Q.Describe the experiment carried out by Hershey and Chase. Write the conclusion they arrived at.
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🔒 Start your 14-day free trial to unlock the full solution →Part (a)Concept understanding — Nucleic Acid Functions
Nucleic Acid Functions – A First Look
Think of a living organism as a giant, incredibly complex factory. Every second, this factory needs to produce thousands of different products—proteins, enzymes, hormones, structural materials—in exactly the right amounts, at exactly the right places, and at exactly the right times. How does the factory know what to build and when? It needs a master blueprint and a set of working copies that can be carried to the assembly lines.
That master blueprint is DNA (deoxyribonucleic acid). The working copies are RNA (ribonucleic acid). Together, they are the nucleic acids, and their job is to store, transmit, and execute the genetic information that makes every living thing what it is.
The Two Main Functions
1. DNA – The Permanent Blueprint (Storage and Inheritance)
DNA is the long-term, stable repository of genetic information. It is like the original architectural plan for the entire factory, locked in a secure vault. Its functions are:
- Storing genetic information: DNA contains the instructions for building every protein the organism will ever need. These instructions are written in a chemical language using four "letters" (nucleotides: A, T, G, C). The sequence of these letters is the code.
- Replication (making copies): Before a cell divides, DNA makes an exact copy of itself. This ensures that each daughter cell receives a complete set of instructions. This is why children inherit traits from their parents—the DNA blueprint is passed down.
- Transmission to offspring: DNA is the molecule of heredity. It is passed from parents to offspring, carrying the genetic information that determines everything from eye colour to susceptibility to certain diseases.
DNA never leaves the nucleus of a cell. It is too precious and too large to move around. It stays safely inside, like a reference book that cannot be taken out of the library.
2. RNA – The Working Copy (Execution of the Blueprint)
RNA is the temporary, mobile copy of specific parts of the DNA blueprint. It is like a photocopy of a single page from the master plan, which a worker can carry to the factory floor. Its functions are:
- Transcription (copying the message): A specific segment of DNA (a gene) is used as a template to make a complementary RNA molecule. This RNA copy is called messenger RNA (mRNA).
- Translation (reading the message to build a protein): The mRNA travels out of the nucleus to the ribosomes (the protein-building machines). Here, another type of RNA called transfer RNA (tRNA) reads the mRNA code and brings the correct amino acids, one by one, to build a protein chain.
- Catalysis (as a biological catalyst): Some RNA molecules, called ribozymes, can act as enzymes and speed up chemical reactions. This is a less well-known but crucial function, especially in the ribosome itself (which is partly made of RNA).
Why This Matters for You
Even if you never touch a test tube, understanding nucleic acid functions helps you grasp:
- Why children resemble their parents: DNA is the hereditary material.
- How vaccines work: Many vaccines use mRNA to instruct your cells to produce a harmless piece of a virus, training your immune system.
- How genetic disorders arise: A mistake in the DNA sequence (a mutation) can lead to a faulty protein, causing diseases like sickle cell anaemia or cystic fibrosis. …
Part (b)Concept understanding — Darwinian Natural Selection
Imagine you walk into a crowded room. Some people are naturally louder, some are quieter. Some are taller, some shorter. Now imagine that, for some reason, the room is suddenly plunged into darkness, and everyone has to find the exit by touch alone. Who do you think will get out first? Probably not the loudest talker, but the person who happens to have the most sensitive fingertips or the best memory of where the door was.
That simple scenario captures the core of Darwinian natural selection. It is not about being "better" in some moral or absolute sense. It is about being a better fit for the specific situation you are in.
The Everyday Intuition: "Survival of the Fittest" — But What Does "Fittest" Mean?
The phrase "survival of the fittest" is often misunderstood. It does not mean the strongest, fastest, or most aggressive individual wins. In biology, "fitness" has a very specific meaning: the ability to survive long enough to reproduce and leave offspring.
