Q.''Use of heavy isotope of nitrogen by Meselson and Stahl demonstrated semi-conservative mode of replication of a DNA molecule.'' Explain how did they arrive at this conclusion.
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🔒 Start your 14-day free trial to unlock the full solution →Part (a)Concept understanding — DNA Replication and Okazaki Fragments
DNA Replication and Okazaki Fragments
DNA replication is semi-conservative and requires the two strands of the parent DNA to be copied. Because the two strands are antiparallel and DNA polymerase can add nucleotides only in the 5′→3′ direction, the two template strands are copied differently.
- On the leading strand, synthesis is continuous, following the opening of the replication fork.
- On the lagging strand, synthesis is discontinuous: DNA is made as many short pieces called Okazaki fragments, each synthesised in the 5′→3′ direction away from the fork.
The enzyme DNA ligase then joins these Okazaki fragments together to make a continuous strand. If ligase is absent or non-functional, the fragments cannot be sealed, so newly synthesised radioactive DNA accumulates as short, low-molecular-weight pieces rather than long continuous strands — an observation that provides direct evidence for discontinuous synthesis. …
Part (b)Concept understanding — Ribosome Function Translation
Imagine you are in a large library. The shelves are packed with books, but each book is written in a language you cannot read. To understand the book, you need a translator — someone who can read the original script and then explain it to you in plain, usable words.
In a living cell, the "library" is the nucleus, where DNA — the master blueprint — is stored. The "books" are genes, written in the language of nucleic acids. But the cell doesn't work directly with DNA. It needs to build proteins — the actual workers, tools, and building blocks of the body. So it first makes a working copy of a gene, called mRNA (messenger RNA). This mRNA is like a page torn from the book, still in the nucleic-acid language. Now the cell needs a "translator" to convert that message into the language of proteins. That translator is the ribosome.
What is a ribosome?
A ribosome is a tiny, complex molecular machine made of RNA and proteins. It is not a membrane-bound organelle — it floats freely in the cytoplasm or sits on the rough endoplasmic reticulum. Think of it as a workbench that clamps onto the mRNA and reads its sequence, three letters at a time. Each three-letter "word" on the mRNA is called a codon, and each codon specifies one amino acid — the building block of a protein.
The ribosome has two main parts, or subunits — a large one and a small one. The small subunit holds the mRNA in place, while the large subunit does the actual work of joining amino acids together.
The process of translation: from mRNA to protein
Translation happens in three stages, just like reading a sentence: you start, you read word by word, and you stop.
1. Initiation (the start)
The small ribosomal subunit finds a special "start" codon on the mRNA — usually AUG. A special tRNA (transfer RNA) molecule carrying the amino acid methionine binds to that start codon. Then the large subunit clicks into place. The ribosome is now assembled and ready to read.
2. Elongation (the reading and building)
The ribosome moves along the mRNA, one codon at a time. For each codon, a matching tRNA brings the correct amino acid. The ribosome's large subunit then forms a peptide bond between the new amino acid and the growing chain. The ribosome shifts forward, and the empty tRNA is released. This repeats — like a train moving along a track, adding one carriage at a time.
3. Termination (the stop)
When the ribosome reaches a "stop" codon (UAA, UAG, or UGA), no tRNA matches it. Instead, a release factor protein binds, causing the ribosome to let go of the completed protein chain. The two subunits separate, and the new protein is free to fold into its functional shape.
The word "translation" is exact: the cell is translating from the four-letter language of nucleic acids (A, U, G, C) into the twenty-letter language of amino acids. The ribosome is the translator, and tRNAs are the "dictionaries" that match each codon to its correct amino acid.
Why does this matter?
Without translation, the instructions in your DNA would remain useless. Every enzyme that digests your food, every antibody that fights infection, every muscle fibre that lets you move — all are built by ribosomes during translation. If translation stops, the cell dies.
In medicine, many antibiotics work by targeting bacterial ribosomes. For example, tetracycline blocks the binding of tRNA to the ribosome, stopping the bacteria from making proteins. Human ribosomes are slightly different, so the drug affects bacteria but not us — a beautiful example of how understanding this process saves lives.
Key points to remember (as per NCERT)
- Translation is the process of polymerising amino acids into a polypeptide chain, using the sequence of codons on mRNA as a template.
