DNA Replication

Overview

DNA replication is the process of duplicating the DNA molecule. This would happen in the eukaryotic cell’s nucleus before the cell divides, whether by mitosis or meiosis. Prokaryotes replicate their DNA in the cytoplasm. Recall that, for eukaryotes, replication occurs during the S phase of Interphase.

Preview the process of DNA replication by watching the following short video. Pay close attention to how the DNA is copied. Do the two strands of DNA remain attached or break apart to create a copy?

The replication of DNA depends on one main idea: Nitrogen bases of the nucleotides are complementary to each other on opposite sides of the molecule. This pairing is due to the shapes of the nucleotide bases, which allow weak hydrogen bonds to form between the complementary bases, holding the two halves of the DNA molecule together.

DNA Replication is Semiconservative

Initially, scientists were unsure of how replication of DNA occurred and proposed three possible mechanisms. Based on experiments, we now know that DNA Replication is semiconservative.

Look at the diagram below. Three ideas were proposed to explain how DNA Replication occurs. What makes up each new daughter helix in semi-conservative replication?

DNAReplicationModels.png

DNA replication is called semi-conservative because the parent helix splits and each half of the parent helix serves as a template for a new strand. Therefore each daughter helix that results is a hybrid of new and old strands.  

Steps to DNA ReplicationDNADetailReplication.jpg

 

  1. The helicase enzymes unwind the parent DNA and then unzip it by breaking the hydrogen bonds between the complementary base pairs. This exposes the two halves of the DNA molecule and creates a replication fork, which moves down the duplicated DNA as helicase continues to unwind it.
  2. DNA polymerase enzymes attach free nucleotides to the open strands, following base pairing rules. On one strand, DNA polymerase can work continuously toward the replication fork. This is called the leading strand (top strand in the picture). On the other strand, DNA polymerase must work in the opposite direction in short bursts, creating fragments. This is called the lagging strand (bottom strand in the picture).
  3. Ligase enzymes (not shown) connect sugars to phosphates along the newly created backbones.
  4. Nuclease enzymes check for and removes the majority of base pairing mistakes, which could result in mutations. A mutation is any change in the DNA. DNA polymerase revisits those errors and adds the correct base.
  5. Replication occurs in multiple locations along a chromosome to speed the process along; these areas are called replication bubbles. The replication bubbles enlarge in both directions and join. Each new double helix consists of one old and one new strand, making this semi-conservative replication.

 

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