As helicase unwinds the double helix, DNA polymerase adds new strands of DNA complementary to each old strand.

DNA replication requires the enzymes DNA polymerase and primase (an enzyme is a protein molecule which speeds up a reaction), free nucleotide bases, and a template strand. After helicase unwinds DNA from its original position, DNA polymerase adds free nucleotide bases complementary to the single template strand. As shown in the picture, one of each old template strand is combined with a new strand formed by DNA polymerase to form The polymerase is actually stationary, and only the strand moves. DNA polymerase can only travel in one direction, so as shown in the picture on the lower strand, it must continuously back up and add nucleotides in fragments. Not shown in the picture is the enzyme DNA ligase, which connects the DNA fragments created by DNA polymerase on the lower strand.

Prior to sequencing, a technique called the Polymerase Chain Reaction (abbreviated PCR) is often used to amplify a purified sample of DNA, or make many copies of it. In PCR, a DNA double helix is heated until it separates (denaturation), and a special heat-resistant DNA polymerase repeatedly makes copies of the DNA strands, increasing the number of copies of the strands at an exponential rate. However, it must be noted that this method can have occasional errors and are limited to a certain number of copying cycles.  Several DNA sequencing methods use alternate methods which also rely on the copying ability of DNA polymerase. (8)

Do you think you have mastered DNA replication? Play a game to test your sequencing skills here.

In PCR, the double helix separates (1), the DNA polymerase attaches to each strand (2), and new copies of the old strands are made (3). The process is then repeated, leading to quick replication.

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