — is the complement written 3′ to 5′, and it is marked wrong. Learn the two-step method, not the answer: the letters in exam questions change, the method does not.

Figure 2. Antiparallel strands: the top strand runs 5′ to 3′ left to right; the bottom strand runs 3′ to 5′. Worked example: a strand reading 5′-ATGCGT-3′ has the complement 5′-ACGCAT-3′ — paired base by base, then written 5′ to 3′ by reading from right to left.

Why does it matter which way the strands run? Because every enzyme that reads or copies DNA moves along a strand in the 5′ to 3′ direction, so the two strands are read in opposite directions. That is also why a new strand can only grow at its 3′ end — a fact that shapes the whole mechanism of DNA replication, which is a later lesson, not this one.

Why the helix is the same width all the way along

Look at the rungs of the ladder: every one of them is the same length. That is not a coincidence of the drawing. Adenine and guanine are purines, built from two fused rings — the large bases. Thymine and cytosine are pyrimidines, built from a single ring — the small bases. Every base pair is one large base plus one small one: A with T, G with C. Two purines would be too wide for the helix, two pyrimidines too narrow, and the width would wobble from rung to rung. Franklin's X-ray images showed a constant width, and only purine–pyrimidine pairing explains it.

The pairs are held by hydrogen bonds: two between adenine and thymine, three between guanine and cytosine. Hydrogen bonds are weak individually — which is exactly what the molecule needs, because the strands must be able to separate when the DNA is copied — but there are thousands of them, and together they hold the helix shut. The strands themselves are held together by nothing stronger than that. The covalent bonds are all along each strand, in the backbone; the link between the strands is hydrogen bonding alone. Saying the strands are held together by covalent bonds is one of the commonest wrong answers on this topic, and it is corrected again in the mistakes section below.

The backbone sits on the outside for a related reason. The sugar-phosphate chain is the frame of the ladder, and the base pairs are the rungs. Put the bases outside and the information would be exposed to damage; put them inside and the helix protects them, while the charged phosphate backbone faces the watery cell. That charge also explains a laboratory fact worth knowing: DNA is negatively charged, so in gel electrophoresis it moves towards the positive electrode.

The grooves: where proteins read the sequence

The helix is not a smooth tube. As the two backbones twist around each other, they leave two grooves running along the outside of the molecule — a wide major groove, about 2.2 nm across, and a narrow minor groove, about 1.2 nm across.

The major groove is where the reading happens. The edges of the bases that stick out into it carry a pattern of hydrogen-bond donors and acceptors, and the pattern is different for each of the four base pairs. A protein sitting in the major groove can tell an A–T pair from a G–C pair without opening the helix at all — it reads the pattern of the exposed edges, like reading the spine of a book without opening it. The minor groove carries less information, so proteins read from it less often.

This is why the grooves matter beyond the diagram: every protein that switches a gene on or off, every enzyme that copies DNA, finds its place by sitting in a groove and reading the pattern there. The shape of the molecule is the address system of the genome.

Three forms of the same molecule: B, A and Z

B-DNA is the form in cells most of the time, and it is the form Watson and Crick modelled. But DNA is flexible, and under different conditions it takes other shapes. Two matter for exams:

Feature	B-DNA	A-DNA	Z-DNA
Handedness	Right-handed	Right-handed	Left-handed
Base pairs per turn	10	11	12
Rise per base pair	0.34 nm	0.26 nm	0.37 nm
Diameter	2.0 nm	2.6 nm	1.8 nm
When it appears	The usual form in cells	Dehydrated DNA; DNA–RNA hybrids	Stretches rich in G and C

A-DNA is what DNA becomes when it loses water: it shortens and widens, with 11 base pairs per turn. It also appears where DNA is paired with RNA, because the RNA strand forces the double helix into the A form — which is why the A form matters in transcription, even though B is the form students meet first. Z-DNA is the odd one out in every column: it twists to the left, its backbone zigzags — the Z in its name comes from that zigzag — and it appears in stretches rich in guanine and cytosine. Only one form is left-handed, and it is Z.

