DNA and the genome & DNA structure (biology only) — AQA GCSE Biology

SPEC 4.6.1.4 4.6.1.5 PAPER 2

DNA & The Genome & DNA Structure

The chemical of life. Polymer, double helix, nucleotides, bases. What every gene is made of — and how the order of four letters spells out every protein in your body.

Why This Matters

Mark schemes for DNA questions are strict on terminology. The 4-mark "describe a nucleotide" question appears repeatedly — and the 1-mark "why is DNA a polymer" expects a single phrase: "repeating nucleotide units". The mark schemes are explicit about what wording scores and what doesn't.

2

Spec sub-topics covered

4

Bases (A, C, G, T)

3

Bases code 1 amino acid

Why This Lesson

Module 6 is built on this content. Every later lesson — inheritance, genetic disorders, mutations, evolution — assumes you can use DNA, gene, chromosome, and genome precisely. Get this lesson right and the rest of Module 6 is much easier.

Learning Objectives

By the end of this lesson, you'll be able to:

Describe the structure of DNA — polymer, two strands, double helix — and define genome. (Spec 4.6.1.4)

Discuss the importance of understanding the human genome — three spec uses. (Spec 4.6.1.4)

Describe a nucleotide — sugar, phosphate, base — and explain how a sequence of three bases codes for one amino acid. (Spec 4.6.1.5, biology only)

Spec Note

4.6.1.5 is biology only — Combined Science students don't do it. AQA also states: "Students should be able to interpret a diagram of DNA structure but will not be required to reproduce it." Focus on reading the diagram, not drawing it.

Key Definitions (1)

Two terms students confuse constantly. The mark scheme is strict on the distinction.

Definition

DNA (deoxyribonucleic acid)

The chemical that makes up the genetic material in the nucleus of a cell. DNA is a polymer made up of two strands forming a double helix.

Definition

Chromosome

A structure made of DNA that carries the genetic material in the nucleus. Each chromosome contains a long DNA molecule, and humans have 23 pairs (46 total) in each body cell.

Mark Scheme Phrase (Nov 2020 P2 Q4.1)

"Why is DNA described as a polymer?" 1 mark. Answer: "many joined nucleotides" or "a long molecule made of repeating units". The word "repeating" is what scores the mark.

Key Definitions (2)

Definition

Gene

A small section of DNA on a chromosome. Each gene codes for a particular sequence of amino acids, which makes a specific protein.

Definition

Genome

The entire genetic material of an organism — every chromosome, every gene, every base.

Mark Scheme Trap

For "genome", the mark scheme requires the word "entire" (or all / whole / complete) genetic material. Saying "genome is DNA" or "genome is the genetic material" misses the mark — the answer has to capture all of it. One missing word, one lost mark.

The Hierarchy — Zooming In

Four terms at four different scales. Students lose marks by mixing the levels. From biggest to smallest:

The Hierarchy

Cell contains a nucleus → nucleus contains chromosomes → each chromosome is made of a long DNA molecule → a gene is a small section of that DNA. Use the precise term at each scale.

Where DNA Is Found

DNA is in different places depending on cell type. Mark schemes test this in synoptic Module 1 + Module 6 questions.

Eukaryotic Cells

(animal, plant, fungal cells)

DNA is in the nucleus.

Packaged into chromosomes.

Human body cells contain 23 pairs of chromosomes.

Prokaryotic Cells

(bacteria)

No nucleus.

DNA is a single circular loop in the cytoplasm.

May also have small extra rings of DNA called plasmids.

Synoptic Link (Module 1)

This connects to Lesson 1.01 (Eukaryotes vs Prokaryotes). The Module 1 question "Where is DNA found in a bacterium?" gets "cytoplasm" as the answer — not "nucleus", because bacteria don't have one.

The Human Genome — Why It Matters

The entire human genome has been sequenced (the order of all 3 billion bases has been read). The spec lists exactly three uses — learn them word-for-word.

Spec 4.6.1.4 — Three Uses (Memorise)

Search for genes linked to different types of disease.

Understanding and treatment of inherited disorders.

Tracing human migration patterns from the past.

"The whole human genome has now been studied and this will have great importance for medicine in the future." — AQA Spec 4.6.1.4

Mark Scheme Trap

Students often write generic answers like "to find out about DNA" or "to cure diseases". These score zero. The mark scheme wants the specific three uses: disease genes, inherited disorders, migration. Anything else — even if true — is off-spec.

Nucleotide Structure Biology Only

DNA is a polymer made from four different nucleotides. A nucleotide is the repeating unit of DNA.

Three Components Of A Nucleotide

A phosphate group

A sugar (deoxyribose / pentose — 5-sided)

One of four bases attached to the sugar

Why "Four Different" Nucleotides?

The spec says DNA is made from four different nucleotides. The phosphate and sugar are the same in every nucleotide — the only difference is which of the four bases (A, C, G, or T) is attached. So there are exactly 4 nucleotide types, one for each base.

Examiner Alert (MS Jun 2021 P2 Q1.2)

The June 2021 mark scheme for identifying the sugar in DNA explicitly says: "allow deoxyribose / pentose; do NOT accept ribose". Ribose is the sugar in RNA, not DNA. One wrong vowel loses one mark.

The Four Bases Biology Only

DNA contains four different bases. Each nucleotide has one of these four attached to its sugar. The full names and letter codes:

LetterFull NameMark Scheme Note

AAdenineDo NOT write "adenosine"

CCytosine—

GGuanine—

TThymineDo NOT write "thiamine" (a vitamin!)

