Module 7 Lesson 2 - 2

DNA Replication

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In the previous module, you learned that, when a cell divides to form two new cells, it must replicate, or make a copy of all of its DNA, also known as the genome. In the previous lesson, you began to learn about this DNA replication method by studying the structure of the DNA molecule. The double stranded structure of DNA leads to a special copying mechanism that Watson and Crick noticed.

When a cell creates copies of its DNA by way of DNA replication, the new molecules of DNA each contain one of the original strands of the DNA. In other words, the double stranded DNA is separated and complementary nucleotides are assembled onto each strand. The two new daughter DNA have one original strand and one newly assembled strand. This makes the DNA replication a semi-conservative process.

Examine figure 18.8 on page 631 to see how one strand of the original "blue" DNA is found in each of the new copies of DNA.

Initiation of DNA Replication

DNA replication begins at the replication origin, a specific nucleotide sequence on the DNA to which the enzyme helicase can bind. The helicase enzyme cuts (cleaves) the DNA and unravels part of the double helix. The oval-shaped area created by the unwound double helix is called the replication bubble, and at each end of this oval is a Y-shaped area called a replication fork. View the figures above and below to visualize these areas. The single strands in the replication bubble act as a template for the new copy strands of DNA.

Elongation 

Elongation of the new DNA strand occurs when the enzyme DNA polymerase adds nucleotides to the template strands inside the replication bubble. An RNA primer first must be constructed by the enzyme primase before DNA polymerase can do its task. This is because DNA polymerase can add nucleotides to only an existing free 3' hydroxyl end of a nucleotide chain. When the primer is in place, DNA polymerase is able to attach a nucleotide to the free 3' hydroxyl end of the primer. You can see the -OH (hydroxyl group) on carbon 3' in the figure above. Then, DNA polymerase removes the RNA primer.

Okazaki Fragments

Think of the structure of DNA. The two complementary strands are joined in opposite directions. Consider the figure above and notice how one strand is in the 3' to 5' direction while the complementary strand is in the 5' to 3' direction. Because DNA polymerase can add nucleotides only in the 5' to 3' direction, only one strand can be added to continuously. This strand is called the leading strand.

The other strand of DNA is called the lagging strand and must be replicated in short segments called Okazaki fragments. These fragments are spliced or glued together by an enzyme called DNA ligase. Multiple primers are needed on the lagging strand. Eventually, DNA polymerase removes the RNA primers and fills the space to attach the neighbouring DNA strands.

Termination

DNA polymerase also is responsible for proofreading as each nucleotide is added to the new strand. The process described above is very well illustrated in the animations on the next page. DNA replication stops when the newly completed DNA strands separate from one another. This is called termination. Read pages 629 to 632 of your textbook, and look at the figure below for more details.