Figure 3. Three forms of the double helix: B-DNA (right-handed, 10 base pairs per turn, the usual form), A-DNA (right-handed, 11 per turn, shorter and wider, in dehydrated DNA and DNA–RNA hybrids), Z-DNA (left-handed, 12 per turn, in GC-rich stretches). The panels carry "Panel 1/2/3" labels, and the Z-DNA drawing reads as a regular helix at a glance — its left-handed zigzag is in the backbone, and the labels carry the handedness.

Worked example 2 — how long is a DNA molecule?

Practice figures, not real data. A DNA molecule is 1,000 base pairs long. How long is it, and how many turns does it make?

Step 1 — length: each base pair adds 0.34 nm, so 1,000 × 0.34 nm = 340 nm, or 0.34 micrometres. Step 2 — turns: one full turn holds 10 base pairs, so 1,000 ÷ 10 = 100 turns. The same arithmetic scales up. A human haploid genome has about 3 billion base pairs: 3,000,000,000 × 0.34 nm is about 1 metre of DNA in each set of chromosomes, and about 2 metres in a diploid cell — which is the starting point for the packaging problem in the next section. Learn the method, not the numbers: the numbers in exam questions change, the method does not.

Packaging: how two metres fit into six micrometres

Here is the problem that makes DNA structure a story about packing, not just about shape. A human cell holds about 2 metres of DNA in a nucleus about 6 micrometres across. Unpacked, the DNA would be roughly 300,000 times longer than the nucleus is wide. It fits because it is packaged in levels, each level folding the last one down — and the first level is the most important structure in this whole lesson.

Level 1 — the nucleosome. About 147 base pairs of DNA wrap about 1.65 turns around a disc of eight histone proteins — two copies each of H2A, H2B, H3 and H4. The disc is the histone octamer, and the DNA-wrapped disc is a nucleosome. The structure was worked out by Roger Kornberg in 1974 and solved at atomic detail by Timothy Richmond's group in 1997. The DNA in a nucleosome is bent far more sharply than a free double helix would ever bend on its own, and that bending is paid for by the attraction between the negatively charged DNA backbone and the positively charged histones.

Level 2 — beads on a string. Nucleosomes are joined by short stretches of linker DNA, usually 10 to 80 base pairs long, where the fifth histone, H1, binds. A chain of nucleosomes looks like beads on a string under the electron microscope, and it is about 10 nanometres wide — the 10 nm fibre.

Level 3 — the 30 nm fibre. With H1's help, the beads-on-a-string fibre coils into a thicker fibre about 30 nanometres wide. Whether it coils as a regular solenoid or folds as a zigzag is still argued about — the honest answer is that both pictures have evidence behind them, and the fibre is probably less regular than either drawing suggests.

Level 4 — loops and chromosomes. The 30 nm fibre folds into loops attached to a protein scaffold, and the loops condense into the chromosome you see in a dividing cell. In a metaphase chromosome, the DNA is compacted roughly 10,000-fold compared with the naked double helix.

Figure 4. Packaging in five steps: double helix → nucleosome (147 base pairs, ~1.65 turns, histone octamer) → beads on a string (10 nm fibre) → 30 nm fibre with H1 → loops on a scaffold (chromosome). About 2 metres of DNA fits in a nucleus about 6 µm across. Two notes on the drawing: the "roughly 10,000-fold" packing figure is the compaction in a metaphase chromosome — it is not the ratio of 2 metres to a 6-micrometre nucleus, which is far larger — and the Step 4 drawing is schematic, a twisted ribbon standing in for the coiled stack of nucleosomes. Euchromatin is loosely packed with genes active; heterochromatin is tightly packed with genes silent.

