Genetics

Name: Anthony A         October 27th, 2016

Biology 2416-81501        

Dr. Alejandro D’Brot

BIOL 2416 Homework #7 (Chapter 10and some 8 and 9)

1. Describe the context, experimental design, results, and conclusions of the Avery, MacLeod and McCarty experiment.

1. Background (previous knowledge):

1. Experimental Design:

Their experimental work was based on the noncapsulated pnemococcal strain transorming into capsulated pneumococcal strain causing deadly infection and killing mice (Griffith experiment). To prove the DNA as transforming agent they killed the bacteria by heat and extracted the bacterial components into saline suspension.

The bacterial protein was precipitated using chloroform. The polysaccharide capsules were hydrolysed by an enzyme. Immunological precipitation of capsule was acheived by specific antibodies. Complete destruction of capsule was taken place and active component of capsule was extracted by alcohol fractionation

1. Results:

 The chemical analysis of elements like carbon, hydrogen, phosphorous, nitrogen was like composition of DNA. To prove the transformation agent was DNA, Avery and his colleagues used different enzymes like trypsin, chymotrypsin and ribonuclease. These agents could not affect the active transforming portion. Then, they used deoxyribonucleodepolymerase enzyme preparation that could destroy the DNA or extract of transforming power.

1. Conclusions: The results have been proved that DNA or the genetic material undergoes transformation (from avirulent to virulent pneumococcal strain).

But the critics could not accept the results and posed challenge of the purification and crystallization of DNA destructing enzyme and Moses Kunitz in 1948, prepared DNA depolymerase (deoxyribonuclease I), and precise work by Rollin Hotchkiss showing that virtually all the detected nitrogen in the purified DNA came from glycine, a breakdown product of the nucleotide base adenine finally proved the work of Avery, MacLeod, and McCarty and success paved their way with huge number of citation of their scentific publication in various fields of biology.

1. Describe the context, experimental design, results, and conclusions of the Hershey-Chase experiment.

1. Context (previous knowledge): Alfred D. Hershey and Martha Chase (1952) performed several experiments that indicated that DNA was the genetic material in the T2 bacteriophage. Some luck was involved in their discovery, for the genetic material of many viruses is RNA and the researchers happened to select a DNA virus for their studies

1. Experimental Design: Hershey and Chase made the virus DNA radioactive with 32P or labeled the viral protein coat with 35S. They mixed radioactive bacteriophage with E. coli and incubated the mixture for a few minutes. The suspension was then agitated violently in a Waring blender to shear off any adsorbed bacteriophage particles
2. Results: After centrifugation, radioactivity in the supernatant and the bacterial pellet was determined. They found that
   most radioactive protein was released into the supernatant, whereas 32P DNA remained within the bacteria. Since genetic material was injected and T2 progeny were produced, DNA must have been carrying the genetic information for T2.

1. Conclusions: [pic 1]

1. Describe the context, experimental design, results, and conclusions of the Meselson-Stahl experiment.

1. context (previous knowledge): The Meselson and Stahl experiment was that they used the isotopes of nitrogen to determine the nature of DNA replication. They used the heavy isotope of nitrogen N 15 in the culture medium. After one round of replication, the samples were tested for the heavy isotope.They put out 3 models:

Conservative: In conservative mode of replication, after one round of replication two cells are formed. One cell is the parent cell with the normal N14 while the other cell contains N15 in both the strands of DNA.

Dispersive: In dispersive mode of replication, the theory was that the original parent strands and the two new strands contains parts of the original parent strand and parts of the new strand. This would give the strand a composition of part N14 and part N15.

Semi conservative: In this mode, each strand of the parent cell synthesizes a new complementary strand. This would give a cell with one parent strand and one new strand.