OCR · A-Level · Biology
Course version: H420 · Version from Sept 2025
(d) (i) the structure of DNA (deoxyribonucleic acid)
Full specification wording
Nucleotides and nucleic acids: (d) (i) the structure of DNA (deoxyribonucleic acid)
DNA is a polymer of nucleotides, and each nucleotide has three parts: a phosphate group, a deoxyribose sugar and a nitrogenous base.
Checked against the OCR H420 specification (opens in a new tab) on .
- Point 4 of 8
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
DNA is a polymer of nucleotides, and each nucleotide has three parts: a phosphate group, a deoxyribose sugar and a nitrogenous base. The bases are adenine, thymine, cytosine and guanine. Nucleotides join through the sugar–phosphate backbone, with phosphodiester bonds linking the phosphate of one nucleotide to the sugar of the next. Two strands run in opposite directions (antiparallel) and wind into a double helix. The bases on opposite strands pair by complementary base pairing: adenine pairs with thymine (two hydrogen bonds) and cytosine pairs with guanine (three hydrogen bonds). The sugar–phosphate backbone lies on the outside and the paired bases on the inside. Recognise that deoxyribose, not ribose, is the sugar, and that thymine replaces uracil found in RNA.
What earns the marks
- DNA is a polymer of nucleotides, each made of a phosphate group, a deoxyribose sugar and a nitrogenous base.
- The four bases are adenine, thymine, cytosine and guanine.
- Nucleotides are joined by phosphodiester bonds forming a sugar–phosphate backbone.
- DNA consists of two antiparallel strands forming a double helix.
- Complementary base pairing occurs: adenine pairs with thymine and cytosine pairs with guanine.
- Adenine–thymine pairs have two hydrogen bonds; cytosine–guanine pairs have three.
Common mistakes and how to put them right
Naming ribose as the sugar in DNA.
Put it right: DNA contains deoxyribose.
Including uracil among DNA bases.
Put it right: DNA contains thymine instead of uracil.
Saying the strands are parallel.
Put it right: The two strands are antiparallel, running in opposite directions.
Placing the bases on the outside of the helix.
Put it right: The sugar–phosphate backbone is on the outside and the bases are on the inside.
Mismatching base pairs. Correction: adenine pairs with thymine and cytosine pairs with guanine.
Exam tips
- Learn the three components of a DNA nucleotide and the four bases.
- Use complementary base pairing rules to work out the partner base on the opposite strand.
- Remember the hydrogen-bond counts: two for A–T and three for C–G.
- Describe the double helix as two antiparallel strands with a sugar–phosphate backbone outside and bases inside.
Try the lesson
The start of the Nucleotides and nucleic acids: (d) (i) the structure of DNA (deoxyribonucleic acid) lesson, made from checked teaching notes for this specification point.
Have a go first
Which pentose sugar is found in a DNA nucleotide?
Listen
DNA is a polymer of nucleotides, and each nucleotide has three parts: a phosphate group, a deoxyribose sugar and a nitrogenous base. The bases are adenine, thymine, cytosine and guanine. Nucleotides join through the sugar–phosphate backbone, with phosphodiester bonds linking the phosphate of one nucleotide to the sugar of the next. Two strands run in opposite directions (antiparallel) and wind into a double helix. The bases on opposite strands pair by complementary base pairing: adenine pairs with thymine (two hydrogen bonds) and cytosine pairs with guanine (three hydrogen bonds). The sugar–phosphate backbone lies on the outside and the paired bases on the inside. Recognise that deoxyribose, not ribose, is the sugar, and that thymine replaces uracil found in RNA. A strong answer earns marks for these points. DNA is a polymer of nucleotides, each made of a phosphate group, a deoxyribose sugar and a nitrogenous base. The four bases are adenine, thymine, cytosine and guanine. Nucleotides are joined by phosphodiester bonds forming a sugar–phosphate backbone. DNA consists of two antiparallel strands forming a double helix. A common slip: Naming ribose as the sugar in DNA. Instead, DNA contains deoxyribose. Exam tip: Learn the three components of a DNA nucleotide and the four bases.
See it
Key words
Say what each word means before you turn the card.
Nucleotide structure
Try answering aloud or in your head before turning the card.
DNA double helix
Try answering aloud or in your head before turning the card.
Complementary base pairing
Try answering aloud or in your head before turning the card.
Phosphodiester bonds and backbone
Try answering aloud or in your head before turning the card.
See every definition
- Nucleotide structure
- Each nucleotide is made of a phosphate group, a deoxyribose sugar and a nitrogenous base.
- DNA double helix
- Two antiparallel strands run in opposite directions and wind into a double helix, with the sugar–phosphate backbone on the outside and the paired bases on the inside.
- Complementary base pairing
- Adenine pairs with thymine and cytosine pairs with guanine, with two hydrogen bonds for adenine–thymine and three for cytosine–guanine.
- Phosphodiester bonds and backbone
- Nucleotides join through the sugar–phosphate backbone, with phosphodiester bonds linking the phosphate of one nucleotide to the sugar of the next.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
There is more to this lesson
The full lesson goes on to check your understanding and help you earn the marks. Create a free account to carry on, with your progress saved.
A week of Premium from your first lesson · Then the free plan continues · No card, no charge
Test your understanding of Nucleotides and nucleic acids: (d) (i) the structure of DNA (deoxyribonucleic acid)
Active recall beats re-reading. Take an instant quiz now to test what you’ve learned and lock it into long-term memory.
A week of Premium from your first lesson · Then the free plan continues · No card, no charge