Packaging is not just storage. It is regulation. DNA wrapped in a nucleosome cannot be read until it is unwrapped, so the cell controls its genes partly by controlling how tightly each stretch of DNA is packed. Loosely packed chromatin, with its genes accessible and switched on, is euchromatin. Tightly packed chromatin, with its genes mostly silent, is heterochromatin — the dark-staining regions in a nucleus, and the state of whole chromosomes that are switched off, like the inactive X chromosome in female mammals.

Bacteria do it differently, and the contrast is worth one paragraph. Bacteria have no nucleus and no histones. Their DNA is usually a single circular chromosome, supercoiled — twisted on itself like an over-wound rubber band — and held in a region called the nucleoid by DNA-binding proteins. The full bacterial picture is in the components of bacterial cells lesson; here, the point is only that histones are a eukaryotic solution, not a universal one.

Melting DNA: denaturation and renaturation

Heat a DNA sample and the two strands come apart. The hydrogen bonds between the bases break, one by one, until the double helix has become two single strands. That is denaturation, and it is not the same as destroying the DNA: the covalent backbone of each strand stays intact, so when the sample cools, the strands can find each other and pair up again — renaturation. The same separation happens in the cell every time DNA is copied or read; in the laboratory, heating is the way to do it on demand.

The temperature at which half of a DNA sample has separated is its melting temperature, written Tm. Two things set it. The first is length — longer molecules melt hotter. The second is base composition, and it is the one exams ask about: a G–C pair has three hydrogen bonds against two for A–T, and G–C steps also stack more strongly on their neighbours, so DNA rich in G and C needs more heat to pull apart.

Worked example 3 — which fragment melts first? (practice figures)

Practice figures, not real data. Two DNA fragments of the same length: one is 70% A–T, the other is 70% G–C. Which melts at the higher temperature, and why?

The GC-rich fragment melts hotter. Every G–C pair carries three hydrogen bonds where an A–T pair carries two, and the G–C steps stack more strongly, so the GC-rich fragment needs more heat to separate its strands. The AT-rich fragment, with more two-bond pairs, comes apart first. The length is the same in both, so composition is the only difference that matters — which is exactly why the question gives you the percentages.

One laboratory detail is worth a line, because it is how melting is actually measured. A double helix absorbs less ultraviolet light than the same bases would as single strands, because the stacked bases shield each other. When DNA melts, the absorption at 260 nm rises — the hyperchromic effect — so a spectrophotometer watching the absorption can follow the melting as it happens, and the Tm is read off the curve.

Confusable pairs — keep these apart

Pair	How to keep them apart
Base vs base pair vs nucleotide	A base is one letter (A, T, G or C). A base pair is two complementary bases, one on each strand, joined by hydrogen bonds. A nucleotide is one base plus its sugar and phosphate, on a single strand.
Antiparallel vs parallel	DNA's strands are antiparallel — opposite directions, 5′ to 3′ against 3′ to 5′. Parallel would mean both running the same way, which DNA does not do.
Major vs minor groove	The major groove is wide (about 2.2 nm) and is where proteins read the sequence. The minor groove is narrow (about 1.2 nm) and carries less information.
B-DNA vs A-DNA vs Z-DNA	B is the usual form, 10 base pairs per turn. A is the dry form, 11 per turn. Z is the left-handed form, 12 per turn, in GC-rich stretches.
Nucleosome vs chromosome	A nucleosome is one disc of histones with 147 base pairs wrapped around it — the smallest unit of packaging. A chromosome is the whole packaged structure, millions of base pairs long.
Heterochromatin vs euchromatin	Heterochromatin is tightly packed, genes mostly silent. Euchromatin is loosely packed, genes active.
Denaturation vs degradation	Denaturation separates the two strands; the backbones survive and the strands can rejoin. Degradation breaks the molecule apart — the backbone is cut and the DNA is destroyed.

