Download Print 1. Recall the Meselson and Weigle experiment that used two strain

Question

Download Print 1. Recall the Meselson and Weigle experiment that used two strains of bacteriophage with unique genetic marks ("A and B" or "a and b"). The location of these marks at the end of the chromosome and in close proximity to each other was important for them to get a robust experimental result (configuration 1 below) Imagine that these genetic marks were located in the two additional configurations listed below. Draw the distribution of recombinants (Ab and aB) in a CsCl gradient for all three configurations. A B Heavy 1 Light Heavy Light A B Light

Explanation / Answer

The experiments conducted by Matthew Meselson and Jean Weigle in 1961 gave evidence about chromosomal breakage and reunion event must be occured during recombination of chromosomes. They simultaniously infected bacteria with two strains of bacteriophases one was heavy which has c and mi markers located at the end of its chromosome and the other was light which has c+ and mi+ markers at the end of its chromosome. Heavy strains were obtained from the cells grown in heavy isotopes of 13 C and 15N. When cells were lysed and dencity gradient cesium density gradent gel electrophoresis was carried, a wide range of bands were obtained. The light and heavy parental value bands were considered nothing but parental strands. The recombinants were also resolved with closar density value with heavy parental strain. This was suggested that the recombination has taken place between the heavy and light strains of bacteria through the physical breakage and reunion between the two makers. For experimental convienience markers used were located at the end regions on the chromosome in close proximity with each other.

The dencity grdient tube have bands from the top to bottom as fallowed

1.at the top light parental bands

2.at the intermidiate region recombinants

3.at the end heavy parents.

In the present scenario the heavy strain is represented by A and B markers and light strain represented by a and b markers. irrespective of the position of the markers on the chromosome and distance between them for all three configarations include 1, 2 and three the distribution of the recombinants Ab and aB will be at the intermediate possition in density gradient cesium chloride gels.

The order of the bands obtained were fallows. From the top to bottom

1. at the top ab parents (light)

2. at the intermediate possition Ab and aB recombinants

3. at the bottom AB parents (heavy)

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