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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 :
Step 1: Theoretical Foundation of the Hershey-Chase Experiment
The Hershey-Chase experiment (1952) sought to determine whether $DNA$ or protein was the hereditary material transmitted by the bacteriophage $T2$ to its host, Escherichia coli. They utilized the fundamental biochemical differences between the molecular compositions of these two biomolecules:
- DNA: Contains Phosphorus ($P$) in its phosphodiester backbone but lacks Sulfur ($S$).
- Proteins: Contain Sulfur ($S$) in amino acids like cysteine and methionine but lack Phosphorus ($P$).
Step 2: Differential Radioactive Labeling
To distinguish between the two, Hershey and Chase cultivated viruses in two separate media containing radioisotopes:
| Labeling Component | Radioisotope Used | Target Macromolecule |
|---|---|---|
| Medium A | $^{32}P$ (Radioactive Phosphorus) | $DNA$ |
| Medium B | $^{35}S$ (Radioactive Sulfur) | Protein Coat (Capsid) |
[Justification: Per the selective incorporation of elements into specific molecular structures, radioactive decay tracking allows for the spatial localization of these isotopes within the host-virus system.]
Step 3: Infection, Blending, and Centrifugation
The experiment proceeded through three critical physical phases:
- Infection: Labeled phages were allowed to infect non-radioactive E. coli cells.
- Blending: A blender was used to agitate the culture, mechanically shearing the viral protein coats (ghosts) from the surfaces of the bacterial cells.
- Centrifugation: The mixture was spun in a centrifuge. The heavier bacterial cells formed a pellet at the bottom, while the lighter viral protein coats remained in the supernatant.
Step 4: Analytical Data Interpretation
The results provided definitive evidence based on the localization of the radioisotopes:
- Supernatant Analysis: The radioactive sulfur ($^{35}S$) was found in the supernatant. [Conclusion: The viral protein coat did not enter the bacterial cell.]
- Pellet Analysis: The radioactive phosphorus ($^{32}P$) was found in the bacterial pellet. [Conclusion: The viral $DNA$ entered the bacterial cell to direct the synthesis of new progeny viruses.]
Step 5: Theoretical Synthesis
The experiment established that the genetic information required for viral replication was contained within the $DNA$ molecule, as only the radioactive $DNA$ was detected within the host cell post-infection. This confirmed the Central Dogma's premise regarding the hereditary molecule's identity [By the Hershey-Chase Conclusion].
Final Solution: Hershey and Chase differentiated DNA from protein by selectively labeling them with radioactive isotopes ($^{32}P$ for DNA and $^{35}S$ for protein). By observing that only $^{32}P$ entered the host bacteria while $^{35}S$ remained outside in the protein coat, they demonstrated that DNA, not protein, is the hereditary 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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