Identify the correct statements about jumping genes.
(a) These are genes that can change their positions.
(b) These are also called transposons.
(c) These were firstly discovered by Barbara McClintock

Question

Identify the correct statements about jumping genes.
(a) These are genes that can change their positions.
(b) These are also called transposons.
(c) These were firstly discovered by Barbara McClintock in 1965.

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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.

Why the other options are wrong — (a) 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.  (b) 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.

Concept: 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.

State PCS 2026 Q60 — Science & Technology PYQ with Answer

State PCS 2026 Q60 — Science & Technology PYQ with Answer