Q.To separate repetitive DNA from bulk genomic DNA, which of the following method is used ? (A) Elution (B) Southern Blotting (C) Polymerase Chain Reaction (D) Density Gradient Centrifugation

Showing the 12 most recent of 18 on this concept.

1. CBSE 2026Set 57/2/11 markMCQ
   Q.To separate repetitive DNA from bulk genomic DNA, which of the following method is used ? (A) Elution (B) Southern Blotting (C) Polymerase Chain Reaction (D) Density Gradient Centrifugation

   ›Reveal solutionSolution

   Density gradient centrifugation is the method used to separate repetitive DNA from bulk genomic DNA based on differences in buoyant density.

   To understand why density gradient centrifugation is the correct choice, we need to step back and think about what repetitive DNA actually is and how it differs from the rest of the genome. In eukaryotic cells, a large portion of the DNA consists of sequences that are repeated many times — these are called repetitive DNA. They can be highly repetitive (like satellite DNA) or moderately repetitive. The rest is unique or "bulk" genomic DNA, which contains most of the genes.

   The key physical property that distinguishes repetitive DNA from bulk DNA is its base composition. Repetitive sequences, especially satellite DNA, often have a different ratio of guanine-cytosine (GC) to adenine-thymine (AT) pairs compared to the rest of the genome. This difference in base composition leads to a difference in buoyant density — GC-rich DNA is denser than AT-rich DNA.

   Now, density gradient centrifugation exploits exactly this property. In this technique, genomic DNA is placed in a solution of a dense substance like cesium chloride (CsCl) and spun at very high speeds in an ultracentrifuge. Over time, the CsCl forms a gradient of increasing density from top to bottom. The DNA molecules migrate to the position in the gradient where their own density matches that of the CsCl solution. Because repetitive DNA has a different density from bulk DNA, it forms a separate band — often called a "satellite band" — distinct from the main band of bulk DNA. This is why such repetitive DNA is also called satellite DNA.

   Note

   The term "satellite DNA" comes directly from this technique — the extra band appears as a "satellite" next to the main DNA band in the centrifuge tube.

   Let us briefly examine why the other options are not suitable for this purpose:

   • Elution is a general term for washing or extracting a substance from a solid support, often used in chromatography. It does not separate DNA based on density or sequence repetition.
   • Southern Blotting is a technique for detecting specific DNA sequences after they have been separated by gel electrophoresis and transferred to a membrane. It is used for identification, not for the initial separation of repetitive from bulk DNA.
   • Polymerase Chain Reaction (PCR) amplifies specific DNA sequences. It cannot separate repetitive DNA from bulk DNA; in fact, it would amplify both if primers are not carefully designed.

   Important

   Density gradient centrifugation is the classical method by which satellite DNA (repetitive DNA) was first discovered and separated from the rest of the genome. This is a standard fact in NCERT.

   ✓Final answer

   In short, density gradient centrifugation is the method used to separate repetitive DNA from bulk genomic DNA because it separates DNA molecules based on their buoyant density, which differs due to variations in base composition between repetitive and non-repetitive sequences.

2. CBSE 2026Set 57/2/11 markMCQ
   Q.Which of the following is used to visualize DNA bands under UV light after gel electrophoresis ? (A) Acetocarmine (B) Safranine (C) Ethidium Bromide (D) Potassium Iodide

   ›Reveal solutionSolution

   Ethidium bromide is the fluorescent dye used to visualize DNA bands under UV light after gel electrophoresis, as it intercalates between DNA base pairs and fluoresces orange when exposed to ultraviolet radiation.

   DNA fingerprinting and recombinant DNA technology rely on a fundamental technique called gel electrophoresis to separate DNA fragments by size. The process itself is elegant: DNA molecules, being negatively charged because of their phosphate backbone, migrate through an agarose gel matrix when an electric field is applied. Smaller fragments move faster and travel farther than larger ones, creating a pattern of separated bands along the gel.

   But here's the challenge — DNA is invisible to the naked eye. Even after successful separation, you're staring at a clear gel with nothing to see. The fragments are there, sorted by size, but completely transparent. This is where visualization becomes critical.

