In the start, people want to know that the control the characteristics of individuals.
Aristotle gives the pangenesis theory, many theories were given some of them are as follows:
Blending theory of Inheritance:
Hereditary material is a liquid which comes for both parents and when the liquid is mixed up an intermediate is obtained so-called blending theory of inheritance.
When we cross black and white a skin an intermediate color is obtained.
Characteristics are mix and cannot be separated.
Particulate theory of Inheritance:
In 1866, Mendal proposed a particulate theory as heritance materials are solid or disaster particles which brought together in an offspring and segregate after fertilization.
His approach was slightly different so he concluded such result as the selected correct plant like pea plant and different characteristics were studied such as cross & pollination, Quantitative which he selected were easier to study.
On the basis of his observation, he gave two Laws:
- Law of Segregation.
- Law of Independent Assortment.
Chromosomal Theory of Inheritance:
In 1950, the chromosomal, theory of inheritance is discovered that “Genes, the unit of inheritance are found in the chromosome, chromosomes act as carriers of hereditary. “ Discovery of chromosomes is a work of German embryologist, Fleming, which means colored bodies.
- In 1910, I.H Morgan while on drosophila which respect to gene linkage claimed that Mendal law of Independent assortment is not applicable.
- Mendal never observed linkage because he studies one gene on one allele or studied seven characteristics which were present on seven different chromosomes initial people did not accept the particulate theory.
Discovery of Nuclein:
In 1869, a germen Fredrick Meischer isolate nuclein Solid bandaged which contain pus cell, to prove that it is universal hereditary material. He said that the compound is acidic in nature and contain phosphorous because on burning specific phosphorous like smell appear. It is soluble in water and precipitate in alcohol + Salt solution.
- A lot of experiments was carried out and the microscope invented After microscope people start seeing the cell, and cell theory were given.
According to cell theory, the cell is the basic unit of structure in all organism, and also the basic unit of reproduction,
In biology cell theory is a scientific theory which describes the property of the cell, in an organism, the division of the cell into two daughter cell occur with the same genetic material during their material, chromosome was observed, Chromosomes are made up of DNA and Protein.
DNA consist of 4 Nucleotide, Each nucleotide is composed of Sugar, phosphate, nitrogenous base.
Sugar and phosphate are same except base which is different in each nucleotide, scientists thought that its simple cannot be inherited or hereditary material.
Protein is made up of 20 amino acid, so scientist thought that it is more complex and maybe the proteins are hereditary material.
- Griffith Experiment:
In 1928, A Microbiologist, Griffith describes a transforming principle. He used Streptocpcaus Pneumoniae to infect mice. He studied two types of strains of this pneumonia.
One of the strain has a Polysaccharides capsule. Its colony appears as smooth or shiny and hence it is called S-TYPE.
The other strain from a rough colony due to the absence of polysaccharides capsule and are reformed as R-TYPE.
- When no injection healthy mice with a strain of S-TYPE all the mice died because mice immune system can’t kill them due to their polysaccharides conquering, so there are called Virulent.
- He injected the strain of R-TYPE to the similar healthy mice showed no ill effect because it is killed by mice immune system due to lack of capsule and is appeared non-Virulent.
- As a control experiment, He injected heat-killed S-TYPE into another group of mice to see if the polysaccharide capsule itself had a virulent effect. The mice remained perfectly healthy.
- As a Final control, he blended living R-TYPE with heat-killed S-TYPE and injected into mice the injected mice died and Blood of mice was found to contain a large number of living S-TYPE Virulent Bacteria.
Griffith concludes that information specifying the polysaccharide capsule and virulence had passed from the head killed S-TYPE bacteria to living R-TYPE. “Transfer of genetic material from one organism to another, by which genetic makeup of recipient is altered, is called Transformation.”
- In Vitro, when R-TYPE heat killed S-TYPE is mixed and grow in a petri dish we got a lot of R-TYPE strain and a few S-TYPE strain because in vivo R-TYPE is killed by Mice Immune system and only live S-TYPE extract or present or isolated. But in vitro R-TYPE strain is in abundance in culture media.
In 1944, Avery (Student of Griffith) works on transforming material.
They took an extract of R-TYPE live cells and two types of the extract of S-TYPES. One in aqueous solution and the rest in an organic solvent.
- Extract of R-TYPE live cells + aqueous extract of heat-killed S-TYPE a Host died a Live S-TYPE cell isolated.
- Extract of R-Type live cells organic extract of meat killed S-TYPE a Host Survive a No live S-TYPE cell isolated.
Avery concludes that transforming material is a chemical compound.
And this chemical compound is soluble in water but not in the organic solvent. Protein is soluble in the organic solvent which means protein is not transformed. Avery when adding proteases to 2nd experiment, He got a result in which R-TYPE is changed into S-TYPE. When he adds nucleases, R-TYPE is not changed into S-TYPE.
He concludes that transforming material is not protein because if transforming material is protein iron. They must be degraded in the solution containing proteases and no transforming will occur.
Hershey and Chase Experiment:
After Avery’s result, in 1952, Hershey and Chase performed an experiment on bacteriophages.
Bacteriophages are the viruses that attack upon bacteria; their body consists of DNA and Protein.
They labeled the DNA of bacteriophages with the radio-active isotope of phosphorous (32p) and also labeled their protein coat with the radioactive isotope of (25S). They labeled viruses were allowed to infect bacteria. After that infection, bacterial cells were separated from media content with the help of centrifugation technique. They were analyzed, In the analysis, 32p was found in the bacterial cell while 35S was found in the medium.
This observation clearly showed that during infection, 32P labeled bacterial cell while Its 35S labeled protein coat remained outside.
This all demonstrates that he: “DNA is a universal genetic material”.