Common mistakes

• "The two strands are held together by covalent bonds." No — by hydrogen bonds between the bases. The covalent bonds run along each strand, in the backbone. If the strands were joined covalently, they could never separate for copying.
• "Adenine pairs with guanine." No — adenine pairs with thymine, guanine with cytosine. A purine always pairs with a pyrimidine, and only those two pairings fit the constant width of the helix.
• "The backbone holds the helix together." The backbone holds each strand together. The helix — the two strands as a pair — is held together by the hydrogen bonds between the bases.
• "The two strands are identical." No — they are complementary. Where one strand has A, the other has T; where one has G, the other has C. Identical strands would be useless as templates.
• "The complement of 5′-ATGCGT-3′ is TACGCA." That is the complement written the wrong way round. Written 5′ to 3′, it is ACGCAT. Always do the second step.
• "DNA is single-stranded in bacteria." No — bacterial DNA is double-stranded, like ours. What bacteria lack is a nucleus and histones, not the double helix.
• "A base pair is the same as a nucleotide." No — a nucleotide is one unit on one strand; a base pair is two nucleotides, one from each strand, facing each other.
• "The helix is 3.4 nm wide." 3.4 nm is the pitch — the length of one turn. The width is 2 nm, and the rise per base pair is 0.34 nm. Three numbers, three different measurements.

How to answer an exam question on this

Most exam questions on DNA structure are one of three kinds. Learn the pattern for each, and the marks follow.

Pattern 1 — "Describe the structure of DNA." Work from the outside in: two antiparallel strands; sugar-phosphate backbones outside; base pairs inside; A–T with two hydrogen bonds, G–C with three; constant width because purine pairs with pyrimidine; major and minor grooves; the measurements — 2 nm wide, 0.34 nm rise, 10 base pairs per turn, 3.4 nm pitch. That paragraph earns the marks; the history and the packaging are extra, not the answer.

Pattern 2 — "Explain how the structure of DNA allows it to carry out its function." The answer is complementary base pairing: because A pairs with T and G with C, each strand is a template for the other, so the molecule can be copied accurately; because the strands are held by hydrogen bonds, they can separate for copying and rejoin afterwards; because the bases are inside, the information is protected. Structure, then consequence, then consequence again — never just a list of features.

Pattern 3 — a calculation. Length = number of base pairs × 0.34 nm. Turns = number of base pairs ÷ 10. Complement = pair each base, then write 5′ to 3′. Show the working: a bare number with no method earns less than a worked answer, and a wrong number with the right method still earns most of the marks.

Model paragraph (Pattern 2). "DNA is a double helix of two antiparallel strands, with sugar-phosphate backbones on the outside and base pairs on the inside. Adenine pairs with thymine by two hydrogen bonds and guanine pairs with cytosine by three, so the two strands are complementary: the sequence of one strand determines the sequence of the other. Because the strands are held together only by hydrogen bonds, they can separate when the DNA is copied, and each strand then acts as a template for building a new partner strand. The bases sit inside the helix, protected by the backbones, which keeps the information safe between divisions."

Frequently asked questions

Why is DNA a double helix and not a single strand? A single strand could carry a sequence, but it could not be copied accurately — there would be nothing to check the copy against. Two complementary strands mean each one is a template for the other, so copying is self-checking. The helix shape itself comes from the way the stacked base pairs twist to fit together.

Is the double helix the same in all organisms? The B form is the same in essentially all cells — the measurements in this lesson hold for a bacterium and a human alike. What differs is the amount of DNA and how it is packaged, not the shape of the helix.

Does DNA ever leave the nucleus? In a eukaryotic cell the DNA stays in the nucleus, except for the small amounts in mitochondria and chloroplasts. What leaves the nucleus is the working copy — messenger RNA — which is why RNA gets its own lesson.

What is the difference between a gene and a chromosome? A gene is a stretch of DNA that carries the instructions for one product, usually one protein. A chromosome is the packaged structure that holds many genes — a human chromosome carries hundreds or thousands of them, wrapped around histones into the structures this lesson describes.

