DNA looks complicated in textbook pictures, but it is really one small unit repeated millions of times. Learn that one unit properly and the double helix, the base pairing and the exam diagrams all make sense straight away.
📚 What you need to know
DNA and RNA are polymers. The repeating unit is a nucleotide.
Every nucleotide has three parts: a phosphate group, a pentose sugar and a nitrogenous base.
DNA uses the sugar deoxyribose and the bases A, T, C, G. RNA uses ribose and swaps T for U.
Nucleotides join by condensation reactions, making a phosphodiester bond and a sugar–phosphate backbone.
Each strand has a 5′ end and a 3′ end. The two DNA strands run in opposite directions — they are antiparallel.
Bases pair up in a fixed way: A with T (2 hydrogen bonds) and C with G (3 hydrogen bonds).
The whole thing twists into a double helix.
What a nucleic acid actually is
A nucleic acid is a long chain built from thousands of small identical-looking units, in the same way a necklace is built from beads. The unit is called a nucleotide, and a chain of them is called a polynucleotide.
There are two nucleic acids you need for SL:
DNA (deoxyribonucleic acid) — the long-term store of genetic information. In eukaryotes it sits mainly in the nucleus, with smaller amounts in mitochondria and chloroplasts.
RNA (ribonucleic acid) — the short-lived working copy used to actually build proteins. It is found in the nucleus and the cytoplasm.
Some viruses, such as the coronavirus that causes COVID-19, carry RNA instead of DNA. That is a favourite exam fact, because it shows the genetic material does not have to be DNA — it just has to be a nucleic acid.
The three parts of a nucleotide
Every single nucleotide, in every living thing, is built the same way. A pentose sugar sits in the middle. A phosphate group is attached to one side of it, and a nitrogenous base is attached to the other. Both are joined to the sugar by covalent bonds.
Notice the phosphate and the base are on opposite sides of the sugar. That is why nucleotides can only join in a line — the phosphate of one reaches the sugar of the next.
The word pentose just means the sugar has five carbon atoms. Those five carbons are numbered 1 to 5, and the numbers matter more than you would expect:
The base is attached at carbon 1.
The phosphate is attached at carbon 5.
Carbon 3 is the spot the next nucleotide will attach to.
Deoxyribose vs ribose in one line: ribose has an –OH group on carbon 2; deoxyribose has just an –H there. “Deoxy” literally means “missing an oxygen”. That is the whole difference, and it is enough to make DNA the more stable of the two.
The bases — and the two families they fall into
The sugar and the phosphate are the same in every nucleotide. The base is the only part that changes, so the base is the part that carries the information.
Family
Bases
Shape
Purines
Adenine (A), Guanine (G)
Bigger — two rings of atoms
Pyrimidines
Cytosine (C), Thymine (T, DNA only), Uracil (U, RNA only)
Smaller — one ring of atoms
A quick way to keep them apart: pyrimidine and the bases C, T and U are the “thin” ones with one ring. Purines are pure big — A and G, two rings each. Later this explains why a purine always pairs with a pyrimidine: one big plus one small always spans the same distance.
Joining nucleotides into a strand
To build a strand, the phosphate group of one nucleotide bonds to the sugar of the next one. This happens by a condensation reaction, which means a molecule of water is released every time a bond is made. The bond formed is called a phosphodiester bond.
Repeat that thousands of times and you get a long chain of alternating sugars and phosphates, with a base hanging off every sugar. That chain is the sugar–phosphate backbone.
Because the phosphate always joins at carbon 5 and the next sugar always joins at carbon 3, the strand can only grow one way round. That is what gives it a 5′ end and a 3′ end.
Building a strand
nucleotide + nucleotide → phosphodiester bond + H2O
Why the 5′ and 3′ labels matter
One end of a strand has a free phosphate on carbon 5 — that is the 5′ end. The other end has a free –OH on carbon 3 — the 3′ end. Strands are always written and read 5′ to 3′, so if an exam question hands you a sequence, check the labels before you answer.
Two strands, base pairing and the double helix
DNA is not one strand but two, lying side by side. Crucially, they run in opposite directions: one goes 5′ to 3′ down the page while its partner goes 3′ to 5′. This is what antiparallel means.
The two strands are held together by hydrogen bonds between the bases, which point inwards from each backbone. And the bases cannot pair up randomly — only two combinations fit:
A pairs with T, held by two hydrogen bonds.
C pairs with G, held by three hydrogen bonds.
This is complementary base pairing, and it is the single most useful fact on this page.
The green arrows show the direction of each strand. Notice one points down and the other points up — that is the detail examiners look for in a drawn answer.
