MPSC 2026 Prelims Paper-I · Question 60 of 100

Correct — option (4), "All of the above". Statement (a) is the definition of the thing. Jumping genes are segments of DNA that are able to move from one position in the genome to another, either by excising themselves and reinserting elsewhere or by copying themselves and inserting the copy. Movement is what makes them what they are, and it was the mobility that made them so hard to believe when they were first described, at a time when the chromosome was pictured as a fixed and orderly string of genes. Statement (b) is the standard synonym. 'Jumping gene' is the informal name; the technical terms are transposon and transposable element. Two broad classes are recognised. Class II elements, the DNA transposons, move by a cut-and-paste mechanism using an enzyme called transposase encoded by the element itself. Class I elements, the retrotransposons, move by copy-and-paste through an RNA intermediate that is reverse-transcribed back into DNA and inserted at a new site, so the original stays where it was and the copy number grows. Transposable elements are abundant: in the human genome a large fraction of the DNA, commonly put at around 45 per cent, is derived from them, with the LINE-1 and Alu families the most numerous. Statement (c) attributes the discovery to Barbara McClintock, and the Commission's final key counts all three statements among the correct ones, which is why option (4) is the credited answer. The attribution itself is solid and is the part of the statement that carries the examinable content. McClintock worked on maize, and it was the maize kernel that gave her the evidence: kernels on a single ear showed speckled and streaked patterns of pigment rather than uniform colour, and she traced the pattern to genetic elements that moved into and out of pigment genes during the development of the kernel, switching them off and on. She named the two she characterised Dissociation and Activator. The finding was received with considerable scepticism for many years, until molecular work in bacteria and in other organisms during the 1960s and 1970s showed that mobile elements were general. She was awarded the Nobel Prize in Physiology or Medicine in 1983 for the discovery of mobile genetic elements, as the sole recipient of the prize that year. On the option structure, note what is and is not available. Each of the three statements appears in exactly three of the four choices, so no single statement eliminates the field by itself. What does narrow it quickly is a pair: statements (a) and (b) are the definition and the synonym, and a candidate confident of both is left with only option (1), which takes just those two, and option (4), which takes all three. The question then reduces to whether statement (c) is counted, and on the Commission's final key it is, so the answer is option (4). A general word about this option, since it appears eleven times in this booklet. 'All of the above' is not a safe default and not a suspect one; it is credited exactly when the key counts every listed item as correct, and it fails the moment one item is excluded. Treat it as a claim to be checked item by item like any other.

• (1)(a) and (b) — Option (1) takes the definition and the synonym, both of which are correct, but excludes statement (c) and so denies the attribution of the discovery to Barbara McClintock. That attribution is not in doubt — she identified mobile genetic elements in maize, the work for which she received the Nobel Prize in Physiology or Medicine in 1983 — and the Commission's final key counts statement (c) among the correct ones. This is the closest rival to the credited answer, and the whole question turns on whether statement (c) is admitted.
• (2)(b) and (c) — Option (2) drops statement (a), which is the definition of a jumping gene — a genetic element able to change its position in the genome. Nothing about transposons is more securely established than their mobility; it is the property that names them, the property McClintock inferred from the speckled maize kernel, and the property that distinguishes them from ordinary genes at fixed loci. An option that excludes the defining characteristic of the subject cannot be right.

Transposable elements are DNA sequences capable of relocating within a genome. They fall into two classes by mechanism. DNA transposons, Class II, encode a transposase that excises the element and inserts it at a new site — cut and paste, leaving the copy number unchanged. Retrotransposons, Class I, are transcribed into RNA, reverse-transcribed back into DNA and inserted elsewhere — copy and paste, so each round of movement adds a copy and the family expands. Their effects on the genome are substantial. An element landing inside a gene can disrupt it, producing an insertional mutation; landing near a gene it can alter that gene's expression by bringing its own regulatory sequences alongside. Repeated copies scattered through a genome provide sites for unequal crossing over, generating deletions, duplications and rearrangements. Over evolutionary time transposable elements have been a major engine of genome change, and a good deal of the repetitive DNA in large eukaryotic genomes is transposon-derived. In the laboratory they are used as tools — for insertional mutagenesis, for gene tagging, and as vectors, the P element of Drosophila being the classic example.

