---
title: "DNA Structure and Base Pairing"
description: "DNA is two strands of nucleotides wound into a double helix, with A always pairing to T and C to G. That pairing rule is what makes an exact copy possible before a cell divides."
canonical: https://lightmysky.com/learn/science/dna-structure-and-base-pairing-mt_PK4uNJeg_k
source: https://lightmysky.com/learn/science/dna-structure-and-base-pairing-mt_PK4uNJeg_k.md
retrieved: 2026-09-12
---

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# DNA Structure and Base Pairing

DNA is two strands of nucleotides wound into a double helix, with A always pairing to T and C to G. That pairing rule is what makes an exact copy possible before a cell divides.

Subject: Science · Area: Genetics & Evolution · Ages 15 to 16
Page: https://lightmysky.com/learn/science/dna-structure-and-base-pairing-mt_PK4uNJeg_k

## Ready when they can

- Names the three parts of a nucleotide and states the pairing rule between the bases.
- Writes the complementary strand for a short given sequence.
- Explains how base pairing lets one strand act as a template for its own copy.

## Lesson: The twisted ladder that copies itself

DNA is two strands wound into a double helix, like a twisted ladder. Each strand is a chain of units called nucleotides. One nucleotide has three parts: a sugar, a phosphate group and a base. Four bases exist: A, T, C and G.

**Example.** The rungs obey a fixed pairing rule: A always pairs with T, and C always pairs with G. The sequence GATTACA pairs with CTAATGT, rung by rung. Only one answer is possible because each base has exactly one partner.

That rule makes copying automatic. The helix unzips and each old strand attracts matching free nucleotides, so two identical ladders grow from one. Enzymes proofread as they build and fix most slips. Each new ladder keeps one original strand.

**Tip.** Write a complement left to right, pairing each base alone: G with C, A with T, T with A, C with G. Then check every rung backwards before you commit.

**Recap.** Fixed pairing turns each strand into a template for its own exact copy.

## Practice

8 questions on this page, each with its working shown.

## Needs first

- [DNA & Genes](https://lightmysky.com/learn/science/dna-and-genes-mt_0sELh0MYWb)
- [Meiosis and Why Gametes Are Haploid](https://lightmysky.com/learn/science/meiosis-and-why-gametes-are-haploid-mt_hvu0bczcfX)

## Opens up

- [Chromatin: Nucleosomes and Higher-Order Packing](https://lightmysky.com/learn/science/chromatin-nucleosomes-and-higher-order-packing-mt_5q3Hsdqm9x)
- [Recombinant DNA and Gene Transfer](https://lightmysky.com/learn/science/recombinant-dna-and-gene-transfer-mt_L9yncU_x2P)
- [From Gene to Protein](https://lightmysky.com/learn/science/from-gene-to-protein-mt_PdNyzTl8Ye)
- [Semi-conservative DNA Replication](https://lightmysky.com/learn/science/semi-conservative-dna-replication-mt_wIcBfUw295)