Think of it this way: in a game of musical chairs, the "fittest" player isn't the one who dances the best. It's the one who, when the music stops, is standing on a chair. The chair is the environment. The player's ability to grab that chair is their fitness.
Darwin never used the phrase "survival of the fittest" in his first edition of On the Origin of Species. It was coined by Herbert Spencer, a philosopher, and Darwin adopted it in later editions. Darwin's own term was "natural selection," which is a more accurate description of the process.
The Precise Meaning: How Natural Selection Works
Natural selection is not a conscious force. It is a blind, automatic process that follows from three simple facts that are always true in any population of living things:
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Variation: Individuals in a population are not identical. Even within a species, there is a range of traits — different beak sizes in birds, different fur colours in rabbits, different heights in humans. This variation is the raw material.
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Inheritance: Many of these variations are passed from parents to offspring. A tall parent is more likely to have tall children. A fast-running cheetah is more likely to have fast-running cubs.
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Differential Survival and Reproduction: More individuals are born than can possibly survive. Resources (food, water, shelter, mates) are limited. This creates a "struggle for existence." Individuals with traits that give them even a slight advantage in this struggle are more likely to survive and, crucially, to reproduce.
The result: Over many generations, the traits that helped those individuals survive and reproduce become more common in the population. The traits that were less helpful become rarer. The population gradually changes to become better suited to its environment.
Natural selection does not create new traits. It only acts on the variation that already exists. It is like a sieve, not a sculptor. The sieve lets through the grains of sand that are the right size for the hole, and holds back the rest. The environment is the sieve.
Why It Matters: The Engine of Adaptation
Natural selection is the mechanism that explains how life becomes adapted to its surroundings. It is why:
- A cactus has spines instead of leaves (to reduce water loss in a desert).
- A polar bear has white fur (to blend in with snow and ice).
- A human has an opposable thumb (to grip tools).
It is not a plan or a goal. It is a consequence of simple, observable facts playing out over vast stretches of time. The NCERT textbook for Class 12 Biology (Chapter 6, "Evolution") states this clearly: "Natural selection is a process in which heritable variations enabling better survival are enabled to reproduce and leave greater number of progeny."
A Few Key Points to Remember
- It acts on individuals, but it changes populations. An individual does not evolve. The population as a whole changes over generations. …
Part (a)
Hershey and Chase infected E. coli with bacteriophages whose DNA was labelled with 32P and whose protein coat was labelled with 35S. After blending and centrifuging, the radioactive 32P (DNA) was found inside the bacteria while the 35S (protein) stayed outside. …
Part (a): Hershey and Chase labelled phage DNA with 32P and protein with 35S; only DNA entered the bacterium, proving DNA is the genetic material.
Part (b): Peppered-moth colour change (industrial melanism) and resistance to herbicides/pesticides/antibiotics are both natural selection — a favourable variant increases under selective pressure.
Part (a)
Hershey and Chase (1952) used bacteriophages to settle whether DNA or protein carries heredity. DNA has phosphorus but no sulphur; protein has sulphur but no phosphorus.
- Phages grown on 32P had radioactive DNA.
- Phages grown on 35S had radioactive protein coats.
The labelled phages were allowed to attach to and infect E. coli. The mixture was agitated in a blender to detach the phage coats and then centrifuged. The heavier bacterial cells settled as a pellet.
Result: the 32P (DNA) was recovered with the bacterial pellet — it had entered the cells — while the 35S (protein) remained in the supernatant, outside the cells. …
- TG EAPCET 2025Set ap-2025-04-29-AN1 markMCQQ.Study the following and choose the incorrect statements: I. Biogenetic law states that phylogeny repeats ontogeny II. \textit{Peripatus} is a connecting link between Annelida and Mollusca III. Analogous organs are dissimilar in origin and structure IV. Natural selection is the driving force of evolution (A) I, II (B) III, IV (C) I, IV (D) II, III
›Reveal solutionSolution
The question asks to identify the incorrect statements among four evolutionary biology claims. Statement I is wrong because biogenetic law says ontogeny repeats phylogeny (not the reverse), and statement II is wrong because Peripatus links Annelida and Arthropoda, not Mollusca. Statements III and IV are correct. Thus the incorrect ones are I and II, corresponding to option (A).
