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

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DNA Replication

DNA replication is a complicated, but fairly well understood mechanism that begins the process of protein synthesis. Before we continue on with the general features of DNA replication, let us first consider what the goal of a gene is. In other words - what do genes do and how do they work?

Ggenes actually perform more than one function:

1. They replicate themselves faithfully
2. They direct RNA transcription and result in protein sysnthesis
3. They mutate and evolve an organism

General Replication Mechanism

DNA replication actually has several parts that will be discussed in detail later, but first you must fully understand the general mechanism.

DNA replication is semi-conservative

DNA replication is semi-discontinuous

DNA replication is bi-directional

What is the Difference Between Conservative and Semi-conservative Replication?

In the diagram below the left side represents the "parent genes" and the right side represents the "daughters".

Semi-conservative replication gives two daughter duplex DNAs. Each of those daughters contain one parent strand (blue) and one daughter strand (red)

Conservative replication gives two daughter duplex DNAs. One duplex contains ALL parental DNA (blue) and one duplex contains all daughter DNA (red)

Dispersive replication gives two daughter duplexes that contain a mixture of parent and daughter DNA

DNA Replication is also Semi-discontinuous

Picture yourself driving on a freeway. People like the freeway because there are no stop lights or stop signs. It can be called a continuous road. There are other roads, called highways, that are also pretty cool becasue while they do have some stop lights, there aren't as many as on a side street. A highway could be called semi-discontinuous becasue you might have to make a few stops, but generally speaking you can go farther faster because the stops are fewer. A typical city street then, woud be discontinuous. There are stop lights at every block and there is no way aroudn them. It takes many stops and starts to get where you are going.

DNA replication is semi-discontinuous because one strand is continuous and one strand is discontinuous. The continuous strand reads the DNA in only one direction, from 3' to 5' and this strand is called the leading strand.

The other strand, called the lagging strand, also goes from 3' to 5', HOWEVER, because the two strands are opposites, the lagging strand must start replication not on the end, like the leading strand, but in the replication fork. The result is that the lagging strand is made is segments. These segements are called Okazaki fragments.

Watch the video below for further clarification:

DNA Replication Requires Primers

In order for DNA replication, or synthesis, to begin, it must be given the proper signal. That signal is DNA primer. The primer creates a place for the replication enzyme, DNA polymerase, to grab a hold of. DNA polymerase is the enzyme responsible for synthesis. It turns out that the primer is not DNA itself, but a short piece of RNA.

In this process a replication fork opens, then short RNA sequences (primers) are made. Next, the DNA polymerase adds nucleotides to the primers, and DNA is formed.

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