MOLECULAR GENETICS - DNA Replication
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Introduction

DNA replication can be easily conceived as the unzipping, and re-zipping of a coat when preparing to leave. Except, when the zipper is unzipped, to other zipper halves attach to the unzipped parts to form 2 full zippers. Think about this as we move into replication. Intricate material for testing will be the function of all the enzymes at work. The basic process is essential, constant knowledge.

Type of Replication

It is important to understand DNA's method of replication and how it was determined. Scientists, Meselson and Stahl performed special DNA centrifugation experiments to determine DNA's style of replication. The details of Meselson and Stahl's experiment is not extremely important, but the application of it is!

DNA's replication style is known to be semi-conservative, or 'half and half' replication. This entails that when 1 DNA double helix forms 2 DNA helices, half of the original helix is in each of the new helices. To put it another way, when the DNA unzips half of one side moves away and a complementary strand is built for it as is the other half.

Method of Replication

Replication begins with an enzyme called topoisomerase. This enzyme makes cuts in the supercoil level of the chromosome to relieve tension in the supercoil and allow it to unwind through the cuts since DNA can only occur when the lowest structural level of the chromosome is free for the process to take place, and enzymes to work. After it makes a cut and the supercoil partially unravels, it reseals the cut it has made. Gyrase another enzymes has a similar function and makes two cuts in the supercoil.

Once the coil unwinds sufficiently to allow DNA to be less obstructed, proteins called Single Stranded Binding Proteins (SSBPs) which bind to the DNA helix. Another word for binding is annealing, the complementary base binding with hydrogen bond attraction. Next, DNA helicase (an enzyme) disrupts hydrogen bonding between the complementary base pairs of the DNA strands effectively separating the strands. This is when the SSBPs are of use, they are bonded to the bases and keep them apart during replication. During this unzipping, DNA gyrase (a specialized enzyme) is constantly making and resealing cuts in the DNA to relieve tension brought about the unwinding of DNA.

As DNA is being separated another enzyme primase, must lay down what are called RNA primers. These are complementary triplet nucleotides, and act as the starting points for replication. Then an enzyme called DNA polymerase 3 uses the base rules to read each base and produce the complementary base (starting from each RNA prime). It does this until it forms the entire complementary base. As the DNA is being unzipped, it must be zipped because there is not enough room in the cell for the entire DNA to be unwound.

Strands and Direction

DNA is synthesized in the 5' to 3' direction. If you do not remember these directions return to DNA structure, and review 'Carbon Direction'. The complementary strand that is being formed is called the leading strand, since it is being formed in the same direction in which DNA helicase is disrupting hydrogen bonds between the DNA. The template strand of DNA, the strand being copied always runs antiparallel (away from the direction of replication) from 3' to 5'. Its strand is fragmented since it can only be copied once helicase has unzipped it, and must be copied rapidly as it is being unzipped. This fragmented strand is called the lagging strand. The fragments which are replicated are called the Okazaki fragments.



Once the replication is complete and the new strands are formed, two enzymes remove the original RNA primers (and replaces them with the appropriate nucleotides) and join the breaks in the Okazaki fragments. It does this by reinforcing and catalyzing the phosphodiester bonds (refer to DNA structure section) in the sugar phosphate backbone.

Conclusion

Replication is prokaryotes (what is described) can be complex at first, but with constant reading a strong comprehension of the material is not far away. Ensure you understand the differences between replication and transcription (in the following) section.

For a interactive aid in comprehension visit the interactive page on the toolbar below.