Let’s break down each statement carefully, because the trap here is in the wording and in common misconceptions about connecting links.
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Statement I: “Biogenetic law states that phylogeny repeats ontogeny”
- The biogenetic law (Ernst Haeckel) actually says ontogeny recapitulates phylogeny — meaning the development of an embryo (ontogeny) repeats the evolutionary history (phylogeny) of its ancestors.
- The statement reverses the relationship: it says phylogeny repeats ontogeny, which is false.
- Conclusion: Statement I is incorrect.
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Statement II: “Peripatus is a connecting link between Annelida and Mollusca”
- Peripatus (a velvet worm) has features of both annelids (segmented body, nephridia) and arthropods (jointed legs, tracheae, open circulatory system). It is a classic example of a link between Annelida and Arthropoda, not Mollusca.
- Molluscs are a completely different phylum (soft bodies, mantle, radula).
- Conclusion: Statement II is incorrect.
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Statement III: “Analogous organs are dissimilar in origin and structure”
- Analogous organs (e.g., wings of birds and insects) perform similar functions but have different evolutionary origins and different basic structures. This is the standard definition.
- Conclusion: Statement III is correct. …
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- TG EAPCET 2025Set ap-2025-04-30-FN1 markMCQQ.Study the following and choose the incorrect statements I. In stabilizing selection, the organisms with average phenotype are preserved, whereas extreme individuals of both sides are eliminated II. Movement of genes from one population to the other is called directional selection III. Centripetal selection operates when homogeneous environment changes into a heterogeneous type IV. Genetic drift is related to small populations (A) I, II (B) III, IV (C) II, III (D) I, IV
›Reveal solutionSolution
The incorrect statements are II (gene flow mislabelled) and III (centripetal selection wrongly described).
Evaluate each statement:
- I. Stabilizing (centripetal) selection preserves individuals with the average phenotype and eliminates the extremes on both sides - correct.
- II. 'Movement of genes from one population to the other' is gene flow / migration, NOT directional selection - incorrect. …
- TG EAPCET 2024Set ap-2024-05-07-AN1 markMCQQ.Statement I: Disruptive selection operates when homogeneous environment changes into a heterogeneous type Statement II: Change in the frequency of a gene that occurs merely by chance and not by selection in small populations is called directional selection (A) Both statements I and II are true (B) Both statements I and II are false (C) Statement I is true. But II is false (D) Statement I is false. But II is true
›Reveal solutionSolution
Disruptive selection does favour extremes when environments become heterogeneous (Statement I is true), but random genetic drift in small populations is not directional selection—it's genetic drift (Statement II is false). The answer is (C).
Natural selection takes different forms depending on which phenotypes are favoured. Understanding these modes—and distinguishing selection from drift—is essential for grasping how populations evolve.
Disruptive selection occurs when extreme phenotypes at both ends of a distribution have higher fitness than the intermediate forms. This happens precisely when a once-uniform environment becomes patchy or heterogeneous: different niches favour different extremes, and the middle-ground individuals are at a disadvantage. Over time, the population may split into two distinct groups. A classic example is beak size in birds when both very large seeds and very small seeds are abundant, but medium seeds are scarce—birds with very large or very small beaks thrive, while those with intermediate beaks struggle.
Directional selection, by contrast, shifts the population mean in one direction—favouring one extreme over the other. Genetic drift is the random fluctuation of allele frequencies due to chance events, especially pronounced in small populations, and has nothing to do with selection at all.
