Molecular Biology Online Tutorials

What is Molecular Biology
Mendelian Genetics
Molecular Genetics
DNA Replication
DNA Replication Enzymes
DNA Replication Mechanism

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Initiation

In the initiation step, several key factors are recruited to an origin of replication. This origin of replication is unwound, and the partially unwound strands form a "replication bubble", with one replication fork on either end. Each group of enzymes at the replication fork moves away from the origin, unwinding and replicating the original DNA strands as they proceed.

These things are called the pre-replication complex and consists of the following:

Helicase unwinds and splits the DNA ahead of the fork

Single-strand binding proteins (SSB) bind to the separated DNA to prevent the strands from reforming the helix

Primase generates an RNA primer to be used in DNA replication

DNA holoenzyme or polymerase complex performs the actual replication

Replication

After the helicase unwinds the DNA, the SSBs are used to hold the DNA strands apart. RNA primase is them able to bind to the DNA and allow the polymerase complex to start replication. As already discussed, DNA polymerase can ONLY synthesize new DNA from the 5' to 3' direction. This isn't a problem for the leading strand, but because the two strands are anti-parallel, the lagging strand must be discontinuously replicated. Anti-parallel means that they run side by side, but in opposite directions. At the end of each replication, DNA ligase, is used to connect the Okazaki fragments.

Termination

Remember that the strands are anti-parallel, so when the polymerase reaches the end there is one more problem with replication. The RNA primer on the leading strand sits on a small portion of the DNA. This small portion is not copied because polymerase never comes into contact with it. If this little problem was never fixed there would be a gap at the end of the new DNA strand. Luckily, the 3' end sticks out and only has non-coding DNA on it, so it is just cut off. This non-coding end is called the telomere.

Before everything is finally done, the new DNA strand must be proofread for mistakes. The replication mechanism is highly accurate, but mistakes do happen. When found, they are immediately corrected by other enzymes.

Watch the following video that ties this section on molecular biology together

This video includes an animation of transcription and translation as well.

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What is Cell Biology
Cellular Components
The Endomembrane System
Golgi Apparatus
Vesicles and Cellular Transport
The Mitochondria
The Plasma Membrane
Plant Biology
Photosynthesis
Parts of a Plant
Plant Life Cycle 1
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Cell Cycle
Cell Cycle Mitosis
Cell Cycle Control