Module 7 Molecular Genetics

Module 7—Genetics at the Molecular Level: DNA and RNA at Work

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In Module 1, you learned that when a cell divides to form 2 new cells, it must replicate, or make a copy of all of its DNA – the entire genome! In the last lesson you began learning 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 which Watson and Crick did notice. 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. This makes 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 that the enzyme helicase can bind to on the DNA. 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 figure 18.9 on page 631 to visualize these areas. The single strands in the replication bubble act as a template for creating the new copy strands of DNA (the pink strands in "Figure 18.9").

Elongation and Termination

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 must first be constructed by the enzyme primase before DNA polymerase can do its job. This is because DNA polymerase can only add nucleotides to an existing free 3’ hydroxyl end of a nucleotide chain. Once 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 18.3 on page 626. DNA polymerase then removes the RNA primer.

Think back to the structure of DNA. The two complimentary strands are joined together in the opposite directions. Take a look at "Figure 18.10" on page 632 and notice how one strand is in the 3’ to 5’ direction while the complimentary strand is in the 5’ to 3’ direction. Since DNA polymerase can only add nucleotides 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 together by an enzyme called DNA ligase.

Multiple primers are needed on the lagging strand. Eventually, DNA polymerase will remove the RNA primers and fill in the space to attach the neighbouring DNA strands. DNA polymerase is also responsible for proof reading as each nucleotide is added to the new strand. The process described above is very well illustrated in the animations below. DNA replication stops when the new completed DNA strands separate from one another. This is called termination. You should also read pages 629 to 632 of your textbook for more details.

You should make summary notes, a flow chart of events, or labeled diagrams to illustrate these processes. This would be important information to add to your course folder for review.

Watch and Listen

 Watch the following video to view a thorough overview of DNA replication. Begin this video at the section titled “DNA replication – part I”, and end at the “Mutation” section.

Self-Check

SC 2. What would the complementary RNA primer be if the DNA single strand had the following nucleotide sequence →   CGA