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DNA An overview
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DNA An overview • Visit our website: http://www.sliderbase.com/ • Free PowerPoint Presentations for teaching and learning • The physical and functional unit of heredity that carries information from one generation to the next • DNA sequence necessary for the synthesis of a functional protein or RNA molecule. • Gene were first detected and analyzed by Mendel and subsequently by many other scientist (Mendel stated that physical traits are inherited as “particles”) • Mendel did not know that the “particles” were actually Chromosomes DNA • Subsequent studies shows the correlation between transmission of genes from one generation to generation (Segregation and independent assortment) and the behavior of chromosomes during sexual reproduction, specifically the reduction division of meiosis and fertilization. • These and related expt. provided a strong early evidence that genes are usually located on chromosomes. • What are the requirements to fulfill as a genetic material? • 1. The genotype function or replication: • The genetic material must be capable of storing genetic information and transmitting this information faithfully from parents to progeny, generation after generation. • 2. The phenotype function or gene expression • The genetic material must control the development of phenotype of the organism, be it a virus, a bacterium, a plant or animal. • That is, the genetic material must dictate the growth and differentiation of the organism from single celled zygote to the mature adult. • Chromosomes are composed of two types of large organic molecules (macromolecules) called proteins and nucleic acids. • The NA are of two types: DNA and RNA • For many years there was considerable disagreement among scientists as to which of these macromolecules carries genetic information. • During the 1940s and early 1950s, several elegant experiments were carried out that clearly shows that NA is genetic material rather than protein. • More specifically these expt. shows that DNA is genetic material for all living organism except for RNA viruses. • Griffith experiment • The phenomenon of transformation was first discovered by Frederick Griffith in 1928. • Pneumococci, like all other living organisms, exhibit genetic variability that can be exhibit with different phenotype • The two phenotypic characteristic of importance in Griffith experiment were: • 1. presence or absence of a surrounding polysaccharide capsule, and • 2. the type of capsule, that is, the specific molecular composition of the polysaccharide present in the capsules. • Bacteriophage T2 is composed of 50% protein and about 50% DNA. • Experiments prior to 1952 had shown that all bacteriophage T2 reproduction takes within E.coli cell. • Therefore, when Hershey and Chase showed that the DNA of the virus particle entered the cell, where as most of the protein of the virus remained absorbed to the outside cell. • This is strongly implied that the genetic information necessary for viral reproduction was present in DNA. • T2 phages labeled with S35 were mixed with E.coli cells for few minutes. • It was then subjected to shearing forces by placing infected cells in a Waring blender • It was found that most of the radioactivity could be removed from the cells without affecting progeny production. • When T2 phages labeled with P32, radioactivity was found inside the cells, that is, it was not subject to removal by shearing in a blender.
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