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. …
Showing the 12 most recent of 40 on this concept.
- CBSE 2026Set 57/1/11 markMCQQ.Assertion (A) : The population of melanized moths increased in industrial areas after Industrial Revolution. Reason (R) : In Industrial environment lichen covered trees were replaced by soot-covered trees offering better camouflage to dark coloured moths. (A) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation for Assertion (A). (B) Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation for Assertion (A). (C) Assertion (A) is true, but Reason (R) is false. (D) Assertion (A) is false, but Reason (R) is true.
›Reveal solutionSolution
The assertion and reason are both true, and the reason correctly explains why melanized moths became more common in industrial areas after the Industrial Revolution.
This question draws on one of the most famous and well-documented examples of natural selection in action — the peppered moth (Biston betularia) in England. The story is a classic illustration of how environmental change can drive evolutionary change, and it is directly covered in the NCERT Class 12 Biology textbook under the chapter on Evolution.
Before the Industrial Revolution, the typical peppered moth had a light, speckled appearance that blended beautifully with the lichen-covered bark of trees. This camouflage protected them from predatory birds. A dark, melanic form of the same species existed but was very rare, because it stood out starkly against the pale lichen and was easily spotted and eaten.
The Industrial Revolution changed the landscape dramatically. Soot and smoke from coal-burning factories coated trees and killed the lichen, turning the bark dark. Now the situation reversed: the light-coloured moths became highly visible against the soot-blackened trees, while the dark (melanized) moths were well-camouflaged. Birds now ate more of the light moths, and the dark moths survived and reproduced in greater numbers. Over several decades, the population shifted from mostly light to mostly dark in industrial areas.
NoteThis phenomenon is called industrial melanism. It is a textbook case of natural selection acting on a heritable trait, not a change acquired during the moth's lifetime.
Now look at the Assertion (A): "The population of melanized moths increased in industrial areas after Industrial Revolution." This is exactly what happened — the dark form became dominant in polluted regions. …
- CBSE 2026Set 57/3/11 markMCQQ.Appearance of antibiotic-resistant bacteria is an example of evolution due to : (A) Adaptive radiation (B) Divergent evolution (C) Artificial selection (D) Anthropogenic action
›Reveal solutionSolution
Antibiotic-resistant bacteria evolve through anthropogenic action — human use of antibiotics creates the selective pressure that favours resistant strains, a direct example of evolution driven by human activity.
Darwin's theory of natural selection rests on a simple but powerful idea: organisms with traits better suited to their environment survive and reproduce more successfully, passing those advantageous traits to the next generation. Over time, populations change. The environment acts as the selective agent, and the result is evolution.
Now consider what happens when we introduce antibiotics into bacterial populations. Before antibiotics, a bacterial colony contains natural genetic variation — most individuals are susceptible to the drug, but a tiny fraction carry random mutations that confer resistance. These resistant bacteria have no particular advantage in the absence of antibiotics; they're just different.
The moment we administer an antibiotic, the landscape shifts entirely. The drug kills or inhibits the susceptible bacteria, but the resistant ones survive. Suddenly, resistance is no longer a neutral quirk — it's a life-or-death advantage. The resistant bacteria reproduce freely in the now-emptied niche, and within generations the population is dominated by resistant strains. The antibiotic didn't create the resistance; it simply selected for bacteria that already possessed it.
This is evolution in fast-forward, and the selective pressure is unmistakably human in origin. We manufacture the antibiotics, we prescribe them, we use them in agriculture, and we often misuse them by stopping treatment early or deploying them unnecessarily. Each use is an evolutionary experiment, and bacteria — with their rapid generation times — respond with breathtaking speed.
Let's see why the other options don't fit:
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Adaptive radiation describes the rapid diversification of a single ancestral species into multiple forms, each adapted to different ecological niches — think Darwin's finches spreading across the Galápagos. Antibiotic resistance doesn't involve branching into diverse new species; it's a single trait spreading through existing populations.
