AQA · GCSE · Biology
Course version: 8461 · Version from Sept 2018
DNA is a polymer made up of two strands forming a double helix.
Full specification wording
DNA and the genome: DNA is a polymer made up of two strands forming a double helix.
DNA is a polymer, meaning it is a large molecule built from many repeating smaller units called nucleotides.
Checked against the AQA 8461 specification (opens in a new tab) on .
- Point 3 of 9
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
DNA is a polymer, meaning it is a large molecule built from many repeating smaller units called nucleotides. Each nucleotide has a sugar, a phosphate group and a base. In DNA, two strands of nucleotides run in opposite directions and are held together by complementary base pairing: adenine pairs with thymine, and cytosine pairs with guanine. The two strands twist around a shared axis to form a double helix, which looks like a twisted ladder. The sugar and phosphate groups form the uprights of the ladder, and the paired bases form the rungs. This structure allows the base sequence to store information and allows the molecule to be copied accurately.
What earns the marks
- DNA is a polymer built from repeating nucleotide units.
- Each nucleotide contains a sugar, a phosphate group and a base.
- Two strands of nucleotides make up the DNA molecule.
- The strands are held together by complementary base pairing.
- The two strands twist to form a double helix.
- The sugar-phosphate backbone forms the sides and the paired bases form the rungs of the helix.
Common mistakes and how to put them right
Saying that DNA is made of two separate molecules.
Put it right by stating that DNA is one molecule made of two strands.
Writing that the strands are joined by strong covalent bonds between all bases.
Put it right by stating that complementary bases pair by hydrogen bonds.
Describing the double helix as a single strand that is twisted.
Put it right by stating that two strands twist together.
Exam tips
- Use the word polymer and the phrase double helix in your answer.
- State the base pairing rule precisely: A with T and C with G.
- If asked to describe the structure, work from nucleotide to strand to double helix in a logical order.
Try the lesson
The start of the DNA and the genome: DNA is a polymer made up of two strands forming a double helix. lesson, made from checked teaching notes for this specification point.
Have a go first
Which shape describes the two strands of a DNA molecule?
Listen
DNA is a polymer, meaning it is a large molecule built from many repeating smaller units called nucleotides. Each nucleotide has a sugar, a phosphate group and a base. In DNA, two strands of nucleotides run in opposite directions and are held together by complementary base pairing: adenine pairs with thymine, and cytosine pairs with guanine. The two strands twist around a shared axis to form a double helix, which looks like a twisted ladder. The sugar and phosphate groups form the uprights of the ladder, and the paired bases form the rungs. This structure allows the base sequence to store information and allows the molecule to be copied accurately. A strong answer earns marks for these points. DNA is a polymer built from repeating nucleotide units. Each nucleotide contains a sugar, a phosphate group and a base. Two strands of nucleotides make up the DNA molecule. The strands are held together by complementary base pairing. A common slip: Saying that DNA is made of two separate molecules. Instead, stating that DNA is one molecule made of two strands. Exam tip: Use the word polymer and the phrase double helix in your answer.
Key words
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Biological polymers
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Complementary base pairing
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Double helix structure
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Storage of genetic information
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See every definition
- Biological polymers
- Large molecules built from many repeating smaller units, such as DNA, which is built from repeating nucleotide units.
- Complementary base pairing
- The pairing that holds the two strands together, where adenine pairs with thymine and cytosine pairs with guanine.
- Double helix structure
- The shape formed when the two strands twist around a shared axis, looking like a twisted ladder.
- Storage of genetic information
- The function allowed by the structure, in which the base sequence stores information and the molecule can be copied accurately.
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