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Lagging Strand of DNA: Definition & Synthesis

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Instructor: Katie Chamberlain

Katie has a PhD in Microbiology and has experience preparing online education content in Biology and Earth Science.

A lagging strand is the name for one of the two DNA strands in a double helix that is undergoing replication. This lesson will explain which strand is lagging and how it is replicated.

Lagging Strand: Definition

A lagging strand is one of two strands of DNA found at the replication fork, or junction, in the double helix; the other strand is called the leading strand. A lagging strand requires a slight delay before undergoing replication, and it must undergo replication discontinuously in small fragments.

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  • 0:01 Lagging Strand: Definition
  • 0:25 Understanding DNA
  • 1:13 DNA Replication
  • 2:30 Replication Bubble
  • 3:24 Lagging Strands: Replication
  • 4:17 Lesson Summary
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Understanding DNA

DNA, or deoxyribonucleic acid, is a complex molecule, typically found in a cell's nucleus, that contains an organism's genetic information. The two strands of DNA form a double helix. The strands are complementary because they contain matching base pairs of the four chemicals found in DNA.

One end of a DNA strand is not the same as the other. One end contains a 5' phosphate group, and the other contains a 3' hydroxyl group. Biologists refer to the two ends of a DNA strand as 5' and 3'. In the double helix structure, the two strands point in opposite directions, with the 5' end of one strand paired with the 3' end of the other strand and vice versa.

DNA Replication

DNA replication begins when the DNA double helix unwinds. The open region containing the separated DNA strands is called the replication bubble. Imagine a replication bubble where the top strand has its 5' end on the left and its 3' end on its right. The bottom strand will have an opposite orientation, with 3' on the left side and 5' on the right side.

DNA polymerase is the enzyme that builds the new DNA strands. It reads the base on the original strand and then adds the corresponding base pair to the new strand. DNA polymerase can only add bases to the 3' end of a DNA chain. This means the new strand can only be built by adding bases to the 3' side of the growing chain.

In order to build the new strand in the 5'-to-3' direction, the polymerase must read the original strand in the 3'-to-5' direction. This is because the newly built strand must be complementary and in the opposite orientation to the original strand. During replication, the DNA polymerase will move from right to left on the top strand and from left to right on the bottom strand.

Replication Bubble

Let's stop at this point and take a look at our replication bubble.

Each side of a replication bubble is called a replication fork because the DNA helix splits into two strands, like a fork. The polymerase on the top strand moves to the left, and as it reaches the replication fork, the DNA magically unwinds ahead of it so it can proceed without stopping. The strands that can replicate continuously are called the leading strands.

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