   Ethidium bromide solves this problem through a clever chemical property. It's a planar molecule that slips between the stacked base pairs of the DNA double helix, a process called intercalation. Once nestled between those bases, ethidium bromide undergoes a dramatic change when exposed to ultraviolet light: it fluoresces bright orange. The more DNA present in a band, the more ethidium bromide molecules bind, and the brighter that band glows under UV illumination.

   The standard protocol involves either mixing ethidium bromide into the agarose gel before it sets, or soaking the gel in an ethidium bromide solution after electrophoresis is complete. When you place the gel on a UV transilluminator, the DNA bands light up as glowing orange stripes against a dark background, allowing you to photograph and analyze the banding pattern.

   Watch out

   Ethidium bromide is a potent mutagen because it intercalates into DNA. Laboratories handle it with strict safety protocols — gloves, designated containers, and proper disposal procedures are non-negotiable.

   Let's quickly rule out the other options. Acetocarmine and safranine are both biological stains used in microscopy — acetocarmine stains chromosomes during cell division studies, while safranine is a counterstain in Gram staining of bacteria. Potassium iodide is used in iodine tests for starch, turning blue-black in its presence. None of these compounds bind to DNA or fluoresce under UV light.

   ✓Final answer

   In short, Ethidium Bromide (C) is the correct answer. It intercalates into DNA and fluoresces under UV light, making the separated DNA bands visible after gel electrophoresis — an essential step in DNA fingerprinting and molecular biology techniques.

3. CBSE 2026Set 57/3/11 markMCQ
   Q.Assertion (A) : Repetitive sequences are stretches of DNA sequences that are thought to have no direct coding functions. Reason (R) : They shed light on chromosome structure, dynamics and evolution.

   ›Reveal solutionSolution

   Repetitive DNA sequences do not code for proteins, but they are far from useless — they provide crucial insights into how chromosomes are organised, how they behave during cell division, and how genomes evolve over time.

   The assertion in this question touches on a fascinating and often misunderstood part of our genome. When we think of DNA, we usually picture genes — those stretches that carry instructions for making proteins. But a very large portion of the DNA in any eukaryotic cell, including our own, does not code for any protein at all. Among this non-coding DNA, a significant chunk consists of repetitive sequences.

   These are exactly what they sound like: short stretches of nucleotides (the building blocks of DNA) that are repeated over and over again, sometimes hundreds or thousands of times. The NCERT textbook explains that these sequences do not have any direct, known coding function — meaning they are not transcribed into messenger RNA and then translated into a protein. So the assertion (A) is perfectly correct.

   But here is where it gets interesting. Just because these sequences don't code for proteins does not mean they are "junk" or useless. The reason (R) points to their real importance: they shed light on chromosome structure, dynamics, and evolution.

   Think of repetitive sequences as the "scaffolding" or "packaging" of the genome. They are found in large concentrations at certain key regions of chromosomes — for example, at the centromeres (the pinched-in middle part of a chromosome that helps in its movement during cell division) and at the telomeres (the protective caps at the ends of chromosomes). Without these repetitive sequences, chromosomes would be unstable, would not segregate properly during cell division, and would fray at the ends.

   Note

   The NCERT textbook specifically mentions that repetitive sequences are used in DNA fingerprinting. Because the number of repeats varies greatly between individuals, these sequences act like a genetic barcode — unique to each person.

   The reason (R) is therefore also correct. By studying how these repetitive sequences are arranged, how many copies exist, and how they change over time, scientists can understand:

   • How chromosomes fold and pack inside the nucleus
   • How chromosomes pair up and exchange segments during meiosis
   • How genomes have expanded or contracted over evolutionary time
   • How certain diseases (like Huntington's disease) arise when a repetitive sequence expands beyond a normal range

   Important

   The key point is that "no direct coding function" does not mean "no function at all." Repetitive sequences are essential for chromosome stability, and they are powerful tools for studying genetics and evolution.

   Now, does the reason (R) correctly explain the assertion (A)? Not exactly. The assertion states a fact about what repetitive sequences do not do (code for proteins). The reason states a fact about what they do reveal (chromosome structure, dynamics, evolution). The reason does not explain why they have no coding function — it simply adds another true statement about them. So while both statements are individually true, the reason is not the correct explanation of the assertion.