The story of the discovery is itself examinable and is worth knowing as a story rather than as a date. Barbara McClintock was a maize geneticist, and maize was the ideal organism for the work because each kernel on an ear is a separate offspring whose pigmentation can be inspected directly. She observed kernels that were not uniformly coloured but speckled and streaked, and she showed that the patterns arose because genetic elements were moving into and out of pigment genes during kernel development, silencing a gene in one cell lineage and releasing it in another. She characterised two interacting elements, which she called Dissociation and Activator. The conclusion — that the genome was not a fixed map but contained elements that moved — ran against the settled view of the chromosome and was resisted for a long time. Confirmation came from the finding of mobile elements in bacteria and in other organisms in the following decades, and the recognition eventually followed: the Nobel Prize in Physiology or Medicine in 1983, awarded to her alone, for the discovery of mobile genetic elements. She remains the only woman to have received an unshared Nobel Prize in that category.

• Jumping genes, formally transposons or transposable elements, are DNA sequences that can change their position within the genome.
• Two classes are recognised: Class II DNA transposons move by cut and paste using a transposase they encode, while Class I retrotransposons move by copy and paste through an RNA intermediate that is reverse-transcribed, so their copy number grows with each round.
• Barbara McClintock discovered mobile genetic elements while working on maize, tracing the speckled pigmentation of maize kernels to elements moving into and out of pigment genes; she named the two she characterised Dissociation and Activator.
• She was awarded the Nobel Prize in Physiology or Medicine in 1983 for the discovery of mobile genetic elements, as the sole recipient of the prize in that category that year.
• Transposable elements are abundant in eukaryotic genomes — a large fraction of human DNA, commonly put at around 45 per cent, is transposon-derived, with the LINE-1 and Alu families the most numerous — and they cause insertional mutations, alter gene expression and drive genome rearrangement.

• Dates attached to a discovery vary widely across exam sources, and they are the least stable part of a statement of this kind. Anchor the fact on what every source agrees on — the name Barbara McClintock, the organism maize, and the 1983 Nobel Prize in Physiology or Medicine.
• Assuming that a statement asserting two names refer to the same thing must be a trick. 'Jumping gene' and 'transposon' are simply the informal and technical names for the same class of elements.
• Confusing the two classes of transposable element. DNA transposons cut and paste, leaving copy number unchanged; retrotransposons copy and paste through an RNA intermediate, so their numbers grow.
• Treating 'All of the above' as either a safe default or an automatic trap. It is credited exactly when every listed statement is counted correct, and it must be checked item by item like any other option.

Molecular genetics enters MPSC's general science section mostly through discoveries, discoverers and definitions rather than through mechanism. Expect the scientist-and-discovery pairing — McClintock and jumping genes, Watson and Crick and the double helix, Mendel and the laws of inheritance — often with a year attached. Expect definitional statements of the kind in statements (a) and (b) here, which are the reliable half of such questions. And expect application questions on recombinant DNA, genetically modified crops and gene therapy. Since the name-and-discovery pairing is the stable element and the year is the volatile one, the efficient preparation is to build a list of scientist, discovery and organism, and to attach the Nobel year where there is one, because Nobel years are documented precisely and are the dates most often asked.

No directly related past PYQ was found.

• practice — not a real PYQ

  Barbara McClintock's discovery of mobile genetic elements was made through her work on which organism ?

  • (a)Drosophila melanogaster
  • (b)Maize
  • (c)Escherichia coli
  • (d)Pisum sativum

  Answer(b) Maize. Each kernel on an ear of maize is a separate offspring whose pigmentation can be inspected directly, and it was the speckled and streaked colour patterns of maize kernels that led McClintock to elements moving into and out of pigment genes during kernel development. Drosophila and E. coli were the workhorses of other classical genetic discoveries, and Pisum sativum, the garden pea, was Mendel's material.

• practice — not a real PYQ

  Which of the following best describes the mechanism of a retrotransposon ?

  • (a)It is excised from one site and inserted at another, leaving the copy number unchanged
  • (b)It is transcribed into RNA, reverse-transcribed into DNA and inserted at a new site, increasing the copy number
  • (c)It moves between cells through a viral particle
  • (d)It exchanges position with a homologous gene on the sister chromatid

  Answer(b) It is transcribed into RNA, reverse-transcribed into DNA and inserted at a new site, increasing the copy number — the copy-and-paste route of Class I elements. Option (a) describes the cut-and-paste mechanism of Class II DNA transposons, which use a transposase and do not multiply as they move. The distinction between the two classes, and its consequence for copy number, is the standard discriminating point in questions on transposable elements.