OCR · A-Level · Biology
Course version: H420 · Version from Sept 2025
(a) the structure of a nucleotide as the monomer from which nucleic acids…
Full specification wording
Nucleotides and nucleic acids: (a) the structure of a nucleotide as the monomer from which nucleic acids are made
A nucleotide is the repeating monomer that polymerises into nucleic acids.
Checked against the OCR H420 specification (opens in a new tab) on .
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
A nucleotide is the repeating monomer that polymerises into nucleic acids. Teach the three-part structure: a pentose sugar (deoxyribose in DNA, ribose in RNA), a phosphate group and a nitrogenous base. The base is either a purine (adenine, guanine) or a pyrimidine (cytosine, thymine in DNA, uracil in RNA). The sugar and base join by a glycosidic bond to form a nucleoside; adding phosphate gives the nucleotide. In DNA the sugar is deoxyribose and bases are A, T, C, G; in RNA the sugar is ribose and uracil replaces thymine. Recognise that the phosphate and sugar form the backbone while bases project inward. A common MCQ asks which component differs between DNA and RNA nucleotides, or which bond links sugar to base.
What earns the marks
- A nucleotide has three components: a pentose sugar, a phosphate group and a nitrogenous base.
- DNA nucleotides contain deoxyribose; RNA nucleotides contain ribose.
- Purine bases are adenine and guanine; pyrimidine bases are cytosine, thymine (DNA) and uracil (RNA).
- The sugar and base are joined by a glycosidic bond, forming a nucleoside; the phosphate is attached to the sugar.
- Nucleotides are the monomers that polymerise to form polynucleotides (nucleic acids).
Common mistakes and how to put them right
Confusing nucleotide with nucleoside.
Put it right by stating a nucleotide has sugar, base and phosphate.
Claiming uracil occurs in DNA.
Put it right: Uracil is found in RNA, replacing thymine; DNA uses thymine.
Stating the base joins the phosphate directly.
Put it right: The base bonds to the sugar, and the phosphate also attaches to the sugar.
Exam tips
- Learn the three components and be ready to identify them on a diagram.
- Memorise which sugar and which bases distinguish DNA from RNA nucleotides.
- Practise naming the bond between sugar and base (glycosidic) and between nucleotides (phosphodiester).
Try the lesson
The start of the Nucleotides and nucleic acids: (a) the structure of a nucleotide as the monomer from which nucleic acids are made lesson, made from checked teaching notes for this specification point.
Have a go first
Which nitrogenous bases found in DNA are classified as purines?
Listen
A nucleotide is the repeating monomer that polymerises into nucleic acids. Teach the three-part structure: a pentose sugar (deoxyribose in DNA, ribose in RNA), a phosphate group and a nitrogenous base. The base is either a purine (adenine, guanine) or a pyrimidine (cytosine, thymine in DNA, uracil in RNA). The sugar and base join by a glycosidic bond to form a nucleoside; adding phosphate gives the nucleotide. In DNA the sugar is deoxyribose and bases are A, T, C, G; in RNA the sugar is ribose and uracil replaces thymine. Recognise that the phosphate and sugar form the backbone while bases project inward. A common MCQ asks which component differs between DNA and RNA nucleotides, or which bond links sugar to base. A strong answer earns marks for these points. A nucleotide has three components: a pentose sugar, a phosphate group and a nitrogenous base. DNA nucleotides contain deoxyribose; RNA nucleotides contain ribose. Purine bases are adenine and guanine; pyrimidine bases are cytosine, thymine (DNA) and uracil (RNA). The sugar and base are joined by a glycosidic bond, forming a nucleoside; the phosphate is attached to the sugar. A common slip: Confusing nucleotide with nucleoside: a nucleoside lacks the phosphate group. Instead, stating a nucleotide has sugar, base and phosphate. Exam tip: Learn the three components and be ready to identify them on a diagram.
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Key words
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Monomer structure
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Pentose sugars
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Nitrogenous bases
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DNA versus RNA nucleotides
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See every definition
- Monomer structure
- A nucleotide has three components: a pentose sugar, a phosphate group and a nitrogenous base.
- Pentose sugars
- DNA nucleotides contain deoxyribose; RNA nucleotides contain ribose.
- Nitrogenous bases
- Purine bases are adenine and guanine; pyrimidine bases are cytosine, thymine (DNA) and uracil (RNA).
- DNA versus RNA nucleotides
- In DNA the sugar is deoxyribose and bases are A, T, C, G; in RNA the sugar is ribose and uracil replaces thymine.
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