The Search for Genetic Material – Transformation and Bacteriophage Experiment

The Search for Genetic Material: Mendel’s experiment on the pea plant explained the patterns of inheritance of different characteristics from their parents to offspring. Based on his experimental evidence, Mendel stated that there are certain factors responsible for the transfer of hereditary information or characteristics from their parents to offspring. These factors were later identified as genes, but their chemical nature role in regulating the inheritance was not known at the time of Mendel.  Further investigations were carried out in different laboratories after the establishment of the gene concept to find out the chemical composition of the chromosome and to search for the genetic material found in the chromosome. Let’s browse through the article to know about the scientist’s contribution to the search for genetic material.

What is Genetic Material?

The substance that stores the biological information in the coded form and transfers it to the next generation, and causes its expression in the offspring is called genetic material.

Characteristics of Genetic Material

The genetic material should have the following characteristics:

1. The genetic material should be able to duplicate itself, forming its exact copies precisely, and should be able to pass its copies into the progeny faithfully.
2. It should be able to develop occasional inheritable changes called mutations to allow adaptation and evolution to occur in the organism.
3. The storage and expression of biological information to determine heritable characteristics are some of the key features of genetic material.
4. All the somatic cells of a species should have a similar genetic material in order to express a specific phenotypic character of an organism.

Study Properties of Genetic Material Here

DNA is a Genetic Material

Sutton and Boveri proposed that genetic information is passed between generations by way of chromosomes. But it took many years to find out whether the genetic material was the DNA or the protein of chromosomes. Both have the structural diversity as great as expected of the genetic material.

Deoxyribonucleic Acid: DNA also shows a wide structural variety because of the four types of nucleotide units. Beyond this, it has the ability of self-replication, mutation, etc. It has now been experimentally proved that DNA is the genetic material.

Why is Protein not a Genetic material?

Earlier proteins were thought to play the role of genetic material because they are the macromolecules that show enormous variety due to the combinations of 20 amino acids in different sequential manners and exhibit a great specificity of function.

However, they have no mechanism for duplication. Hence they do not qualify to act as the genetic material. Moreover, they cannot store the genetic information from the parents to the offspring. Therefore, they cannot be considered a genetic material.

Experimental Evidence for DNA as Genetic Material

DNA is a unique molecule that directs its own replication. The resemblance of offspring to their parents results from the precise replication of parental DNA and the transfer of the replicated molecules of DNA to the offspring. The following experimental efforts by scientists have proven that DNA is a fully justified genetic material:

1. Griffith’s Experiment: Transformation Experiment

This experiment was performed by Frederick Griffith in 1928, suggesting that bacteria are capable of transferring genetic information. The scientist performed an experiment with the bacterium Streptococcus pneumonia (Diplococcus) that causes pneumonia in mammals, including humans. There are two different strains of this bacterium:

i. S-type: It is a smooth, capsulated, or virulent strain of bacterium that causes pneumonia. It forms smooth glistening colonies when growing on an agar medium.
ii. R-type: It is a rough, non-capsulated, or non-virulent strain of bacterium that does not cause pneumonia.

Observation: Griffith chose mice as a subject for his experiment. The results obtained by introducing different strains to separate groups of mice are discussed as follows:

I. By injecting S-type strain: A group of mice when infected with s-type strain, it has been observed that all the mice died.

II. By injecting R-type strain: The group of mice infected with R-type strain of bacteria remain alive and do not show any symptoms of pneumonia.

III. By injecting heat-killed S-type bacterial strain: The s-strains, when heated, becomes inactive and thereby cannot cause pneumonia in the mice group.

IV. By injecting a mixture of heat-killed S-type and live R-strain: Surprisingly, when a mixture of inactive and non-pathogenic bacterial strains is introduced in a group of mice, all the mice are found to have pneumonia.

Conclusion: Griffith’s found that smooth bacterial strain has some chemical substance called transforming principle that is released by the killed S-type strain transforms the R-type bacterial strain into S-type strain. This was a permanent genetic change as s-type bacteria continued to produce similar cells. It is the first known example of bacterial transformation. This finding proved that some component of the bacterial cell, rather than the whole, was a transforming agent.

Fig: Demonstration of Griffith’s Experiment

2. The Experiment of Avery, MacLeod, and McCarty

Oswald Avery, Colin MacLeod, and McCarty revealed the chemical nature of the transforming substance to be DNA. They showed that DNA isolated from the S-type bacterial strain itself could confer the pathogenic properties to R-type bacterial strains. Their evidence can be discussed as follows:

Method: Scientists separated the protein, carbohydrate, and DNA contents of smooth and pathogenic bacteria. The separated protein, carbohydrates, and DNA content of S-type bacteria were then added separately to the culture medium of R-type bacteria.

Observation:

I. It has been observed that when fractions of protein and carbohydrate of S-type bacterium are added to the culture of R-bacterium, there is no transformation of R-bacterial strain to S-bacterial strain.

II. When the DNA extract of the S-type is added to the culture medium of the R-type, it has been observed that the R-type bacterial strain transformed into an S-type bacterial strain.

III. Moreover, when the DNA extract of an S-type bacterium is treated with an endonuclease enzyme called Dnase (an enzyme that digests DNA).

IV. In the enzyme-treated DNA extract, when introduced to the R-type culture medium, no transformation occurred, and the disease was not caused.

This ensures that DNA is the genetic material that carries biological information.