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Divergent evolution occurs when related species accumulate differences over time as they adapt to different environments, eventually becoming distinct. Again, we're not watching bacterial species diverge into separate lineages here; we're watching one trait sweep through a population. …
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- CBSE 2026Set 57/3/11 markMCQQ.Identify the statements that correctly describe Darwin's Theory of Evolution.(i) Overproduction of organisms leads to competition.(ii) Variation is inherited and causes evolution.(iii) Acquired characters are inherited.(iv) Survival depends on favourable traits.(v) New species arise due to accumulation of favourable variations. Choose the correct option. (A) (i), (ii),(iv) and(v) are correct (B) (ii),(iii) and(iv) are correct (C) All are correct (D) Only(i) and(iv) are correct
›Reveal solutionSolution
Darwin’s theory rests on overproduction, inherited variation, survival of the fittest, and the gradual accumulation of favourable variations — but it explicitly rejects the inheritance of acquired characters.
Darwin’s theory of evolution by natural selection is one of the most elegant and powerful ideas in biology. It explains how life changes over time without needing any guiding hand or purpose. The core logic is simple: organisms produce more offspring than can possibly survive, those offspring vary in their traits, and the environment “selects” the variants best suited to it. Over many generations, this process can give rise to entirely new species.
Let’s examine each statement in the question against what Darwin actually proposed.
Statement (i): Overproduction of organisms leads to competition.
This is absolutely correct. Darwin observed that every species has the potential to produce far more young than the environment can support. A single pair of elephants, for example, could in theory produce millions of descendants in a few centuries — yet elephant numbers remain roughly stable. The inevitable result is a struggle for existence: competition for food, shelter, mates, and safety. Overproduction sets the stage for natural selection.
Statement (ii): Variation is inherited and causes evolution.
Also correct. Darwin knew that individuals within a species differ from one another, and that many of these differences are passed from parents to offspring. He called this “inherited variation.” Without heritable variation, natural selection would have nothing to act upon — all individuals would be identical, and no change could occur. Evolution happens precisely because favourable variations are inherited and spread through a population.
Statement (iii): Acquired characters are inherited.
This is the one statement that does not belong to Darwin’s theory. The idea that traits acquired during an organism’s lifetime (like a blacksmith’s strong arm or a giraffe’s stretched neck) can be passed to offspring was proposed by Jean-Baptiste Lamarck, not Darwin. Darwin himself was unsure about the mechanism of inheritance, but he never made the inheritance of acquired characters a pillar of his theory. Modern genetics has shown that such acquired traits are not inherited — changes in body cells do not affect the DNA in eggs or sperm.
Watch outA common exam mistake is to confuse Lamarck’s idea with Darwin’s. Remember: Darwin said variation arises by chance and is then selected; Lamarck said organisms change in response to need and pass those changes on. They are fundamentally different.
Statement (iv): Survival depends on favourable traits. …
- CBSE 2026Set ANNUAL1 markQ.Which theory was given by Charles Darwin?
›Reveal solutionSolution
Darwin's theory of natural selection explains that organisms with heritable traits better suited to their environment survive and reproduce more successfully, gradually driving evolutionary change.
Charles Darwin, along with Alfred Russel Wallace, proposed the theory of Natural Selection, formally published by Darwin in 1859 in his book On the Origin of Species.
The key ideas of this theory are:
- Individuals within a population show heritable variations.
- Populations tend to produce more offspring than the environment can support, leading to a struggle for existence.
- Individuals with variations better suited ('fitter') to their environment are more likely to survive and reproduce — 'survival of the fittest'. …
- CBSE 2026Set ANNUAL1 markMCQQ.For a long time, it was believed that the organisms were fixed and unchanging. Which theory was proposed by Charles Darwin to challenge this belief?(a) Theory of Pangenesis(b) Theory of Use and Disuse(c) Theory of Natural Selection(d) Theory of Inheritance of Acquired Characters
›Reveal solutionSolution
Darwin's Theory of Natural Selection (published in 'On the Origin of Species', 1859) proposed that organisms change over generations through differential survival and reproduction of the fittest variants.