Can DNA really be 2 metres long in one cell? Yes — a human diploid cell holds about 6 billion base pairs, and at 0.34 nm per base pair that is about 2 metres. It fits because it is packaged into nucleosomes, fibres and loops, compacted roughly 10,000-fold in a metaphase chromosome.

Why does GC-rich DNA melt at a higher temperature? Because a G–C pair has three hydrogen bonds against two for A–T, and G–C steps stack more strongly on their neighbours. More bonds and stronger stacking mean more heat is needed to pull the strands apart.

Continue learning

• Introduction to Nucleic Acids — the foundation: nucleotides, the five bases, Chargaff's rules and the genetic code.
• Types of RNA — the next lesson: messenger, transfer and ribosomal RNA, and how the single strand folds.
• Inheritance and Genetic Variation — what DNA does with its information: genes, alleles and how traits pass from one generation to the next.
• Structural and Functional Components of Bacterial Cells — the bacterial contrast: the nucleoid, plasmids and DNA without histones.
• Biological Method — how a model like the double helix is proposed, tested and accepted, step by step.

Sources

• Watson, J. D. & Crick, F. H. C. (1953). Molecular structure of nucleic acids: a structure for deoxyribose nucleic acid. Nature 171, 737–738.
• Franklin, R. E. & Gosling, R. G. (1953). Molecular configuration in sodium thymonucleate. Nature 171, 740–741.
• Wilkins, M. H. F., Stokes, A. R. & Wilson, H. R. (1953). Molecular structure of deoxypentose nucleic acids. Nature 171, 738–740.
• Chargaff, E. (1950). Chemical specificity of nucleic acids and mechanism of their enzymatic degradation. Experientia 6, 201–209.
• OpenStax, Biology 2e (2018), Chapter 9: Molecular Biology — the structure of DNA and DNA packaging.
• Alberts, B. et al., Molecular Biology of the Cell, 6th edition (2014), Chapters 4 and 5 — DNA structure, chromatin and the nucleosome.
• Luger, K., Mäder, A. W., Richmond, R. K., Sargent, D. F. & Richmond, T. J. (1997). Crystal structure of the nucleosome core particle at 2.8 Å resolution. Nature 389, 251–260.
• Kornberg, R. D. (1974). Chromatin structure: a repeating unit of histones and DNA. Science 184, 868–871.

20 Explained MCQs — answers hidden until opened

Open each question to reveal its answer and explanation. The interactive quiz below uses a different set of questions.

1. What did Watson and Crick propose in 1953?

A. That DNA is made of amino acids joined by peptide bonds   B. That DNA is a triple helix of three parallel strands   C. That DNA is a single strand folded into a ball   D. That DNA is a double helix of two antiparallel strands
Answer: D. In their short 1953 Nature paper, Watson and Crick proposed that DNA is two strands wound around each other into a right-handed double helix. The model explained Chargaff's rules and how DNA could be copied — which is why it is still the picture every textbook uses.

2. Whose X-ray diffraction image gave the key evidence for the double helix?

A. Friedrich Miescher, from his isolation of nuclein   B. Erwin Chargaff, from his base measurements   C. Alfred Hershey and Martha Chase, from their blender experiment   D. Rosalind Franklin, whose image Photo 51 showed the cross pattern of a helix
Answer: D. Rosalind Franklin's X-ray diffraction image, Photo 51, showed the clear cross pattern of a helix and gave Watson and Crick the dimensions they needed. Wilkins showed the image to Watson without Franklin's knowledge; she died in 1958, four years before the Nobel Prize went to Watson, Crick and Wilkins.

3. What is the diameter of the DNA double helix?

A. About 3.4 nm   B. About 0.34 nm   C. About 2 nm   D. About 20 nm
Answer: C. The double helix is about 2 nanometres across — 20 angstroms — and it stays the same width all the way along, because a purine always pairs with a pyrimidine.