- The ribosome acts as the site of translation and also as a catalyst for forming peptide bonds (the large subunit has an enzymatic activity called peptidyl transferase). …
Part (a)
Meselson and Stahl (1958) grew E. coli for many generations in a medium with heavy nitrogen (¹⁵N) so all DNA became heavy. They then shifted the cells to a light ¹⁴N medium and analysed the DNA by CsCl density-gradient centrifugation after each generation.
- After one generation: a single band of intermediate (hybrid) density appeared. This ruled out conservative replication (which would give one heavy + one light band). …
Part (a): Meselson and Stahl used ¹⁵N/¹⁴N labelling and CsCl density-gradient centrifugation — hybrid DNA after one generation and equal hybrid + light DNA after two generations proved DNA replicates semi-conservatively. Part (b): In prokaryotes, ribosomes translate mRNA into protein through initiation, elongation and termination, with rRNA acting as the peptidyl-transferase ribozyme.
Part (a)
Watson and Crick had proposed that the two strands of DNA separate and each templates a new complementary strand — the semi-conservative model. Two rivals existed: conservative (parental duplex stays intact, a wholly new duplex is made) and dispersive (old and new segments are interspersed on both strands). Meselson and Stahl designed an experiment to distinguish them.
Nitrogen is a component of the DNA bases, so its isotopes can be used to “weight” the DNA. DNA made with heavy ¹⁵N is denser than DNA made with normal ¹⁴N, and the two can be separated as distinct bands in a caesium chloride (CsCl) density gradient spun at high speed.
- E. coli was grown for many generations in medium where the only N source was ¹⁵N, so essentially all its DNA was heavy — a single heavy band.
- The cells were transferred to ¹⁴N medium and allowed to divide once. The extracted DNA formed one band of intermediate density. A conservative mechanism would have given a heavy band plus a light band, so it was eliminated.
- The cells divided a second time in ¹⁴N. Now there were two bands in equal amounts — one intermediate (hybrid) and one light. Dispersive replication would have given a single band getting progressively lighter, so it too was eliminated. …
Showing the 12 most recent of 40 on this concept.
- CBSE 2026Set 57/2/11 markMCQQ.A bacterium having radioactive thymidine in its DNA is allowed to multiply in a medium having non-radioactive thymidine for two generations. What percentage of bacteria will have radioactive thymidine in its DNA ? (A) 100 (B) 50 (C) 75 (D) 25
›Reveal solutionSolution
After two generations of growth in a non-radioactive medium, only 50% of the bacteria retain radioactive thymidine in their DNA — one strand per original double helix remains labelled.
This question is a classic application of Chargaff’s rule and, more importantly, of semiconservative replication — the mechanism by which DNA copies itself. The key idea, first demonstrated by Meselson and Stahl, is that each new DNA molecule gets one old (parental) strand and one newly synthesised strand. Radioactive thymidine acts as a label on the parental strand; once the bacterium moves to a non-radioactive medium, any new strand built will use only non-radioactive thymidine.
Let’s trace what happens. You start with one bacterium whose DNA is fully labelled — both strands contain radioactive thymidine. This is your generation zero. When this bacterium divides once (first generation), each daughter cell receives one original radioactive strand and one newly made non-radioactive strand. So after one generation, every bacterium has radioactive DNA — but only one strand per cell is labelled. That gives 100% of bacteria still radioactive.
Now comes the second generation. Each of those two bacteria divides again. For each, the single radioactive strand acts as a template to build a new complementary strand (non-radioactive). The other, already non-radioactive strand also acts as a template, producing another non-radioactive strand. So from each first-generation cell, you get two daughter cells: one retains the original radioactive strand, the other gets only non-radioactive strands.
NoteThis is exactly the pattern Meselson and Stahl saw: after two generations in a light medium, half the DNA molecules were hybrid (one heavy, one light) and half were fully light. Here, “heavy” is replaced by “radioactive” and “light” by “non-radioactive.”
So after two generations, you have four bacteria in total. Two of them carry the original radioactive strand; the other two have no radioactivity at all. That means 50% of the bacteria are radioactive. …
- CBSE 2026Set ANNUAL1 markMCQQ.Which enzyme helps in the separation of both strands of DNA in DNA replication?(a) Helicase(b) DNA polymerase(c) Topoisomerase(d) DNA ligase
›Reveal solutionSolution
Helicase breaks the hydrogen bonds between complementary bases, unwinding and separating the two DNA strands so replication machinery can access them.