Memorise — The Letter Codes Alone Are Enough

The spec accepts the letters A, C, G, T in mark schemes. You don't need the full names unless the question asks for them. The letters alone are enough to score the mark.

Spelling Trap

The Nov 2020 mark scheme explicitly says: "do not accept thiamine / adenosine". Thiamine is vitamin B1. Adenosine is a different molecule. The correct spellings are thymine and adenine.

DNA Strand Structure Biology Only

The long strands of DNA have alternating sugar and phosphate sections forming the "backbone". One of the four bases is attached to each sugar.

Key Features

The backbone is made of alternating sugar and phosphate sections.

One base is attached to each sugar.

DNA has two strands — they twist together into a double helix.

The bases face each other in the middle (between the two backbones).

The Genetic Code Biology Only

The four bases (A, C, G, T) don't act alone. They work in groups of three. Every group of three is a code for one specific amino acid.

The Rule (Spec 4.6.1.5)

A sequence of three bases is the code for a particular amino acid. The order of bases controls the order in which amino acids are assembled to produce a particular protein.

From Bases To Protein Biology Only

The logic chain that connects DNA at one end to a working protein at the other. The mark scheme rewards every link.

DNA Order of BASES

Code Every 3 bases

= 1 amino acid

Protein Order of AMINO ACIDS

Result Specific PROTEIN

The Chain Of Logic

The order of bases in DNA determines...

...the order of amino acids assembled (because every 3 bases = 1 amino acid)...

...which determines the specific protein produced.

Forward Link (Lesson 6.03 — HT only)

HT students will go deeper next lesson: how the order of amino acids determines the protein's shape, and why a mutation (change in base order) can change the protein's shape and stop it working.

Examiner Alerts

Three traps from real mark schemes and examiner reports.

Alert 1 — DNA vs Gene vs Chromosome

Mark schemes consistently distinguish these terms: DNA = the chemical; gene = a section of DNA on a chromosome; chromosome = a structure made of DNA. A chromosome is made of DNA — it's not "a container that holds DNA". Use the precise term required by the question.

Alert 2 — "Genome" definition wording

Mark schemes for the "define genome" question require the word "entire" (or all / whole / complete) genetic material. Saying "genome is DNA" misses the mark — it has to be ALL the DNA. The word "entire" is what scores.

Alert 3 — Sugar spelling (MS Jun 2021 P2 Q1.2)

The June 2021 mark scheme for identifying the sugar in DNA explicitly says: "allow deoxyribose / pentose; do NOT accept ribose". Ribose is in RNA, not DNA. One wrong vowel loses one mark.

PEE Worked Example

Question: Describe how nucleotides are arranged to form a DNA molecule. 4 marks

P

Point 1: Each nucleotide is made of three parts: a phosphate group, a sugar (deoxyribose), and one of four bases (A, C, G, or T).

P

Point 2: Nucleotides are joined together so that the sugars and phosphates form a backbone — an alternating sugar-phosphate-sugar-phosphate chain.

P

Point 3: One base is attached to each sugar in the backbone.

E

Evidence: DNA is a polymer of repeating nucleotide units, and the molecule has two strands that twist together into a double helix.

Why This Scores 4/4

Four distinct mark-scheme points: (1) nucleotide components named (phosphate, sugar, base); (2) alternating sugar-phosphate backbone; (3) base attached to each sugar; (4) polymer / two strands / double helix. This question tests both the nucleotide structure AND the strand arrangement — broader than just "describe a nucleotide".

Practice Question

A scientist analyses a short section of DNA and finds it contains 27 bases.

(a) Name the three components that make up a single DNA nucleotide. 3 marks

(b) Calculate the maximum number of amino acids this section of DNA could code for. Explain your answer. 2 marks

(a)

Three components: (1) a phosphate group; (2) a sugar (deoxyribose / pentose); (3) one of four bases (A, C, G, or T). 1 mark per correctly named component.

(b)

9 amino acids. A sequence of three bases codes for one amino acid, so 27 ÷ 3 = 9. 1 mark for the calculation method (or the rule "3 bases = 1 amino acid"), 1 mark for the correct answer (9).

Mark scheme (5 marks):

•

(a) Phosphate (1), Sugar / deoxyribose / pentose (1), Base / one of 4 bases / A C G T (1)

•

(b) Recognises 3 bases code for 1 amino acid (1)

•

(b) 27 ÷ 3 = 9 amino acids (1)

Recap — 3 Takeaways

The Hierarchy

DNA = the chemical (polymer, double helix). Gene = a section of DNA that codes for one protein. Chromosome = made of DNA, contains many genes. Genome = ALL the genetic material.

Nucleotide = 3 Parts

Every nucleotide = phosphate + sugar + base. Four bases: A, C, G, T. The backbone is alternating sugar-phosphate, with one base attached to each sugar.

3 Bases = 1 Amino Acid

The order of bases in a gene codes for the order of amino acids in a protein. Every 3 bases = 1 amino acid. Different protein = different base order.

One Mark-Scheme Trap To Avoid

For "genome", write "the entire genetic material of an organism". Skipping the word "entire" (or whole/all/complete) is the #1 way students lose this 1-mark question. The genome is everything — not just one chromosome, not just DNA in general.

NEXT: Lesson 6.03 — DNA structure: protein synthesis & mutations (HT only)