Instrumentation 6

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Microscopy is the study of objects or samples that are too small to be seen by the naked eye. There are several types of microscopy, each with its own advantages and limitations. Here are the main types of microscopy: 1. Optical microscopy: This is the most common type of microscopy, which uses visible light to illuminate a sample. Optical microscopy can be further divided into several subtypes, such as brightfield, darkfield, phase contrast, fluorescence, and confocal microscopy. Optical microscopy is a technique that uses visible light to observe the sample under a microscope. It consists of several components, including an objective lens, an eyepiece lens, and a light source. The working of optical microscopy involves the following steps. The sample to be viewed is prepared by fixing it onto a glass slide and adding a stain or dye to enhance its contrast. The light source, located beneath the sample, emits light that is directed through the condenser lens to focus the light o...
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quick revision [12th chapter 6/topic The Meselson-Stahl experiment proof]

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for easy to recall.....
The Meselson-Stahl experiment, conducted by Matthew Meselson and Franklin Stahl in 1958, provided strong evidence for the semi-conservative model of DNA replication, which proposed that each strand of the original DNA molecule serves as a template for the synthesis of a new complementary strand.

 

In their experiment, Meselson and Stahl grew E. coli bacteria in a medium containing a heavy isotope of nitrogen, 15N, for several generations until all the DNA in the bacteria incorporated this heavy isotope. They then transferred the bacteria to a medium containing a lighter isotope of nitrogen, 14N, and took samples of the bacteria at different time points after the transfer.

 


The DNA from each sample was extracted and centrifuged to separate the DNA molecules based on their density. The heavier 15N DNA settled at the bottom of the centrifuge tube, while the lighter 14N DNA formed a band higher up in the tube.

 

After one generation of replication in the 14N medium, the DNA from the bacteria formed a single band of intermediate density, indicating that each DNA molecule consisted of one heavy strand and one newly synthesized light strand. This supported the semi-conservative model of replication.

 

After two generations of replication in the 14N medium, the DNA formed two distinct bands: one at the intermediate density and one at the light density. This showed that DNA replication is semi-conservative, and not conservative or dispersive.

 

The Meselson-Stahl experiment is considered a classic and elegant demonstration of DNA replication and helped to establish the semi-conservative model as the most widely accepted mechanism for DNA replication.



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