   ✓Final answer

   Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of Assertion (A). Repetitive sequences have no direct coding function, and independently, they are valuable for understanding chromosome structure and evolution.

4. CBSE 2026Set ANNUAL1 markMCQ
   Q.The technique of DNA fingerprinting is based on the principle of

   (a) Similarities in DNA sequences

   (b) Differences in DNA sequences

   (c) Polymorphism in DNA sequences

   (d) Differences in proteins

   ›Reveal solutionSolution

   DNA fingerprinting detects differences (polymorphisms) in repetitive DNA sequences that are unique to each individual, allowing identification.

   DNA fingerprinting, developed by Alec Jeffreys, is based on polymorphism — variability — in DNA sequences, specifically in repetitive DNA regions such as satellite DNA (VNTRs, variable number of tandem repeats). These repeat sequences vary hugely in copy number between individuals (except identical twins), even though the surrounding gene sequences may be very similar. A probe complementary to these hypervariable regions, combined with techniques like Southern hybridisation and gel electrophoresis, reveals a pattern of bands unique to each person — a 'DNA fingerprint' — useful in forensics and paternity testing.

   It is not based on similarities in sequence, nor directly on protein differences — the technique works at the DNA level, detecting sequence-length polymorphism.

   ✓Final answer

   (c) Polymorphism in DNA sequences.

5. CBSE 2026Set ANNUAL1 mark
   Q.Give the name of key factor in DNA profiling.

   ›Reveal solutionSolution

   VNTRs (variable-number tandem-repeat sequences) are the polymorphic DNA regions that give each person a unique DNA fingerprint.

   DNA profiling (DNA fingerprinting), developed by Alec Jeffreys, is based on the analysis of highly polymorphic (variable) repetitive DNA sequences called VNTRs (Variable Number of Tandem Repeats), also known as minisatellite DNA. These sequences consist of a short core nucleotide sequence repeated a variable number of times in tandem at specific loci, and the number of repeats differs greatly from person to person (except identical twins). Because this variability is inherited and highly individual-specific, comparing the pattern of VNTR-containing restriction fragments (obtained via restriction digestion, gel electrophoresis, Southern blotting and hybridization with a labelled probe) between individuals generates a distinctive banding pattern — the 'DNA fingerprint' — used in forensic identification, paternity testing and other applications.

   ✓Final answer

   VNTRs (Variable Number of Tandem Repeats), also called minisatellites/satellite DNA — highly variable, repetitive non-coding DNA sequences that differ in copy number between individuals — are the key factor exploited in DNA profiling (DNA fingerprinting).

6. CBSE 2024Set D1 mark
   Q.Fill in the blank: DNA finger printing involves identifying differences in some specific regions in DNA sequence called as ______.

   ›Reveal solutionSolution

   DNA fingerprinting identifies differences between individuals based on the variable number of repeats of specific short DNA sequences, called VNTRs (Variable Number of Tandem Repeats) or satellite DNA.

   Besides the coding, single-copy genes, the genome contains large stretches of repetitive DNA in which a short sequence is repeated many times, forming satellite DNA. These repetitive sequences can be separated from the bulk of genomic DNA as distinct peaks (satellite peaks) during density-gradient centrifugation, and are further classified as satellite, minisatellite, and microsatellite DNA based on base-pair length and number of repetitions.

   The number of repeat units at a given satellite DNA locus (VNTR) is highly variable from person to person — this variability, or polymorphism, forms the basis of DNA fingerprinting, first developed by Alec Jeffreys. A VNTR probe hybridises with these repetitive regions on a Southern blot, producing an individual-specific banding pattern, which is exploited in forensic science, paternity testing, and diversity/evolutionary studies.

   ✓Final answer

   Repetitive DNA sequences called VNTRs (Variable Number of Tandem Repeats) / satellite DNA.

7. CBSE 2024Set ANNUAL1 markMCQ
   Q.The technique of DNA fingerprinting was initially developed by

   (a) G. J. Mendel

   (b) Alec Jeffreys

   (c) Francois Jacob

   (d) Jacques Monod

   ›Reveal solutionSolution

   DNA fingerprinting, which identifies differences in repetitive (satellite) DNA sequences between individuals, was developed by the British geneticist Alec Jeffreys.

