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PinkMonkey Online Study Guide-Biology

CHAPTER 8:GENES AND MOLECULAR GENETICS

8.0 Introduction

Mendel (1865) was the first to put forward clearly the idea of hereditary units, and referred to them as ’factors’. These were later named genes. In 1903, Sutton and Bovery postulated the chromosomal theory of inheritance and stated that the genes were located in the chromosomes. This was subsequently confirmed by the works of Morgan and others and is now accepted universally. Thus, it was established that the chromosomes are the carriers of the genetic material. The chemical of which the genes are composed is called the genetic material.

Quest for the identification of the genetic material

The nucleic acids were discovered by Meischer in 1869. In later years, it was also established that chromosomes are made up of proteins (histones and protamines) and nucleic acids (DNA). Now, the question was, which of these actually represented the genetic material? Was it the proteins or the DNA? The process of identification of genetic material began in 1928 and was successfully concluded in 1952. It was conclusively established that it is the DNA that contains the genetic material and not the proteins in the chromosomes. This fact could be established only through some brilliantly conceived and designed novel experiments. The most notable among these are:


  1. Experiments of Griffith (1928): Through his experiments, Griffith discovered the phenomenon of transformation (a special type of genetic recombination) in which the non-virulent strain of the bacterium Diplococcus pneumoniae was transformed into the virulent strain. Griffith’s experiments started the process of the identification of the genetic material.

  2. Avery, Macleod and Mc Carty (1944) : repeated, elaborated and modified the transformation experiments of Griffith. They provided the first clear evidence the DNA is the genetic material and is responsible for the transformation in bacteria. They also gave evidence of the chemical nature of genes.

  3. Lederberg and Zinder (1952) : confirmed the identity of DNA as the genetic material through a bacteriophage (vector), and the subsequent incorporation of the segment in the recipient DNA thereby causing transformation.

  4. Hershey and Chase (1952) : confirmed that at least in the bacterial virus called T2-phage, DNA is the only genetic material. This fact has been universally accepted since then.

  5. Frankel - Conrai and Singer (1957) : established that in some viruses, RNA functions as the genetic material (and not the DNA).

Table 8.1: Some additional historical events

Year

Scientists

Discovery/Event

1909

Johannsen

Coined the term "gene"

1924

Feulgen

Showed that chromosomes contain DNA

1953

Waston and Crick

Double stranded helical model for structure of DNA molecule

Table of Contents

8.0 Introduction
8.1 Packaging of Hereditary Material
8.2 The Structure of DNA
8.3 Replication Of DNA In Eukaryotes
8.4 Replicatin of Pokaryotic Chromosome
8.5 Plasmids
8.6 RNA: Structure and Types
8.7 The Genetic Code
8.8 The Central Theme of Protein Synthesis

Chapter 9





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