Structure of nucleic acids
Cambridge A-Level Biology (9700) · Unit 6: Nucleic acids and protein synthesis · 8 flashcards
Structure of nucleic acids is topic 6.1 in the Cambridge A-Level Biology (9700) syllabus , positioned in Unit 6 — Nucleic acids and protein synthesis , alongside Protein synthesis. In one line: A nucleotide consists of three components: a pentose sugar (deoxyribose in DNA, ribose in RNA), a phosphate group, and a nitrogenous base. These components are covalently bonded together.
Marked as AS Level: examined at AS Level in Paper 1 (Multiple Choice), Paper 2 (AS Structured Questions) and Paper 3 (Advanced Practical Skills). The same content may also be assumed in Paper 4 (A Level Structured Questions).
The deck below contains 8 flashcards — 5 definitions and 3 key concepts — covering the precise wording mark schemes reward. Use the 5 definition cards to lock down command-word answers (define, state), then move on to the concept and calculation cards to handle explain, describe, calculate and compare questions.
Describe the structure of a nucleotide, including its three components
A nucleotide consists of three components: a pentose sugar (deoxyribose in DNA, ribose in RNA), a phosphate group, and a nitrogenous base. These components are covalently bonded together.
What the Cambridge 9700 syllabus says
Official 2025-2027 spec · AS LevelThese are the exact learning outcomes Cambridge sets for this topic. The candidate is expected to be able to do each of these on the relevant paper.
- describe the structure of nucleotides, including the phosphorylated nucleotide ATP (structural formulae are not expected)
- state that the bases adenine and guanine are purines with a double ring structure, and that the bases cytosine, thymine and uracil are pyrimidines with a single ring structure (structural formulae for bases are not expected)
- describe the structure of a DNA molecule as a double helix, including: • the importance of complementary base pairing between the 5′ to 3′ strand and the 3′ to 5′ strand (antiparallel strands) • differences in hydrogen bonding between C–G and A–T base pairs • linking of nucleotides by phosphodiester bonds
- describe the semi-conservative replication of DNA during the S phase of the cell cycle, including: • the roles of DNA polymerase and DNA ligase (knowledge of other enzymes in DNA replication in cells and different types of DNA polymerase is not expected) • the differences between leading strand and lagging strand replication as a consequence of DNA polymerase adding nucleotides only in a 5′ to 3′ direction
- describe the structure of an RNA molecule, using the example of messenger RNA (mRNA)
Cambridge syllabus keywords to use in your answers
These are the official Cambridge 9700 terms tagged to this section. Mark schemes credit responses that use the exact term — weave them into your answers verbatim rather than paraphrasing.
Tips to avoid common mistakes in Structure of nucleic acids
- › Define a gene precisely as a sequence of nucleotides that codes for the amino acid sequence of a specific polypeptide.
- › Ensure the correct spelling of 'guanine', 'adenine', 'cytosine', 'thymine', and 'uracil' to gain marks in structured questions.
- › Use the exact term 'phosphodiester bond' for the covalent bond in the sugar-phosphate backbone and 'hydrogen bond' for base pairing.
- › Always multiply the number of base pairs by two to determine the total number of individual nucleotides or bases in a double-stranded DNA molecule.
- › Write out the full names of the bases: Adenine, Thymine, Guanine, Cytosine, and Uracil as required by the syllabus.
Describe the structure of a nucleotide, including its three components.
A nucleotide consists of three components: a pentose sugar (deoxyribose in DNA, ribose in RNA), a phosphate group, and a nitrogenous base. These components are covalently bonded together.
Distinguish between purines and pyrimidines, naming the bases that belong to each group.
Purines (adenine and guanine) have a double-ring structure, while pyrimidines (cytosine, thymine in DNA, and uracil in RNA) have a single-ring structure. This structural difference is crucial for proper base pairing in DNA and RNA.
Explain the importance of complementary base pairing in the structure of DNA, and how this relates to the antiparallel nature of the strands.
Complementary base pairing (A with T, and C with G) ensures accurate DNA replication and transcription. The antiparallel nature (5' to 3' strand paired with a 3' to 5' strand) allows for optimal hydrogen bond formation between the base pairs, stabilizing the double helix.
Compare the number of hydrogen bonds between C-G and A-T base pairs in DNA.
A-T base pairs are connected by two hydrogen bonds, while C-G base pairs are connected by three hydrogen bonds. The greater number of hydrogen bonds in C-G pairs contributes to the greater stability of DNA regions with high C-G content.
Describe the role of DNA polymerase in semi-conservative DNA replication.
DNA polymerase is the enzyme responsible for synthesizing new DNA strands by adding nucleotides complementary to the template strand. It can only add nucleotides to the 3' end of an existing strand, hence the 5' to 3' directionality.
Explain the difference between the leading and lagging strands during DNA replication.
The leading strand is synthesized continuously in the 5' to 3' direction towards the replication fork. The lagging strand is synthesized discontinuously in short fragments (Okazaki fragments) because DNA polymerase can only add nucleotides in the 5' to 3' direction, away from the replication fork.
What is the function of DNA ligase in DNA replication?
DNA ligase is an enzyme that joins the Okazaki fragments on the lagging strand during DNA replication, forming a continuous DNA strand. It catalyzes the formation of phosphodiester bonds between the fragments.
Describe the structure of an mRNA molecule.
mRNA is a single-stranded RNA molecule that carries genetic information from DNA to the ribosomes for protein synthesis. It contains a sequence of codons, each of which specifies a particular amino acid.
More topics in Unit 6 — Nucleic acids and protein synthesis
Structure of nucleic acids sits alongside these A-Level Biology decks in the same syllabus unit. Each uses the same spaced-repetition system, so progress in one informs the next.
Key terms covered in this Structure of nucleic acids deck
Every term below is defined in the flashcards above. Use the list as a quick recall test before your exam — if you can't define one of these in your own words, flip back to that card.
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