Why only these two pairs? A and G are the big two-ring purines; C and T are the small one-ring pyrimidines. A big base must always face a small one, otherwise the two backbones would have to bulge apart or squeeze together. Pairing a purine with a pyrimidine keeps the ladder exactly the same width all the way down.
The double helix
Real DNA is not a flat ladder. The whole molecule twists into a double helix, a three-dimensional spiral shape. The backbones form the outside of the spiral and the base pairs are tucked inside, which protects them.
Good news for the exam: you are not usually asked to draw the helix. A flat ladder with antiparallel strands is normally accepted, and it is far easier to draw neatly under time pressure.
Drawing nucleic acids in the exam
🧩 How to draw DNA and pick up every mark
Use simple shapes. Circle with a P for phosphate, pentagon for the sugar, rectangle for the base. Neat beats artistic.
Draw big. A large diagram is easier for the examiner to read, and easier for you to label.
Solid lines for covalent bonds (inside the backbone) and dashed lines for hydrogen bonds (between the bases).
Include all four bases if the question asks for base pairing — A, T, C and G should all appear.
Flip the second strand upside down so the strands are clearly antiparallel, and label 5′ and 3′ at each end.
Check what is being asked. “Label a nucleotide” means circle a phosphate, a sugar and a base together — not just one part.
Worked examples
WORKED EXAMPLE
A section of DNA is 60 base pairs long. 24 of those pairs are C–G. Work out the total number of nucleotides, the number of sugars, and the total number of hydrogen bonds holding the section together.
Step 1: turn base pairs into nucleotides
Each pair has one nucleotide on each strand.
60 × 2 = 120 nucleotidesStep 2: count the sugarsevery nucleotide has exactly one sugar, so sugars = nucleotides120 sugars (and 120 phosphates)Step 3: split the pairsC–G pairs = 24, so A–T pairs = 60 − 24 = 36Step 4: count hydrogen bonds (3 per C–G, 2 per A–T)(24 × 3) + (36 × 2) = 72 + 72 = 144120 nucleotides, 120 sugars, 144 hydrogen bonds
WORKED EXAMPLE
One DNA strand reads 5’–A T G C C T A G–3′. Write out the complementary strand in the 5′ to 3′ direction.
Step 1: swap each base for its partner
A→T, T→A, C→G, G→C
A T G C C T A G gives T A C G G A T CStep 2: remember the strands are antiparallelthat partner strand runs 3′ to 5′ as written, so it must be reversed to be read 5′ to 3′Step 3: reverse itT A C G G A T C reversed is C T A G G C A T5’–C T A G G C A T–3′if the question doesn’t mention 5′ and 3′, writing TACGGATC is usually fine – but check
WORKED EXAMPLE
A student draws a nucleotide with a ribose sugar and the base thymine, and labels it “a DNA nucleotide”. Explain two things that are wrong.
Problem 1: the sugar
Ribose belongs to RNA. DNA nucleotides contain deoxyribose.
Problem 2: the base does not match the sugar
Thymine belongs to DNA. An RNA nucleotide would carry uracil instead.
So the drawing is a mix of the twoEither ribose + uracil (RNA), or deoxyribose + thymine (DNA)the sugar and the base have to come from the same molecule – they are not mix and match
💡 Exam tip
Learn the three parts of a nucleotide as a set: phosphate, pentose sugar, nitrogenous base. Questions often award one mark each.
If you are given the number of one base, you can find every other one, because A = T and C = G.
Covalent bonds hold a strand together; hydrogen bonds hold the two strands together. That single sentence answers a surprising number of questions.
Say “condensation reaction” and “phosphodiester bond” by name — vague answers like “they join up” score nothing.
Hydrogen bonds are individually weak, but there are millions of them, so the molecule as a whole is very stable. Both halves of that idea are worth marks.
In a drawn answer, always label the 5′ and 3′ ends and make the strands point opposite ways.
⚠ Common mix-up
Calling the whole strand a nucleotide. A nucleotide is one phosphate + one sugar + one base. The chain is a polynucleotide.
Saying hydrogen bonds join nucleotides in a strand. They do not — that job belongs to covalent phosphodiester bonds.
Mixing up “antiparallel” with “opposite bases”. Antiparallel is about the direction of the strands, not the base sequence.
Writing that RNA has no thymine but forgetting the sugar. RNA differs in two ways: uracil instead of thymine, and ribose instead of deoxyribose.
Drawing both strands the same way up. Examiners look for one strand flipped — it is an easy mark to lose.
Confusing the number of bases with the number of base pairs. 500 base pairs means 1000 bases in total.
Up next: The Basis of the Genetic Code — now that you know the bases sit in a fixed order along the backbone, we can look at how that order is read in threes to spell out a protein.
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