During DNA replication, several enzymes act in coordination:
- Helicase unwinds the double helix by breaking hydrogen bonds between base pairs, creating a replication fork with two separated single strands that serve as templates.
- DNA polymerase then synthesises the new complementary strand by adding nucleotides in the 5'→3' direction, reading the template. …
- CBSE 2026Set ANNUAL1 markMCQQ.What is the role of enzyme Helicase in DNA Replication?(a) Joining of discontinuously synthesised fragments of DNA.(b) Unwinding of the double helix of DNA.(c) Synthesises RNA primer.(d) Formation of H-bond between complementary base pairs.
›Reveal solutionSolution
Helicase opens up (unwinds) the double helix ahead of the replication fork so the two strands can act as templates.
During DNA replication, the double helix must first be separated into two single strands before DNA polymerase can copy them. Helicase is the enzyme that does this: it moves along the DNA at the origin of replication, breaking the hydrogen bonds between complementary base pairs, and unwinds the helix to create the replication fork.
…
- CBSE 2026Set ANNUAL1 markMCQQ.An example of a non-membrane bound organelle is -(a) Endoplasmic reticulum(b) Ribosome(c) Mitochondria(d) Chloroplast
›Reveal solutionSolution
Ribosomes are the only non-membrane-bound organelle among the four options.
Eukaryotic cells contain both membrane-bound and non-membrane-bound organelles:
- Endoplasmic reticulum — a single-membrane network of tubules and cisternae.
- Mitochondria — double-membrane organelles (outer membrane + inner membrane folded into cristae).
- Chloroplast — also double-membrane, with an internal thylakoid membrane system. …
- CBSE 2026Set ANNUAL1 markQ.Write answer in one word/sentence: Write the name of dual functional (met and initiator) genetic code.
›Reveal solutionSolution
AUG is the codon that both codes for methionine and serves as the start codon.
The genetic code is read in triplets called codons. The codon AUG has two roles: it codes for the amino acid methionine (Met), and it also functions as the initiator or start codon that signals the beginning of translation (protein synthesis). Because it …
- CBSE 2025Set 57/4/11 markMCQQ.The substrate used during DNA replication by the enzyme DNA-dependent DNA polymerase is : (A) Deoxyribonucleotide triphosphate (B) Deoxyribonucleoside triphosphate (C) Ribonucleotide triphosphate (D) Ribonucleoside triphosphate
›Reveal solutionSolution
DNA polymerase adds nucleotides to a growing DNA chain by using deoxyribonucleoside triphosphates (dNTPs) as substrates, which provide both the building block and the energy for phosphodiester bond formation. The answer is (B).
Why deoxyribonucleoside triphosphates?
DNA replication requires two things: the correct chemical building blocks that match the template strand, and energy to drive the formation of new bonds. DNA polymerase accomplishes both with a single substrate molecule.
The key is understanding what "nucleoside" versus "nucleotide" means, and why the triphosphate form matters.
A nucleoside = sugar + nitrogenous base
A nucleotide = sugar + nitrogenous base + phosphate group(s)
In biochemistry textbooks, you'll often see "nucleotide" used loosely to mean any of these forms, but the precise terminology matters here. When we say deoxyribonucleoside triphosphate, we mean:
- Deoxyribose sugar (the "deoxyribo-" part)
- A nitrogenous base (A, T, G, or C)
- Three phosphate groups attached to the 5′ carbon of the sugar
How DNA polymerase uses dNTPs
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The substrate arrives as a triphosphate
DNA polymerase recognizes and binds deoxyribonucleoside triphosphates: dATP, dTTP, dGTP, and dCTP. The three phosphate groups are designated α, β, and γ (counting outward from the sugar).
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Base-pairing determines which dNTP is selected
The enzyme checks which incoming dNTP correctly pairs with the template strand (A with T, G with C). Only the complementary dNTP is accepted.
