MEDICAL BIOLOGY, HUMAN ANATOMY, PHYSIOLOGY AND PATHOLOGY - Ya.I. Fedoniuk 2010

BIOLOGY

CHAPTER 1. BIOLOGICAL FOUNDATIONS OF HUMAN VITAL ACTIVITY

1.3. MOLECULAR-GENETIC AND CELLULAR LEVELS OF LIFE ORGANIZATION

1.3.3. The Hereditary Apparatus of Eukaryotic Cells and its Functioning: Molecular Level

Evidence for the Genetic Role of DNA: Transformation and Transduction

The fact that DNA (RNA in some Viruses) serves as the genetic material and carrier of hereditary information is a universally recognized tenet in modern genetics. However, this was not always the case. Until 1944, geneticists believed that chromosomal Proteins constituted the substance of heredity. This hypothesis was developed in the 1920s by M.K. Koltsov (1872–1940). The genetic role of DNA was ultimately established through studies of microorganisms involving the phenomena of transformation and transduction.

Transformation (Lat. transformatio - change, conversion) is The transfer of Genetic information from one bacterium to another via isolated DNA. The phenomenon of transformation was first discovered in 1928 by the English microbiologist F. Griffith, who worked with two strains of pneumococci (Streptococcus pneumoniae). One strain (S) possesses a polysaccharide capsule, grows on Agar as smooth colonies,

and is virulent (causes Pneumonia), whereas the other strain (R) lacks a capsule, forms rough colonies on agar, and is avirulent (does not cause disease). The ability of S-pneumococci to cause disease is associated with the presence of a capsule, which protects them from phagocytosis within the host Organism. In a series of experiments on white mice, Griffith obtained the following results:

1) injection of living non-capsulated pneumococci resulted in the mice surviving;

2) injection of living capsulated pneumococci resulted in the mice dying from pneumonia;

3) injection of heat-killed capsulated pneumococci resulted in the mice surviving;

4) injection of heat-killed capsulated pneumococci combined with living non-capsulated pneumococci resulted in the death of the majority of mice, and living capsulated pneumococci were isolated from their Blood.

The outcome of the final series of experiments was unexpected. It was concluded that a specific factor was released from the heat-killed capsulated pneumococci, which endowed the living non-capsulated pneumococci with The ability to synthesize a capsular polysaccharide and become virulent, and this new trait was heritable. Consequently, a conversion (transformation) of the avirulent R strain into the virulent S strain had occurred (R -> S).

The Chemical Nature of the transforming factor was identified in 1944 by the American geneticists O. Avery, C. MacLeod, and M. McCarty, who demonstrated that it was DNA. If the DNA of heat-killed capsulated pneumococci is degraded by the enzyme deoxyribonuclease, transformation does not take place. Conversely, even when purified DNA is diluted with 6·10⁸ parts of Water and added to a culture of non-capsulated pneumococci, R -> S transformation still occurs, and the non-capsulated pneumococci become virulent to mice.

Transformation does not occur in all Cells, but only in those capable of taking up DNA; such cells are termed competent. The Mechanism of transformation proceeds as follows: 1) a double-stranded DNA molecule from the donor bacterium is adsorbed onto The surface of the recipient Cell; 2) the recipient cell internalizes the donor DNA; 3) intracellular Nucleases convert the double-stranded donor DNA into single-stranded fragments; 4) the single-stranded DNA fragment integrates into the recipient chromosome; 5) the integrated donor Gene is expressed within The Cell, imparting new, heritable traits to the organism.

Transduction (Lat. transductio - transfer) is the transfer of DNA fragments from one bacterium to another mediated by temperate Bacteriophages. A temperate bacteriophage (p. 29), upon release from a bacterium, inadvertently packaging a fragment of the donor bacterial DNA and transfers it into a recipient bacterium. Transduction was first discovered in 1952 by N. Zinder and J. Lederberg in salmonellae. It is exceptionally widespread among Bacteria. Transduction not only corroborates the genetic role of DNA, but is also widely utilized in Introduction/32.html">Genetic Engineering and for mapping chromosome and gene structures.