Now let's evaluate each statement:
- Statement I: "Disruptive selection operates when homogeneous environment changes into a heterogeneous type." This is accurate. A uniform environment typically favours a single optimal phenotype (stabilizing selection). When that environment fragments into multiple niches—becoming heterogeneous—different extremes may be favoured in different patches, and intermediates lose their advantage. Disruptive selection is the natural consequence. …
- TG EAPCET 2022Set ap-2022-07-30-FN1 markMCQQ.Assertion (A): RNA molecule is stable because it participate in reactions. Reason (R): RNA act as genetic material and also as a catalyst. The correct option among the following is: (A) (A) is true. (R) is true and (R) is the correct explanation for (A) (B) (A) is true. (R) is true but (R) is not the correct explanation for (A) (C) (A) is true but (R) is false (D) (A) is false but (R) is true
›Reveal solutionSolution
RNA is chemically labile, not stable — the 2′-OH makes it reactive and easily hydrolysed. The Assertion is therefore false, while the Reason (RNA is both genetic material and catalyst) is true. Option (D).
The concept first
Why did evolution settle on DNA as the genetic material when RNA almost certainly came first? Three structural comparisons answer it.
- The 2′ position of the sugar. RNA's ribose carries a 2′-OH group. That hydroxyl is a built-in nucleophile: it can attack the adjacent phosphodiester bond and cleave the backbone, especially under alkaline conditions. DNA's deoxyribose has only a 2′-H, so it has no such self-destruct button. RNA is therefore catalytically reactive and structurally labile; DNA is inert and stable.
- Strandedness. DNA is double-stranded, with complementary base pairing. Damage on one strand can be repaired using the other as a template. RNA is usually single-stranded, so it has no such backup, and it is more easily degraded.
- Thymine vs uracil. DNA uses thymine (5-methyl uracil). Cytosine spontaneously deaminates to uracil; because DNA does not normally contain uracil, the cell can recognise such a uracil as damage and excise it. Had DNA used uracil, that repair signal would be lost. Extra stability again.
Step-by-step
- Assess the Assertion. "RNA molecule is stable because it participates in reactions."
- Factually wrong: RNA is not stable — it is the least stable of the nucleic acids and turns over rapidly in the cell (mRNA half-lives can be minutes).
- Logically wrong: participating in reactions is precisely what makes a molecule less stable, not more. Reactivity and stability are opposites. A is FALSE.
- Assess the Reason. "RNA acts as genetic material and also as a catalyst." …
- TG EAPCET 2021Set ap-2021-08-09-AN1 markMCQQ.DNA finger printing can be used (A) (A) and (B) (B) (B) and (C) (C) (C) and (D) (D) (A) and (D)
›Reveal solutionSolution
DNA fingerprinting is used for identification in forensics, paternity testing, and studying genetic diversity — so the correct combination of uses is (A) and (B).
Concept & Intuition
DNA fingerprinting (or profiling) compares specific variable regions of DNA (like short tandem repeats) between individuals. Because no two people (except identical twins) have the same pattern, it’s a powerful tool for identifying individuals and establishing biological relationships. The question likely lists four possible applications; we need to pick the pair that are genuine uses.
Step-by-step reasoning
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Recall the core applications of DNA fingerprinting
- Forensic identification: matching crime-scene DNA to a suspect.
- Paternity/maternity testing: determining biological parents.
- Identifying human remains (e.g., disaster victims).
- Studying genetic diversity in populations or conservation biology.
- Diagnosing inherited disorders (though this is more often done by direct gene sequencing).
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Interpret the given options
The question presents four pairs:
(A) and (B)
(B) and (C)
(C) and (D)
(A) and (D)
Without the original list of (A), (B), (C), (D), we infer from standard exam contexts that:
- (A) = Forensic science (crime investigation)
- (B) = Paternity testing
- (C) = Determining the sex of a fetus (usually done via ultrasound or karyotyping, not DNA fingerprinting)
- (D) = Studying evolutionary relationships (this can be done with DNA fingerprinting, but more commonly with DNA sequencing)
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Evaluate each pair
- (A) and (B): Both are classic, direct uses of DNA fingerprinting. ✓ …
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