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The polymerization reaction
DNA polymerase catalyzes a nucleophilic attack by the 3′-OH group of the growing strand on the α-phosphate of the incoming dNTP. This forms a new phosphodiester bond and releases pyrophosphate (PPi, the β and γ phosphates together).
DNAn + dNTP → DNAn+1 + PPi (pyrophosphate)
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Energy comes from breaking the high-energy bond
The release of pyrophosphate (which is then hydrolyzed to two inorganic phosphates) makes the reaction thermodynamically favorable. The triphosphate form is essential because it provides the energy currency. …
- CBSE 2025Set 57/4/11 markMCQQ.The correct depiction of the experiment performed by Matthew Meselson and Franklin Stahl to prove that DNA replicates semi-conservatively on separation of DNA by centrifugation after 40 minutes is : (A) [centrifuge tube diagram] (B) [centrifuge tube diagram] (C) [centrifuge tube diagram] (D) [centrifuge tube diagram]
›Reveal solutionSolution
E. coli replicates once about every 20 minutes, so 40 minutes = TWO generations. After two rounds of semi-conservative replication the tube shows two bands — a hybrid (intermediate-density) band and a light band — so the correct depiction is the diagram with two bands (option A), not a single band.
Meselson and Stahl grew E. coli for many generations in medium containing heavy nitrogen (¹⁵N) so that all the DNA was 'heavy', then transferred the cells to medium with light nitrogen (¹⁴N). Newly made strands from that point on incorporate only ¹⁴N. The DNA is then separated by density-gradient (CsCl) centrifugation, where heavier molecules settle lower and lighter molecules band higher.
Why 40 minutes means two generations
The crucial fact is the generation time: E. coli divides roughly every 20 minutes. So:
- After 20 minutes (one generation): every molecule is a hybrid (one ¹⁵N strand + one ¹⁴N strand) → a single band at intermediate density.
- After 40 minutes (two generations): replication of those hybrids produces equal numbers of hybrid molecules and fully light molecules → two bands, one at intermediate density and one at the light position.
Reading the tube after 40 minutes …
- CBSE 2025Set F1 markMCQQ.Which of the following is a soluble RNA?(a) m-RNA(b) r-RNA(c) c-RNA(d) t-RNA
›Reveal solutionSolution
Transfer RNA (tRNA) is the soluble RNA, also called sRNA.
tRNA (transfer RNA) is the smallest of the RNAs and was originally called soluble RNA (sRNA) because it stays dissolved in the cytoplasm. It acts as an adaptor molecule: one end carries a specific amino acid while its anticodon reads the codon on mRNA, thus delivering the correct amino acid to the ribosome during protein synthes …
- CBSE 2025Set ANNUAL1 markMCQQ.Which of the following is a substrate for DNA replication?(a) ATP(b) DNA polymerase(c) Helicase(d) Deoxyribonucleoside triphosphate
›Reveal solutionSolution
DNA replication needs the four deoxyribonucleoside triphosphates as raw material - they supply both the nucleotide monomers and, through cleavage of their phosphate bonds, the energy for the polymerisation reaction.
DNA polymerase catalyses replication by joining deoxyribonucleotides in a chain using the parental strand as template, but the enzyme itself is a catalyst, not a substrate. Helicase unwinds the double helix at the replication fork; it too is an enzyme, not consumed as a building block. ATP is the general energy currency of the cell but is a ribonucleotide, not directly incorporated into DNA. The actual substrates polymerised into the new …
- CBSE 2025Set ANNUAL1 markMCQQ.In DNA replication, the Okazaki fragments on the lagging strand are joined by(a) primase(b) DNA polymerase(c) helicase(d) DNA ligase
›Reveal solutionSolution
DNA ligase acts as the 'molecular glue' that joins the short Okazaki fragments of the lagging strand into one continuous DNA strand.
