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Q7:
How did Hershey and Chase differentiate between DNA and protein in their experiment while proving that DNA is the genetic material?
Solution :
Initial Experimental Premises
The Hershey-Chase experiment (1952) sought to distinguish between DNA and protein as the carrier of genetic information by utilizing the bacteriophage $T_2$ virus, which infects Escherichia coli. The differentiation strategy relied on the specific elemental composition of these biomolecules:
- DNA: Contains phosphorus ($P$) but lacks sulfur ($S$).
- Protein: Contains sulfur ($S$) in amino acids like methionine and cysteine, but lacks phosphorus ($P$).
Step 1: Radioactive Labeling
To differentiate the molecules, the researchers prepared two distinct cultures of E. coli bacteria, each grown in a medium containing a specific radioisotope:
- Culture 1: Grown in a medium containing radioactive phosphorus ($^{32}P$). The bacteriophages infecting these bacteria incorporated $^{32}P$ into their DNA [Per the metabolic pathways of nucleotide synthesis].
- Culture 2: Grown in a medium containing radioactive sulfur ($^{35}S$). The bacteriophages infecting these bacteria incorporated $^{35}S$ into their protein coat (capsid) [Per the synthesis of sulfur-containing amino acids].
Step 2: Infection and Blending
The labeled bacteriophages were allowed to infect non-radioactive E. coli bacteria. After a brief period, the cultures were agitated in a blender. This mechanical shearing process detached the viral protein coats (ghosts) from the surfaces of the bacterial cells without disrupting the cell integrity [Methodological technique: Centrifugal separation].
Step 3: Centrifugation and Detection
The mixture was subjected to centrifugation, which separates components based on density. The heavier bacteria formed a pellet at the bottom, while the lighter viral coats remained in the supernatant.
| Isotope | Labeled Molecule | Resulting Location |
|---|---|---|
| $^{32}P$ | DNA | Found in the bacterial pellet |
| $^{35}S$ | Protein | Found in the supernatant |
Conclusion and Logical Inference
The presence of $^{32}P$ inside the bacterial cells indicated that the DNA of the bacteriophage had been injected into the host, whereas the presence of $^{35}S$ in the supernatant confirmed that the protein coat remained external [By the Law of Segregation of Labeled Components].
Furthermore, the progeny viruses produced by the infected bacteria were found to contain $^{32}P$, confirming that the material transmitted to the next generation was DNA. Thus, it was definitively proven that DNA, not protein, serves as the genetic material.
Final Solution: Hershey and Chase differentiated between DNA and protein by labeling them with distinct radioisotopes ($^{32}P$ for DNA and $^{35}S$ for protein). The experiment demonstrated that only the radioactive DNA entered the bacterial cells to direct the production of viral progeny, thereby identifying DNA as the genetic material.
More Questions from Class 12 Biology Molecular Basis of Inheritance EXERCISES
- Q1: Group the following as nitrogenous bases and nucleosides: Adenine, Cytidine, Thymine, Guanosine, Uracil and Cytosine.
- Q1: Group the following as nitrogenous bases and nucleosides: Adenine, Cytidine, Thymine, Guanosine, Uracil and Cytosine.
- Q2: If a double stranded DNA has 20 per cent of cytosine, calculate the per cent of adenine in the DNA.
- Q2: If a double stranded DNA has 20 per cent of cytosine, calculate the per cent of adenine in the DNA.
- Q3: If the sequence of one strand of DNA is written as follows: 5' -ATGCATGCATGCATGCATGCATGCATGC-3' Write down the sequence of complementary strand in 5'\(\rightarrow\)3' direction.
- Q3: If the sequence of one strand of DNA is written as follows: 5' -ATGCATGCATGCATGCATGCATGCATGC-3' Write down the sequence of complementary strand in 5'→3' direction.
- Q4: If the sequence of the coding strand in a transcription unit is written as follows: 5' -ATGCATGCATGCATGCATGCATGCATGC-3' Write down the sequence of mRNA.
- Q4: If the sequence of the coding strand in a transcription unit is written as follows: 5' -ATGCATGCATGCATGCATGCATGCATGC-3' Write down the sequence of mRNA.
- Q5: Which property of DNA double helix led Watson and Crick to hypothesise semi-conservative mode of DNA replication? Explain.
- Q5: Which property of DNA double helix led Watson and Crick to hypothesise semi-conservative mode of DNA replication? Explain.
- Q6: Depending upon the chemical nature of the template (DNA or RNA) and the nature of nucleic acids synthesised from it (DNA or RNA), list the types of nucleic acid polymerases.
- Q6: Depending upon the chemical nature of the template (DNA or RNA) and the nature of nucleic acids synthesised from it (DNA or RNA), list the types of nucleic acid polymerases.
- Q7: How did Hershey and Chase differentiate between DNA and protein in their experiment while proving that DNA is the genetic material?
- Q8(a): Differentiate between the followings: Repetitive DNA and Satellite DNA
- Q8(a): Differentiate between the followings: (a) Repetitive DNA and Satellite DNA
- Q8(b): Differentiate between the followings: mRNA and tRNA
- Q8(b): Differentiate between the followings: (b) mRNA and tRNA
- Q8(c): Differentiate between the followings: Template strand and Coding strand
- Q8(c): Differentiate between the followings: (c) Template strand and Coding strand
- Q9: List two essential roles of ribosome during translation.
- Q9: List two essential roles of ribosome during translation.
- Q10: In the medium where E. coli was growing, lactose was added, which induced the lac operon. Then, why does lac operon shut down some time after addition of lactose in the medium?
- Q10: In the medium where E. coli was growing, lactose was added, which induced the lac operon. Then, why does lac operon shut down some time after addition of lactose in the medium?
- Q11(a): Explain (in one or two lines) the function of the followings: (a) Promoter
- Q11(a): Explain (in one or two lines) the function of the followings: Promoter
- Q11(b): Explain (in one or two lines) the function of the followings: (b) tRNA
- Q11(b): Explain (in one or two lines) the function of the followings: tRNA
- Q11(c): Explain (in one or two lines) the function of the followings: (c) Exons
- Q11(c): Explain (in one or two lines) the function of the followings: Exons
- Q12: Why is the Human Genome project called a mega project?
- Q12: Why is the Human Genome project called a mega project?
- Q13: What is DNA fingerprinting? Mention its application.
- Q13: What is DNA fingerprinting? Mention its application.
- Q14(a): Briefly describe the following: (a) Transcription
- Q14(a): Briefly describe the following: Transcription
- Q14(b): Briefly describe the following: (b) Polymorphism
- Q14(b): Briefly describe the following: Polymorphism
- Q14(c): Briefly describe the following: (c) Translation
- Q14(c): Briefly describe the following: Translation
- Q14(d): Briefly describe the following: (d) Bioinformatics
- Q14(d): Briefly describe the following: Bioinformatics
CBSE Solutions for Class 12 Biology Molecular Basis of Inheritance
Chapters in CBSE - Class 12 Biology
- Reproduction in Organisms
- Human Reproduction
- Reproductive Health
- Principles of Inheritance and Variation
- Evolution
- Human Health and Disease
- Strategies for Enhancement in Food Production
- Microbes in Human Welfare
- Biotechnology: Principles and Processes
- Biotechnology and Its Applications
- Organisms and Populations
- Ecosystem
- Biodiversity and Conservation
- Environmental Issues
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