Fig: Demonstration of Avery, MacLeod, and McCarty Experiment

3. Hershey and Chase Experiment: Bacteriophage Experiment

The scientists perform a transduction experiment using the virus that infects bacteria called the bacteriophage virus. Scientists have chosen T₂ bacteriophage for their experimental material. Hershey and Chase based their experiment on the following two facts: 

I. DNA but not the protein contains phosphorus.
II. Sulphur is present in proteins but not in DNA.

Method:

I. They incorporated radioactive isotopes of phosphorus (P³²) into phage DNA and that of sulphur (S³⁵) into proteins of a separate phage culture. These phages were used independently to infect the bacterium Escherichia coli.

II. The two types of bacteriophages, one labelled with P³² and the other with S³⁵ were allowed to infect different colonies of  E.coli.

III. The two were independently agitated on a blender after some time to separate the empty phage capsids or ghosts from the bacterial cells by centrifugation.

Observation:

I. It has been observed that radioactive P³² was found to be associated with the bacterial cells and appeared in the progeny phage. It can be said that the bacteria infected with virus-containing radioactive DNA were radioactive.

II. However, when S³⁵ was used, all radioactive material was limited to phage ghosts. It can be said that the bacteria infected with virus-containing radioactive proteins were not radioactive.

Conclusion: These results indicate that the DNA of the bacteriophage enters the bacterial cell, not the protein. It has been proven that DNA is the genetic material.

Fig: Demonstration of Hershey and Chase Experiment

RNA as Genetic Material

Later studies determined that RNA also serves as genetic material in some viruses. A majority of plant viruses have RNA as genetic material. Fraenkel and Conrat conducted experiments on tobacco mosaic virus (TMV) to demonstrate that in some viruses, RNA acts as genetic material.

1. Experiment I

I. Tobacco Mosaic Virus is composed of a single molecule of RNA that is encapsulated in a cylindrical protein coat.

II. The protein coat and RNA of TMV were separated by certain chemical treatments.

III. Further, the tobacco plant leaf was introduced separately with the protein coat and the RNA of the tobacco mosaic virus.

IV. It has been observed that the leaf, when infected with RNA fraction of tobacco mosaic virus, shows brown spots on the leaves that indicate the replication of RNA on the infected leaf. In contrast, no brown spots have been identified on the leaf infected with protein extract of tobacco mosaic virus.

It proves that RNA serves as genetic material.

Fig: Demonstration of RNA as Genetic Material

2. Experiment II

I. The two strains of tobacco mosaic virus, namely type-A and type-B, were taken, each representing different symptoms. Their protein and RNA were separated.

II. The hybrid viruses were synthesised by mixing or interchanging the RNA extract of type-A with protein extract of type-B and vice-versa.

III. The tobacco leaf, when infected with these hybrids, symptoms resemble the strain from which RNA is extracted. It can be said that the progeny virus resembles phenotypically and genotypically with the type from which RNA is extracted though it has the protein extract of another type or strain.

It shows that RNA stores and transfers genetic information and is therefore considered as genetic material.

Fig: Demonstration of RNA as Genetic Material in a Hybrid Virus

The above-discussed experiments justify that both DNA and RNA qualify for the properties of genetic material. The stability of the genetic material matters a lot as the stable molecule can only carry the information with altering. The following reasons justify DNA as a more stable and better genetic material than RNA:

1. DNA possesses 2-H groups, whereas RNA possesses 2-OH groups that are more prone to hydrolytic cleavage.

2. The RNA is a single-stranded genetic material that can undergo autohydrolysis and therefore is less stable compared to double-stranded DNA.

3. Thymine in RNA is replaced by the Uracil that is derived from the deamination of cytosine that would lead to a decrease in the G≡C pair and an increase in the A=U base pair. Continuous deamination of cytosine over a period might lead to the elimination of G≡C base pairs and therefore causes the distortion of RNA. Whereas in DNA, the presence of thymine makes it more stable by inhibiting the process of deamination of cytosine.

Summary

The nucleic acid and Mendel’s law of inheritance were both discovered near about the same time, but the discovery of genetic material and its proof took a long time. It has been ensured after the Mendelian law of inheritance that some factors are responsible for the inheritance of characteristics from parents to offspring. Many scientists worked to find out the proof for the genetic material.

Griffith has performed a transformation experiment by involving smooth and rough strains of bacteria. His workings are further carried forward by Avery, MacLeod, and McCarty to prove DNA as genetic material. A bacteriophage experiment was also performed to prove DNA as genetic material. Fraenkel and Cornel conducted their experiment on the tobacco mosaic virus to prove that RNA also serves as genetic material in some viruses.

Frequently Asked Questions (FAQs) on Search for Genetic Material

Q.1. What experiments demonstrated that DNA is genetic material?
Ans: Transformation experiment by Griffith and Bacteriophage experiment by Hershey and Chase demonstrated that DNA is the genetic material.

Q.2. Why did Hershey and Chase use bacteriophage?
Ans: Hershey and Chase used bacteriophage as it contains DNA enclosed in a protein coat.

Q.3. What was the conclusion of Griffith’s experiment?
Ans: Griffith concluded that a transforming substance is found in S-type (pathogenic) heat-killed bacteria that transforms the R-type (non-pathogenic) bacterial strain into S-type bacterial strain.

Q.4. Why is a protein not a genetic material?
Ans: Protein is not a genetic material because it can not replicate.

Q.5. What are the three characteristics of genetic material?
Ans: The three main characteristics of genetic material include:
1. It should be stable and must be present in each cell.
2. It should be able to express itself whenever required.
3. The genetic material should be capable of replication of its own.

Learn About DNA in Detail Here

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