   DNA fingerprinting exploits variable number tandem repeats (VNTRs) in satellite DNA, which differ in copy number from person to person (except identical twins), to generate an individual-specific DNA profile. It was developed by Alec Jeffreys in 1984 and, in India, further refined for forensic use by Dr. Lalji Singh's group at the CCMB, Hyderabad. G. J. Mendel is the father of genetics (laws of inheritance), while Francois Jacob and Jacques Monod proposed the operon model of gene regulation - neither is linked to DNA fingerprinting.

   ✓Final answer

   (b) Alec Jeffreys.

8. CBSE 2023Set 57/1/11 markMCQ
   Q.Study the DNA profiles obtained as a result of DNA fingerprinting of a child 'X' and three individuals 1, 2 and 3. Which one of the following options shows the possible parents of the child 'X' ?

   (a) 1 and 2

   (b) 2 and 3

   (c) 1 and 3

   (d) Only individual 3

   ›Reveal solutionSolution

   DNA fingerprinting works because a child inherits exactly half their DNA bands from each biological parent. Reading the real gel image, the correct parent pair is whichever two individuals' bands together account for every one of child X's bands.

   DNA fingerprinting works on a simple principle: every person's DNA profile is a unique combination of bands, and a child receives exactly 50% of these bands from the mother and 50% from the father. Every single band visible in a child's lane must therefore be traceable to either one parent or the other.

   The key insight: if you line up the child's bands against potential parents, the true biological parents will together account for all of the child's bands, with no band left unexplained. A band in the child that appears in neither of two proposed parents rules out that pairing immediately.

   Here is how to work through it systematically, using the real gel image:

   1. Identify every band in child X's profile. Note the position of each band in the child's lane — these are the markers the child must have inherited from someone.
   2. Check each band against individual 1. For every band in X, see whether it also appears in individual 1's lane.
   3. Check the remaining bands against individual 2. Take the bands in X that did not match individual 1, and see whether individual 2's lane accounts for them. If so, (1, 2) could be the parent pair.
   4. Repeat for the pairing (2, 3) and then (1, 3) — for each, check whether the two lanes together explain every band in X.
   5. Evaluate "only individual 3." A single individual accounting for all of a child's bands would mean uniparental inheritance, which nuclear DNA fingerprinting does not show — this option is only correct if it is the sole lane containing every one of X's bands, which is not how biparental inheritance works, so it is the least likely answer on principle.

   Watch out

   Don't assume the parent with more matching bands is automatically correct — both members of the true pair must together account for all of the child's bands. A single unmatched band disqualifies a pairing.

   Note

   This question is answered by reading the band pattern directly off the gel-profile diagram printed in the paper. That image is not yet part of this platform's transcription of the question, so which specific pair — (a), (b), or (c) — matches cannot be confirmed here without it.

   ✓Final answer

   Apply the band-matching method above directly to the printed gel image: the correct pair of parents is whichever two individuals' lanes, together, account for every band present in child X's profile.

9. CBSE 2023Set 57/3/11 markMCQ
   Q.DNA profiles of the child and three individuals 1, 2 and 3 who claim to be the parents of the child are given below. Select the option that shows the correct actual parent/parents of the child. [Figure: DNA profile gel showing bands for Child X, Individual 1, Individual 2, Individual 3]

   (a) Individual 1 and 3

   (b) Individual 1 and 2

   (c) Individual 2 and 3

   (d) Individual 1 is the only parent of the child amongst 1, 2 and 3

   ›Reveal solutionSolution

   A child inherits exactly one allele per locus from each biological parent, so every band in the child's DNA profile must match a band in one parent or the other. Find which pair of individuals' combined bands account for all of the child's bands — that pair are the true parents.

   Why DNA profiling reveals parentage

   DNA profiling (typically using STR markers — short tandem repeats) produces a banding pattern where each band represents an allele at a particular genetic locus. Because we inherit one chromosome from each parent, every band in a child's profile must come from either the mother or the father.

   The logic is simple: scan each band in the child's lane. If a band appears in the child, at least one parent must also show that band. If a supposed parent pair leaves some of the child's bands unaccounted for by either individual, they cannot be the biological parents. The true parents, together, will account for every single band in the child.

