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In this lesson, we will learn why DNA replicates as well as how.

We’ll also learn about the rates of replication and how DNA proofreads itself. After watching the lesson, you can complete a short quiz to test your knowledge.

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How Does DNA Replicate?

DNA (deoxyribose nucleic acid) carries all the genetic information needed to re-create itself and to pass on the characteristics of the organism. When a cell reproduces, it needs to pass all of this genetic information on to the new cells. Before reproduction can take place, a cell must replicate, or copy, its DNA.

The structure of the DNA molecule makes replication easy. Each side of the double helix run in opposite directions – one up, one down. This gives the structure the ability to unzip down the middle, and each side serves as a template for the other side.

During actual replication, the helix doesn’t unzip completely. A smaller area, called a replication fork, is unzipped instead.

Steps of Replication

What are the steps of replication?

  1. An enzyme, DNA gyrase, separates each side of the double helix.

  2. Another enzyme, helicase, unwinds the double helix.
  3. Several small proteins temporarily bind to each side and keep the sides separated.
  4. An enzyme complex, DNA polymerase, walks down the DNA strands and adds new nucleotides to each strand. The nucleotides pair up: adenine (A) with thymine (T) and guanine (G) with cytosine (C).
  5. A subunit of the DNA polymerase proofreads the new DNA.
  6. An enzyme, DNA ligase, seals up the fragments into long continuous strands.

  7. The new copies automatically wind up again.

Rates of Replication

Different types of cells replicate at different rates. Some, like those in hair, fingernails, and bone marrow, divide constantly.

Others, like those in the heart, muscles, and brain, go through several rounds of replication and cell division and then stop. Some others, such as skin and liver cells, stop replicating and dividing but will start again to repair injuries. For cells that do not constantly divide, the cues to tell them to divide come in the form of chemicals, such as hormones, or from the environment.


The replication of DNA needs to be perfect to preserve genetic information. Occasional mistakes do happen and then a DNA polymerase enzyme proofreads the paired nucleotides. When a mismatch is found – for example, adenine is paired with cytosine – the incorrect nucleotide is removed and replaced.Some mistakes will remain. If the cell has a lot of mistakes, and the DNA is too damaged for the enzymes to fix it, the cell either stops dividing or it self-destructs. Still other mistakes in replication will remain and cause changes in the genetic information carried by the DNA.

Lesson Summary

Let’s sum up.

Before reproduction can take place, a cell must replicate its DNA. The double helix separates, unwinds, adds new nucleotides, seals up into two new long strands, and the new copies wind themselves up into a new double helix.Some cells replicate and divide constantly, like hair, fingernails, and bone marrow. The cells in the heart, muscles, and brain go through several rounds of replication and division, then stop.

Others stop replicating and dividing but will start up again to repair injuries.Mistakes in replication do occur. Some can be repaired, others cause the cell to self-destruct, while still others cause changes in the genetic information carried by the DNA.

Learning Outcomes

After you are finished with this lesson you should be able to:

  • Recall how DNA replicates itself
  • Explain the DNA replication process
  • Explain what happens when there are mistakes in the replication process

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