By the end of this lesson
- Describe DNA as a nucleotide polymer.
- Apply complementary base-pairing rules.
- Relate DNA structure to information storage.
BIG QUESTION
How does DNA structure allow both stable information storage and accurate copying?
A strong sugar-phosphate framework protects a sequence that can be copied through complementary pairing.
DNA is built from nucleotides
Each DNA nucleotideThe unit of DNA, made of a phosphate, a sugar (deoxyribose) and one of four bases (A, T, C or G). contains deoxyribose sugar, phosphate and one base: adenine, thymine, cytosine or guanine.
Nucleotides join into a sugar-phosphate backbone; base sequence carries genetic information.
Two strands form a double helix
Two antiparallel polynucleotide strands twist into a double helixThe shape of a DNA molecule: two strands twisted around each other, held together by complementary base pairs.. Bases face inward and pair through hydrogen bonds.
Adenine pairs with thymine, and cytosine pairs with guanine. This complementary base pairingThe rule that bases in DNA pair in a fixed way, A with T and C with G, held by hydrogen bonds. makes one strand’s sequence predictable from the other.
Structure supports storage and copying
The covalent backbone provides stability while hydrogen bonds can be separated during replication and transcription.
Changes in base sequence are mutations and may alter a gene product; they do not always alter phenotype.
DNA structural components
Each component has a distinct role.
| Component | Structure | Role |
|---|---|---|
| Sugar-phosphate backbone | Covalently linked chain. | Stable outer framework. |
| Base sequence | Order of A, T, C and G. | Stores genetic information. |
| A–T pair | Complementary bases. | Links opposite strands. |
| C–G pair | Complementary bases. | Links opposite strands. |
| Double helix | Two twisted antiparallel strands. | Compact, copyable information store. |
Language that earns precision
- nucleotide
- Sugar, phosphate and nitrogenous base unit of DNA or RNA.
- double helix
- Twisted structure formed by two DNA strands.
- complementary base pairing
- Specific pairing of A with T and C with G in DNA.
- antiparallel
- Running in opposite chemical directions.
Common misconceptions
Claim: DNA contains amino acids.
Correction: DNA contains nucleotides; proteins contain amino acids.
Claim: A can pair with any base.
Correction: In ordinary DNA, A pairs with T and C with G.
Claim: Hydrogen bonds form the sugar-phosphate backbone.
Correction: Strong covalent bonds form the backbone; hydrogen bonds join paired bases.
Match the answer to the command word
Apply A–T and C–G one position at a time.
Two strands, double helix, backbone and complementary bases.
Recall, apply, then justify
Describe the structure of DNA.
Hint
Nucleotides, strands, helix and pairing.
Suggested marking points
- DNA is a polymer of nucleotides.
- It has two polynucleotide strands.
- The strands form a double helix.
- A pairs with T and C pairs with G.
Write the complementary DNA sequence to ACGTTA.
Hint
Pair each base in order.
Suggested marking points
- The complementary sequence is TGCAAT.
- Each A is paired with T and each C with G.
Explain why a high proportion of C–G pairs can increase the temperature needed to separate DNA strands.
Planning hint
Compare hydrogen bonds per base pair.
What a strong design includes
- C–G base pairs form three hydrogen bonds.
- A–T base pairs form two hydrogen bonds.
- A C–G-rich region has more hydrogen bonding for the same length.
- More thermal energy is needed to disrupt those interactions.
- Its strands therefore separate at a higher temperature.
- Base sequence and length also affect the precise melting temperature.