   Step-by-step matching method

   1. List the child's bands. Note every visible band in Child X's lane (count from top to bottom, or note their positions).
   2. Check Individual 1. Which of the child's bands appear in Individual 1? Mark those as "explained by 1."
   3. Check Individual 2. Which of the child's bands appear in Individual 2? Mark those as "explained by 2."
   4. Check Individual 3. Which of the child's bands appear in Individual 3? Mark those as "explained by 3."
   5. Test each pair against the four options:
      • (a) 1 and 3 — do their combined bands cover every band in the child, with none left over?
      • (b) 1 and 2 — same test.
      • (c) 2 and 3 — same test.
      • (d) 1 alone — can Individual 1 account for every band by themself? (Only plausible if the child were a clone/parthenogenetic offspring — biologically implausible in normal human reproduction.)
   6. Eliminate mismatches. Any combination that leaves even one child band unaccounted for is ruled out. Any combination that requires a band not present in either proposed parent is also ruled out.

   Tip

   In a typical gel, look for unique bands in the child — bands that appear in only one of the three individuals. Those unique bands immediately tell you that individual must be a parent.

   Watch out

   A common mistake is to pick a pair simply because they share some bands with the child. You must verify that every band in the child is present in at least one of the two proposed parents, and that no band is left orphaned.

   Applying this to the exam's figure

   The correct pair is determined entirely by the actual band positions printed in the paper's gel diagram — walk through steps 1–6 above against that image to find which of options (a)–(d) is the one combination that fully explains Child X's profile. A confident final letter is deliberately not asserted here, since doing so without the real band pattern in front of you would risk teaching a guess as fact; the method above is exactly what you need to apply to your own printed figure to get the right answer with certainty.

   ✓Final answer

   Use the matching method above against the gel diagram in your paper: the correct parents are the pair whose combined bands explain every band in Child X's profile, with none left unaccounted for.

10. CBSE 2023Set ANNUAL1 markMCQ
    Q.Satellite DNA is a useful tool of

    (a) sex determination

    (b) organ culture

    (c) forensic science

    (d) tissue culture

    ›Reveal solutionSolution

    Satellite DNA underlies DNA fingerprinting, used in forensic science.

    Satellite DNA is made up of short base sequences repeated many times (repetitive DNA), including minisatellites and microsatellites (VNTRs). The number of repeats varies greatly from person to person, so the pattern is essentially individual-specific (except identical twins). This polymorphism is exploited in DNA fingerprinting to establish identity, parentage and to solve crimes, making satellite DNA a powerful tool of forensic science.

    ✓Final answer

    (c) forensic science.

11. CBSE 2022Set HE2201 markMCQ
    Q.Choose the correct answer: The first scientist who developed DNA fingerprint -

    (a) Wilkins

    (b) Eyan Willmoott

    (c) Alex Jeffrey

    (d) Kary Mullis

    ›Reveal solutionSolution

    Alec Jeffreys developed DNA fingerprinting in 1984 by exploiting highly variable repetitive DNA regions unique to each individual.

    In 1984, British geneticist Sir Alec Jeffreys at the University of Leicester developed the technique of DNA fingerprinting, which detects variation (polymorphism) in the number of repetitive DNA sequences (VNTRs/minisatellites) that differ between individuals but are inherited from parents, allowing identification of a person and establishment of parentage. This forms the basis of DNA profiling used today in forensic science, paternity testing, and identifying the source of biological samples. (In India, Dr. Lalji Singh at CCMB, Hyderabad, developed a similar indigenous DNA fingerprinting technique.)

    ✓Final answer

    (c) Alex Jeffrey — correctly, Sir Alec Jeffreys, who pioneered DNA fingerprinting in 1984.

12. CBSE 2022Set ANNUAL1 mark
    Q.If an inheritable mutation is observed in population at high frequency, it is called DNA ______.

    ›Reveal solutionSolution

    An inheritable mutation present at high frequency in a population is called DNA polymorphism.

    When a variation (mutation) at a DNA locus is inherited and is found in a population with a frequency greater than 0.01 (1%), it is called DNA polymorphism. Such variations, especially in non-coding regions, arise and accumulate over generations. They form the basis of DNA fingerprinting and of tracing evolutionary/genetic diversity.

    ✓Final answer

    DNA polymorphism.