DNA Structure | Biology

📖 In-Depth Theory

The Structure of DNA

DNA (deoxyribonucleic acid) is the molecule that carries genetic information in all living organisms.

DNA has a DOUBLE HELIX structure — two strands twisted around each other like a twisted ladder.

Each strand is made of NUCLEOTIDES. Each nucleotide contains:

1. A SUGAR molecule (deoxyribose)

2. A PHOSPHATE group

3. One of four BASES: Adenine (A), Thymine (T), Guanine (G), Cytosine (C)

The two strands are held together by WEAK HYDROGEN BONDS between the bases.

COMPLEMENTARY BASE PAIRING:

Bases always pair in the same way:

A pairs with T (Adenine with Thymine)

G pairs with C (Guanine with Cytosine)

This means if you know one strand's sequence, you can work out the other strand.

This complementary pairing is essential for accurate DNA replication.

Genes and the Genetic Code

A GENE is a sequence of bases on a DNA molecule that codes for a specific protein.

The base sequence acts as a CODE — it determines the sequence of amino acids that make up a protein.

Each group of three bases (a CODON) codes for a specific amino acid.

The order of amino acids determines the protein's shape → which determines its function.

With 4 bases and 3-base codons: 4³ = 64 possible codons — more than enough for 20 amino acids.

NON-CODING DNA:

Large sections of DNA between genes do not code for proteins.

Some non-coding DNA regulates when genes are switched on or off.

PROTEIN SYNTHESIS:

Genes are first TRANSCRIBED to make mRNA (messenger RNA).

mRNA travels out of the nucleus to ribosomes.

At ribosomes, the mRNA is TRANSLATED — amino acids are assembled in the correct sequence.

The chain of amino acids folds into a specific protein shape.

DNA Replication

Before cell division, DNA must be REPLICATED (copied) so each daughter cell gets a full set.

SEMI-CONSERVATIVE REPLICATION:

1. The double helix UNWINDS and the two strands SEPARATE (hydrogen bonds break).

2. Each original strand acts as a TEMPLATE.

3. FREE NUCLEOTIDES from the nucleus pair up with complementary bases on each template strand.

4. New hydrogen bonds form between paired bases.

5. Two new identical double helices are formed — each with one original strand + one new strand.

Why semi-conservative?

Each new DNA molecule keeps ONE original strand and one newly made strand.

'Semi' = half original.

This process is highly accurate but errors (mutations) can occasionally occur.

MUTATIONS: changes in the base sequence → may alter the protein produced → may affect the organism.

⚠️ Common Mistake

A always pairs with T, and G always pairs with C — never A-G or T-C. This complementary base pairing is fixed. Students often mix up which bases pair together — use the memory: 'Apples in Trees, Cars in Garages' (A-T, C-G).

📌 Key Note

DNA: double helix, two strands of nucleotides (sugar + phosphate + base). Bases: A-T, C-G (complementary pairing). Gene = sequence of bases coding for a protein. 3 bases = 1 codon = 1 amino acid. Protein synthesis: transcription (DNA→mRNA) then translation (mRNA→protein). Replication: semi-conservative, each strand acts as template.

🎯 Matching Activity — Complementary Base Pairs

Match each base to its complementary partner. — drag the symbols on the right to match the component names on the left.

Adenine (A)

Drop here

Guanine (G)

Drop here

Template strand

Drop here

Codon

Drop here

Original DNA strand used as a blueprint during replication

Thymine (T) — always paired by two hydrogen bonds

Three consecutive bases that code for one amino acid

Cytosine (C) — always paired by three hydrogen bonds

⭐ Higher Tier Only

Explain semi-conservative replication in detail: double helix unwinds, each strand acts as template, free nucleotides pair by complementary base pairing (A-T, G-C), joined by DNA polymerase. Describe how mutations (substitution, insertion, deletion) alter the base sequence and may change the amino acid sequence and therefore the structure and function of the protein produced.

🔬 Triple Science Only

DNA structure detail (4.6.2.1) is biology-only — Combined Science covers DNA and the genome more broadly without the structural detail of the double helix, nucleotides, base pairing or the mechanism of DNA replication.

🎯 Test Yourself

Question 1 of 2

1. A section of DNA has the base sequence ATCGTA. What is the complementary strand sequence?

2. During DNA replication, why does each daughter cell end up with one original and one new DNA strand?

⭐ How Well Do You Understand This Topic?

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DNA and the Genome

📋 All Inheritance, Variation and Evolution subtopics

Genetic Inheritance