DNA as the Molecule Responsible for Heredity
- In the 1860s, Friedrich Miescher a physician by profession was the first person to isolate phosphate-rich chemicals from white blood cells or leukocytes. He named these chemicals (which would eventually be known as RNA and DNA) nuclein because they were isolated from the nuclei of the cells.
- The research at the end of the 19th century had verified Mendelian inheritance and it was also believed that the genetic material is in the chromosome. However, scientists still didn’t know the true features of the genetic material.
- In the early twentieth century, biologists believed that proteins carried genetic information.
- But the Griffith experiment with Streptococcus pneumoniae (1928), Avery, MacLeod and McCarty experiment (1944) on transforming principle and Hershey–Chase experiment (1952) on bacteriophage T2, confirms that DNA is genetic material.
What is genetic material?
Genetic material is the material that determines the inherited characteristics of a functional organism. It has the following properties:
- It must be stable
- It must be capable of being expressed when needed
- It must be capable of accurate replication
- It must be transmitted from parent to progeny without change
Classic experiments: DNA as the genetic material
1. Frederick Griffith: Bacterial transformation
- The transformation phenomenon was first observed in 1928 by Frederick Griffith, an English physician.
- Frederick Griffith was the first person to show that hereditary information could be transferred from one cell to another “horizontally,” rather than by descent.
- Transformation has now been shown to occur in Haemophilus influenzae, Bacillus subtilis, Shigella paradysenteriae, and Escherichia coli, among many other microorganisms.
- Transformation of numerous genetic traits other than colony morphology has also been demonstrated, including traits involving resistance to antibiotics.
- These observations further strengthened the belief that transformation by DNA is primarily a genetic event rather than simply a physiological change.
After Griffith’s experiment (In 1928) the question still remained: What is the nature of the transforming principle?
Link for Griffith’s experiment: http://easylifescienceworld.com/griffiths-experiment-discovery-of-transforming-principle/
Griffith’s work led other physicians and bacteriologists to research the phenomenon of transformation.
Did this transformation of the bacteria depend on something that happened in the mouse’s body? No.
- In 1931, M. Henry Dawson and his coworkers showed that transformation could occur in vitro (in a test tube). When heat-killed IIIS cells were incubated with living IIR cells, living IIIS cells were recovered. Therefore, injection into mice was not necessary for transformation to occur.
- Years later, another group of scientists discovered that a cell-free extract of heat-killed S cells also transformed R cells. (A cell free extract contains all the contents of ruptured cells, but no intact cells.)
This result demonstrated that some chemical transforming principle from the dead S.pneumoniae S strain could cause a heritable change in the affected R cells. This was an extraordinary discovery.
- Treatment with a substance permanently changed an inherited characteristic.
Question remained: What is the chemical nature of the transforming substance?
2. Avery, McCarty, and MacLeod: Identifying the nature of transforming principle
In 1944 Avery, MacLeod, and McCarty demonstrated that the transforming principle is DNA, providing the first evidence that DNA is the genetic material.
Transformation is the uptake of a free DNA fragment from the surrounding environment and the expression of the genetic information in the recipient cell.
That means by integration of the DNA fragment, the recipient (transformant) has gained some ability it previously lacked. Today, transformation represents a gene transfer process building genetic diversity.
3. Hershey Chase experiment: Viral replication experiments confirm that DNA is the genetic material