Mini_Prep_Gels_Summary - Mini Prep Gels SummaryWhat are...

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Mini Prep Gels Summary- What are plasmids? Super coiled/ relaxed/ linear states: Plasmids are extrachromosomal, double stranded circular DNA molecules generally containing 1,000 to 100,000 base pairs. Even the largest one will be considerably smaller than the chromosomal DNA of the bacterium. Certain plasmids are known to replicate independently of the chromosomal DNA and can therefore be present in hundreds of copies per cell. FIG-1 Relaxed / Supercoiled DNA Plasmid DNA naturally exists as a super-coiled molecule. Supercoiling arises from alterations in the winding of the two DNA strands around each other. In certain areas of the molecules, the DNA stands are wound around each other less frequently than in non- super coiled DNA. The strain caused by these alterations creates deformations in the DNA. These deformations relieve the strain and ultimately lead to super coiling (see fig on left). Supercoiled DNA is folded onto itself and has more condensed and entangled structure than the same DNA, which is relaxed. Again if you can not understand this think of DNA as a rubber band during its relaxed form, and then when you have the rubber band twisted, it eventually will become knotted and collapse onto itself as an entangled ball, this would then be the supercoiled state of that plasmid. Purified DNA must be covalently closed circle to exist as a supercoiled molecule. Super coiled plasmid DNA is often called Form I DNA. Supercoiling in the cell is caused by the action of enzyme called DNA gyrases. These enzymes use chemical energy in ATP to introduce supercoiling into a relaxed molecule. In addition, there are enzymes that relax supercoiled DNA and are called unwinding or relaxing enzymes. Super coiling has important biological consequences. Very large DNA molecules (for example if there is a
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This note was uploaded on 05/18/2008 for the course BIO G 104 taught by Professor Chen,k.c during the Spring '06 term at Cornell University (Engineering School).

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Mini_Prep_Gels_Summary - Mini Prep Gels SummaryWhat are...

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