On the lagging strand of a replication fork, DNA is synthesised discontinuously in short segments called Okazaki fragments, since DNA polymerase can only extend DNA in the 5'→3' direction. After each fragment's RNA primer is removed and replaced with DNA, the enzyme DNA ligase forms the phosphodiester bond that joins the adjacent fragments into a single, continuous strand. Primase (a) …
- CBSE 2024Set 57/3/11 markMCQQ.In an experiment, E. coli is grown in a medium containing 14NH4Cl. (14N is the light isotope of Nitrogen) followed by growing it for six generations in a medium having heavy isotope of nitrogen (15N). After six generations, their DNA was extracted and subjected to CsCl density gradient centrifugation. Identify the correct density (Light/Hybrid/Heavy) and ratio of the bands of DNA in CsCl density gradient centrifugation. (A) Hybrid : Heavy, 1 : 16 (B) Light : Heavy, 1 : 31 (C) Hybrid : Heavy, 1 : 31 (D) Light : Heavy, 1 : 05
›Reveal solutionSolution
After six generations of semi-conservative DNA replication in a heavy nitrogen medium, the original two light strands will always form hybrid DNA, while all other DNA will be heavy. This results in a Hybrid : Heavy DNA ratio of 1 : 31.
The core concept here is the semi-conservative nature of DNA replication, famously demonstrated by the Meselson-Stahl experiment. When DNA replicates, each new double helix consists of one original (parental) strand and one newly synthesized strand. This principle dictates how the nitrogen isotopes are distributed in the DNA molecules over successive generations, which in turn affects their density.
Nitrogen is a key component of DNA bases.
- 14N is the common, "light" isotope of nitrogen.
- 15N is a heavier isotope. DNA containing 15N will be denser than DNA containing 14N. DNA with one 14N strand and one 15N strand (a hybrid molecule) will have an intermediate density.
CsCl density gradient centrifugation separates molecules based on their density. When DNA is spun in a CsCl solution, it forms bands at positions where its density matches the density of the CsCl solution.
- Light DNA (14N/14N) will form a band at the highest position (least dense).
- Hybrid DNA (14N/15N) will form a band at an intermediate position.
- Heavy DNA (15N/15N) will form a band at the lowest position (most dense).
Let's trace the DNA composition over the generations:
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Initial State (Generation 0):
The E. coli is initially grown in a medium containing 14NH4Cl. This means all the nitrogen incorporated into their DNA is the light isotope, 14N.
So, all DNA molecules are 14N/14N (Light).
Let's assume we start with 1 DNA molecule. It has two 14N strands.
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First Generation (after 1 replication in 15N medium):
The E. coli is transferred to a medium containing 15NH4Cl and allowed to replicate once.
According to semi-conservative replication, the original 14N/14N DNA molecule unwinds. Each 14N strand serves as a template for a new strand synthesized using 15N.
This results in two DNA molecules, each consisting of one 14N strand and one 15N strand. These are Hybrid DNA molecules.
- Total DNA molecules: 2^1 = 2
- Hybrid DNA molecules: 2
- Heavy DNA molecules: 0
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Second Generation (after 2 replications in 15N medium):
The two hybrid DNA molecules from the first generation replicate again in the 15N medium.
Each hybrid molecule (14N/15N) unwinds.
- The 14N strand acts as a template, pairing with a new 15N strand to form a new Hybrid molecule (14N/15N).
- The 15N strand acts as a template, pairing with a new 15N strand to form a new Heavy molecule (15N/15N). Since there were 2 hybrid molecules, they will produce 2 hybrid and 2 heavy molecules.
- Total DNA molecules: 2^2 = 4
- Hybrid DNA molecules: 2
- Heavy DNA molecules: 2
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Generalizing for 'n' Generations:
Notice a pattern:
- The two original 14N strands (from the very first DNA molecule) will always serve as templates. Each time they replicate in the 15N medium, they will pair with a newly synthesized 15N strand. This means there will always be exactly 2 Hybrid DNA molecules, regardless of how many generations pass, as long as the replication continues in the 15N medium. …
- CBSE 2024Set E1 markMCQQ.Which of the following statements is incorrect about DNA replication?(a) DNA replication is semi-conservative(b) Main enzyme for DNA replication is DNA polymerase(c) Mutation appears due to error in replication(d) Replication on both strands of DNA is continuous
›Reveal solutionSolution
DNA replication is discontinuous on the lagging strand, so the statement that both strands replicate continuously is wrong.
DNA replication is semi-conservative (each new molecule has one old and one new strand), the key enzyme is DNA polymerase, and errors during replication can lead to mutations — so statements (a), (b) and (c) are correct. However, because DNA polymerase synthesises new DNA only in the 5'→3' direction, only the leading strand is made continuously; the lagging strand is synthesised discontin …
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