Darwin observed that organisms produce more offspring than the environment can support, that individuals show heritable variations, and that individuals better suited to the environment survive and reproduce more successfully ('survival of the fittest'). Over generations, favourable variations accumulate, driving evolutionary change. This directly challenged the older belief that species were fixed and unchanging.
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- CBSE 2026Set ANNUAL1 markMCQQ.Identify the effect brought about by single step large mutation in evolution from the options given below :(a) Saltational speciation(b) Founder effect(c) Gene migration(d) Evolution by special creation
›Reveal solutionSolution
Speciation brought about by a single-step large mutation is called saltation (saltational speciation).
While Darwinian evolution emphasises gradual accumulation of small heritable variations, Hugo de Vries (working on Oenothera, the evening primrose) argued that mutation caused evolution and that large, sudden, single-step mutations — which he called saltation — could produce a new species in one step, rather than by slow, minor variations. Thus the effect of a single-step large mutation is saltational speciation.
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- CBSE 2025Set A1 markQ.Match the correct pair and write the match for 'Darwin'. Column I:(i) Apomixis(ii) Darwin(iii) Toddy(iv) Agarose(v) Detritivorous. Column II:(a) Galapagos Islands(b) Southern India(c) Sea weeds(d) Earthworm(e) Grass family.
›Reveal solutionSolution
Darwin matches with (a) Galapagos Islands, the site of his famous finch observations during the voyage of the Beagle.
Charles Darwin, during his voyage aboard HMS Beagle, observed several closely related but distinct species of finches on the different islands of the Galapagos archipelago, each with beak shapes adapted to the specific food sources available on its island. These observations of adaptive radiation from a common anc …
- CBSE 2025Set ANNUAL1 markMCQQ.The essence of Darwinian theory about evolution is -(a) Mutation(b) Spontaneous generation(c) Natural selection(d) Chemical change
›Reveal solutionSolution
Darwin proposed that heritable variations giving a survival/reproductive advantage are naturally selected and become more common over generations.
Darwin's theory rests on the observations that populations show heritable variation, more offspring are produced than can survive, and individuals best suited to the environment survive and reproduce more successfully ('survival of the fittest'). This differential, non-random survival and reproduction of variants — natural selection — is the essence of Darwinism, driving gradual evolutionary change. Mutation is a source of variation (emphasised later by neo- …
- CBSE 2025Set ANNUAL1 markQ.What kind of charge is present on DNA molecule ?
›Reveal solutionSolution
The phosphate groups in the sugar-phosphate backbone give DNA its negative charge.
Each nucleotide in the DNA backbone contains a phosphate group linked to the 5' and 3' carbons of adjacent deoxyribose sugars via phosphodiester bonds. At physiological pH, these phosphate groups are ionised (each losing a proton), leaving a negatively charged oxygen. Since this repeats along the entire length of both strands, …
- CBSE 2025Set ANNUAL1 markQ.How many types of RNA are there?
›Reveal solutionSolution
RNA occurs in three functional forms — m-RNA, r-RNA and t-RNA — each with a distinct role in protein synthesis.
Unlike DNA, which is essentially one molecule per chromosome, RNA exists in the cell as three chemically similar but functionally different types:
- Messenger RNA (m-RNA) — carries the genetic code copied from DNA (transcription) to the ribosome, specifying the sequence of amino acids to be joined. …
- CBSE 2025Set ANNUAL1 markQ.Who is responsible for heredity?
›Reveal solutionSolution
Heredity is controlled by genes, the functional units of DNA carried on the chromosomes.
Each chromosome in the nucleus of a cell is made of a very long DNA molecule. Specific segments of this DNA, called genes, code for particular traits/proteins. When a cell divides, DNA replicates and an exact copy of these genes is passed on to the daughter cells, and ultimately from parents to offspring during r …
- CBSE 2025Set ANNUAL1 markQ.Define heredity.
›Reveal solutionSolution
Heredity = passing on of hereditary characters from one generation to the next via genes.
Heredity is defined as the biological process by which physical and other characteristics (traits) of parents are transmitted to their offspring. This transmission takes place through genes, which are specific segments of the DNA molecule present on chromosomes, …
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