4. How far apart are neighbouring base pairs along the helix?

A. 0.34 nm   B. 3.4 nm   C. 2 nm   D. 10 nm
Answer: A. Each base pair sits 0.34 nanometres above the next — the rise per base pair. Ten base pairs therefore span 3.4 nanometres, which is the length of one full turn of the helix.

5. How many base pairs are there in one full turn of B-DNA, as Watson and Crick modelled it?

A. About 10.5   B. 10   C. 11   D. 12
Answer: B. Watson and Crick's model has 10 base pairs per turn, giving a pitch of 3.4 nm. In solution the twist is slightly looser — about 10.5 base pairs per turn — but exam questions usually use the model's 10.

6. Why is the double helix the same width all the way along?

A. Because every base is the same size   B. Because the backbone is made of identical sugars   C. Because the hydrogen bonds are all the same length   D. Because a purine always pairs with a pyrimidine, and the two together always span the same distance
Answer: D. Adenine and guanine are large two-ring purines; thymine and cytosine are small one-ring pyrimidines. Every pair is one large base plus one small one, so every rung of the ladder is the same length. Two purines would be too wide, two pyrimidines too narrow.

7. Which groove do proteins mostly read the base sequence from?

A. Neither groove — proteins read the backbone instead   B. Both grooves equally, since they carry the same information   C. The major groove, because it is wide enough to expose the edges of the bases   D. The minor groove, because it is narrower and easier to grip
Answer: C. The major groove is about 2.2 nm wide, and the edges of the bases that stick out into it carry a pattern of hydrogen-bond donors and acceptors that is different for each base pair. Proteins sit in the major groove and read that pattern without opening the helix. The minor groove, about 1.2 nm wide, carries less information.

8. What sits on the outside of the double helix?

A. The sugar-phosphate backbones of the two strands   B. The nitrogenous bases   C. The hydrogen bonds   D. The codons
Answer: A. The two sugar-phosphate backbones run along the outside of the helix, and the base pairs sit inside, like the rungs of a ladder. That arrangement protects the bases and leaves the charged phosphate backbone facing the watery cell.

9. A DNA strand is written 5'-ATGCGT-3'. What is the complementary strand, written 5' to 3'?

A. ATGCGT   B. TACGCA   C. TGCATG   D. ACGCAT
Answer: D. Pair each base — A with T, T with A, G with C, C with G, G with C, T with A — to get TACGCA running the other way. Written 5' to 3', that reads ACGCAT: the complement is written from right to left of the original strand. The commonest mistake is writing TACGCA, which is the complement read in the wrong direction.

10. Practice figures, not real data: a DNA molecule is 1,000 base pairs long. How long is it?

A. 3,400 nm   B. 10,000 nm   C. 340 nm   D. 34 nm
Answer: C. Each base pair adds 0.34 nm, so 1,000 base pairs give 1,000 x 0.34 nm = 340 nm, or 0.34 micrometres. These are practice figures for working out helix lengths, not a measurement from a real molecule.

11. Practice figures, not real data: how many complete turns does that 1,000-base-pair molecule make?

A. 1,000 turns   B. 34 turns   C. 340 turns   D. 100 turns
Answer: D. One full turn holds 10 base pairs, so 1,000 base pairs make 1,000 / 10 = 100 turns. These are practice figures for working out helix turns, not a measurement from a real molecule.

12. Which form of DNA is left-handed?

A. C-DNA   B. B-DNA   C. A-DNA   D. Z-DNA
Answer: D. Z-DNA is the left-handed form, with 12 base pairs per turn and a zigzag backbone. It appears in stretches rich in G and C. B-DNA and A-DNA are both right-handed; C-DNA is another right-handed form that is rarely taught.

