---
title: "Semi-conservative DNA Replication"
description: "Each new DNA molecule keeps one original strand and one newly built one. Helicase, DNA polymerase and the split into leading and lagging strands explain how base pairing gets copied."
canonical: https://lightmysky.com/learn/science/semi-conservative-dna-replication-mt_wIcBfUw295
source: https://lightmysky.com/learn/science/semi-conservative-dna-replication-mt_wIcBfUw295.md
retrieved: 2026-09-12
---

> **Agent view.** This is the Markdown twin of the page, for tools and assistants.
> When to use this site, and the call that answers each job: https://lightmysky.com/agent-instructions.md
> API description (OpenAPI 3.1): https://lightmysky.com/openapi.json · Authentication: https://lightmysky.com/auth.md
> Pricing: https://lightmysky.com/pricing.md · Catalog: https://lightmysky.com/llms.txt · Full catalog: https://lightmysky.com/llms-full.txt
> Every machine-readable file on this domain: https://lightmysky.com/.well-known/ai-catalog.json
> Ask for Markdown with `Accept: text/markdown`, a `.md` address, or `?mode=agent`.

# Semi-conservative DNA Replication

Each new DNA molecule keeps one original strand and one newly built one. Helicase, DNA polymerase and the split into leading and lagging strands explain how base pairing gets copied.

Subject: Science · Area: Genetics & Evolution · Ages 16 to 17
Page: https://lightmysky.com/learn/science/semi-conservative-dna-replication-mt_wIcBfUw295

## Ready when they can

- States what each new molecule inherits from the parent molecule.
- Names the enzymes that unwind and that build, and says what each one needs.
- Explains why one strand is built continuously and the other in fragments.

## Lesson: Half old strand, half new print

DNA replication is semi-conservative. You unzip the parent, keep one old strand in each daughter, and build the partner from free nucleotides. Each new molecule is half history and half new print. The Meselson Stahl experiment proved it: bacteria grown in heavy nitrogen shifted DNA density across generations exactly as half old, half new predicts, ruling out fully kept and fully mixed models.

Meet the enzyme crew and what each one demands. Helicase needs ATP to break the bonds and open the helix. Primase lays the short primer that starts each new stretch, because polymerase cannot start from nothing. DNA polymerase needs that primer plus nucleotides and proofreads as it goes. Ligase later stitches fragments into one unbroken strand.

**Example.** The two strands at each fork behave differently, and the cause is direction. Polymerase only adds bases one way, so the strand pointing the right way grows smoothly as the leading strand. The other must loop back and build in short Okazaki fragments, the lagging strand, which ligase joins together.

**Tip.** Assign each job and confusion ends: unwinding belongs to helicase, starting to primase, building to polymerase, and joining to ligase. Eukaryotes fire many origins along each long chromosome at once, and telomere tips need special handling. When a question asks what an enzyme needs, answer with its fuel.

**Recap.** Unzip, keep one old strand each, and build the partner with primed polymerase and ligase.

## Practice

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

## Needs first

- [DNA Structure and Base Pairing](https://lightmysky.com/learn/science/dna-structure-and-base-pairing-mt_PK4uNJeg_k)

## Opens up

- [Transcription and RNA Splicing in Eukaryotes](https://lightmysky.com/learn/science/transcription-and-rna-splicing-in-eukaryotes-mt_j2vc70vH5t)
- [PCR, Electrophoresis and DNA Profiling](https://lightmysky.com/learn/science/pcr-electrophoresis-and-dna-profiling-mt_Sv1jV0XnsX)
- [Replication Machinery: Origins, Forks and Telomeres](https://lightmysky.com/learn/science/replication-machinery-origins-forks-and-telomeres-mt_v95MJWeV_B)