DNA Replication Fork: Definition & Overview

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  • 0:00 What is the Replication Fork?
  • 0:59 Function of the…
  • 2:35 Lesson Summary
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Lesson Transcript
Instructor: Adrianne Baron

Adrianne has taught high school and college biology and has a master's degree in cancer biology.

DNA defines an individual and is shaped like a spiral staircase. But how is DNA replicated if it is so twisted? Learn what a DNA replication fork is and its importance in this lesson.

What Is the Replication Fork?

Our DNA determines everything about us. And for this reason, a copy of our DNA is needed in every cell of our body, with the exception of our red blood cells.

A copy of the DNA is made just prior to when a cell is going to split to create two cells. In order for this replication of DNA to take place, the DNA has to be in an orientation that will allow the replication machinery to make a copy. Our DNA is double-stranded, with the strands being held together by hydrogen bonds. The normal structure of our DNA when it is not being copied is a double helix. This looks very similar to a winding staircase.

In this normal form, the DNA cannot be copied. DNA helicase is needed in order to open the DNA to expose the nucleotide bases that are used as the template for replicating the DNA. The area of the DNA that is opened by DNA helicase is known as the replication fork because it looks very similar to a fork in the road.

The Function of the Replication Fork

The replication fork is the area where the replication of DNA will actually take place. There are two strands of DNA that are exposed once the double helix is opened. One strand is referred to as the leading strand, and the other strand is referred to as the lagging strand. The leading strand is exposed in the 5' - 3' direction, while the lagging strand is exposed in the 3' - 5' direction. DNA is always copied in the 5' - 3' direction.

As the leading strand is exposed, DNA polymerase will use the leading strand as a template to create a continuous complementary strand of DNA. As the lagging strand is exposed, RNA primers are needed in order to start the replication process. The RNA primer will attach to the most 5' end of the exposed portion of the lagging strand. This primer then allows DNA polymerase to attach and add the complementary strand to the lagging strand in small segments known as Okazaki fragments.

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