13. Which form of DNA appears when DNA is dehydrated?

A. D-DNA   B. A-DNA   C. Z-DNA   D. B-DNA
Answer: B. When DNA loses water it shortens and widens into A-DNA: 11 base pairs per turn, a rise of 0.26 nm and a diameter of 2.6 nm. A-DNA also appears in DNA-RNA hybrids, where the RNA strand forces the DNA into the A form.

14. How many base pairs of DNA wrap around one histone octamer in a nucleosome?

A. About 100   B. About 50   C. 147   D. About 200
Answer: C. A nucleosome core particle holds 147 base pairs of DNA wrapped about 1.65 turns around an octamer of histone proteins — two copies each of H2A, H2B, H3 and H4. Linker DNA of 10 to 80 base pairs joins one nucleosome to the next.

15. Which histones make up the nucleosome core octamer?

A. One copy each of H1, H2A, H2B, H3 and H4   B. Two copies each of H2A, H2B, H3 and H4   C. Eight copies of histone H1   D. Four copies of H3 and four copies of H4
Answer: B. The core octamer is built from two copies of each of four core histones: H2A, H2B, H3 and H4. Histone H1 is not part of the core; it is the linker histone that binds the DNA where it enters and leaves the nucleosome and helps pack the 30 nm fibre.

16. What is the 'beads on a string' fibre called?

A. The 10 nm fibre   B. The solenoid fibre   C. The 30 nm fibre   D. The metaphase chromosome
Answer: A. A chain of nucleosomes joined by linker DNA looks like beads on a string under the electron microscope, and it is about 10 nanometres wide — the 10 nm fibre. Coiling it further, with the help of histone H1, gives the thicker 30 nm fibre.

17. Which chromatin is loosely packed, with its genes switched on?

A. The nucleosome   B. The metaphase chromosome   C. Heterochromatin   D. Euchromatin
Answer: D. Euchromatin is loosely packed chromatin, and its genes are accessible and active. Heterochromatin is tightly packed, its genes mostly silent — the dark-staining regions seen in a nucleus, and the state of whole chromosomes that are switched off.

18. What is the melting temperature (Tm) of a DNA sample?

A. The temperature at which DNA starts to replicate   B. The temperature at which half of the DNA is denatured — half of its base pairs separated   C. The temperature at which DNA is destroyed completely   D. The temperature at which the backbone breaks apart
Answer: B. The melting temperature is the temperature at which half of a DNA sample has separated into single strands. Heating breaks the hydrogen bonds between the bases; the covalent backbone stays intact. The strands can come back together — renature — when the sample cools.

19. Practice figures, not real data: two DNA fragments of the same length — one is 70% A-T, the other is 70% G-C. Which melts at the higher temperature?

A. The GC-rich fragment, because G-C pairs have three hydrogen bonds and stronger base stacking   B. The AT-rich fragment, because A-T pairs are larger   C. They melt at exactly the same temperature   D. Neither melts, because the backbone is covalent
Answer: A. A G-C pair is held by three hydrogen bonds against two for A-T, and G-C steps also stack more strongly on their neighbours. So a GC-rich fragment needs more heat to pull apart and has a higher Tm. These are practice figures for working out melting order, not measurements from real samples.

20. Which claim stays inside the evidence of the practice figures in this lesson?

A. All DNA in every species has exactly 1,000 base pairs   B. A DNA molecule of 1,000 base pairs is 340 nm long and makes 100 turns   C. Every DNA molecule is exactly 340 nm long   D. DNA always makes exactly 100 turns, whatever its length
Answer: B. Only the claim that repeats the practice figures is safe: in the practice figures, a DNA molecule of 1,000 base pairs was 340 nm long and made 100 turns. The others turn one worked example into a general rule the figures do not support — about all DNA, all species, or all lengths.

Separate Interactive Quiz — 20 different questions

These quiz questions are different from the MCQs above. Select an answer for each question. You will not see whether an answer is correct until you submit all 20 questions. Use Reset to try again.