DNA Structure
Build confidence in DNA Structure for GCSE Biology with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA, Edexcel and OCR. A separate (triple) biology topic on nucleotides, complementary base pairing (A–T and C–G) and how the order of bases codes for proteins.
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These worksheets and mark schemes are original, written for Virtus Academy and checked against the current AQA, Edexcel and OCR specifications. Every worksheet comes with a full mark scheme.
Topic overview
DNA is made of repeating units called nucleotides. This is a Triple-only topic.
Each nucleotide consists of a common sugar, a phosphate group, and one of four bases: A, C, G or T. The sugar and phosphate form the backbone of each strand, and the bases attach to the sugar.
The two strands are held together by the bases, which pair in a fixed way: A always pairs with T, and C always pairs with G. This complementary pairing is what allows DNA to be copied accurately. A sequence of three bases codes for one amino acid, and the order of bases therefore controls the order of amino acids in a protein.
Revision notes
Nucleotide structure
Each nucleotide has a sugar, a phosphate group and one of four bases: A, C, G or T.
The sugar and phosphate alternate to form the backbone of the strand, and a base attaches to each sugar.
Complementary base pairing
A always pairs with T, and C always pairs with G.
These pairs hold the two strands together. Because the pairing is fixed, one strand can act as a template for making an exact copy of the other.
The genetic code
A sequence of three bases codes for one amino acid.
The order of bases in a gene therefore determines the order of amino acids in the protein, and it is that order which gives each protein its particular shape and function.
Key points
- DNA is made of repeating nucleotides.
- Each nucleotide has a sugar, a phosphate and a base.
- The four bases are A, C, G and T.
- A pairs with T and C pairs with G.
- Three bases code for one amino acid.
- The base order determines the amino acid order.
Worked examples
Example 1
Name the three components of a DNA nucleotide. [3 marks]
Model answer
A sugar (1 mark), a phosphate group (1 mark) and a base (1 mark).
Example 2
A section of DNA has the base sequence ACGTA. Write the sequence on the complementary strand. [2 marks]
Model answer
TGCAT. Each A is paired with T and each T with A (1 mark), and each C is paired with G and each G with C (1 mark).
Example 3
Explain how the order of bases in a gene determines the protein produced. [3 marks]
Model answer
A sequence of three bases codes for one amino acid (1 mark). The order of bases therefore determines the order in which amino acids are joined together (1 mark), and that order determines the shape and function of the protein produced (1 mark).
Common mistakes
Pairing A with C.
A pairs only with T, and C pairs only with G.
Saying one base codes for one amino acid.
Three bases code for one amino acid.
Forgetting the phosphate in the nucleotide.
All three components are needed: sugar, phosphate and base.
Saying the bases form the backbone.
The sugar and phosphate form the backbone; the bases attach to it.
Exam tips
- Memorise A–T and C–G as fixed pairs.
- Remember three bases per amino acid.
- Name all three nucleotide components when asked.
- Remember this is a Triple-only topic.
Key terms
- Nucleotide
- The repeating unit of DNA, made of a sugar, phosphate and base.
- Complementary base pairing
- The fixed pairing of A with T and C with G.
- Base
- One of the four chemicals A, C, G or T in DNA.
- Genetic code
- The sequence of bases determining the amino acid order.
Related topics
Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.