Which one of the following is the correct chronology of invention/discovery in the field of Biotechnology?
- (a)Discovery of Plasmid, Double Helical structure of DNA, DNA fingerprinting, cloning of Dolly
- (b)Double Helical structure of DNA, Discovery of Plasmid, DNA fingerprinting, cloning of Dolly
- (c)DNA fingerprinting, cloning of Dolly, Double Helical structure of DNA, Discovery of Plasmid
- (d)Double Helical structure of DNA, DNA fingerprinting, Discovery of Plasmid, cloning of Dolly
Correct — A, Discovery of Plasmid, Double Helical structure of DNA, DNA fingerprinting, cloning of Dolly. Fix the four dates and the sequence follows. Joshua Lederberg proposed the term plasmid in 1952 for extrachromosomal genetic elements in bacteria. James Watson and Francis Crick published the double-helical structure of DNA in Nature in April 1953, working from Rosalind Franklin's and Maurice Wilkins's X-ray diffraction data. Alec Jeffreys developed DNA fingerprinting at Leicester in 1984 and it was first used in a criminal case in 1986. Dolly the sheep, the first mammal cloned from an adult somatic cell, was born at the Roslin Institute in July 1996 and announced in 1997. Only this option puts plasmid ahead of the double helix, which is where the item is decided.
- (b)Double Helical structure of DNA, Discovery of Plasmid, DNA fingerprinting, cloning of Dolly — This is the intuitive order, and it is wrong by one year at the front. The double helix is 1953 while the plasmid concept dates from 1952, so the plasmid comes first. Everything after that is correctly placed, which is what makes this the strongest distractor.
- (c)DNA fingerprinting, cloning of Dolly, Double Helical structure of DNA, Discovery of Plasmid — This inverts the whole sequence, putting the 1980s and 1990s work ahead of the 1950s. DNA fingerprinting and cloning both depend on knowing the structure of DNA, so they cannot precede it.
- (d)Double Helical structure of DNA, DNA fingerprinting, Discovery of Plasmid, cloning of Dolly — This places DNA fingerprinting before the discovery of the plasmid, that is 1984 before 1952. It also separates the two 1950s milestones by three decades.
Modern biotechnology rests on a short and well-dated sequence of discoveries. The structure of DNA in 1953 made molecular genetics possible; plasmids gave molecular biologists their first vectors for carrying foreign genes into bacteria; restriction enzymes discovered in the late 1960s and early 1970s gave the cutting tools, and the first recombinant DNA molecules followed in 1972-73. DNA fingerprinting turned the variability of non-coding repeats into an identification method, and somatic-cell nuclear transfer showed that a differentiated nucleus could be reprogrammed to build a whole animal.
Chronology items are decided at the closest pair, and here that pair is plasmid against double helix — 1952 against 1953. Everything else in the list is separated by decades and cannot be confused. The reason the 1952 date is easy to miss is that plasmids became famous only in the 1970s, when they were adopted as cloning vectors, so the concept feels much later than the structure of DNA. Anchor the two dates side by side and the item takes seconds.
- Joshua Lederberg introduced the term plasmid in 1952 for extrachromosomal genetic elements.
- Watson and Crick published the double-helical structure of DNA in April 1953.
- Alec Jeffreys developed DNA fingerprinting in 1984-85; it was first used in a criminal investigation in 1986.
- Dolly the sheep was born in July 1996 by somatic-cell nuclear transfer at the Roslin Institute.
- Plasmids became the standard vectors of gene cloning in the 1970s, two decades after they were named.
- 1952 — Lederberg proposes the term plasmid for extrachromosomal bacterial elements
- 1953 — Watson and Crick publish the double helix, using Franklin and Wilkins X-ray data
- 1984-85 — Jeffreys develops DNA fingerprinting at Leicester
- 1996 — Dolly is born at the Roslin Institute by somatic-cell nuclear transfer
The whole item turns on the single year between 1952 and 1953.
- Assuming the double helix must precede everything, including the naming of plasmids.
- Dating Dolly to 1997, the year of the announcement, rather than 1996, the year of birth.
- Confusing DNA fingerprinting with the Human Genome Project of 1990 to 2003.
A chronology item decided by one adjacent pair. Three options can be struck out on decades, and the fourth comparison is a single year apart.
Assertion (A): Dolly was the first cloned mammal. Reason (R): Dolly was produced by in-vitro fertilization.
- (a) Both A and R are true, and R is the correct explanation of A
- (b) Both A and R are true, but R is not a correct explanation of A
- (c) A is true, but R is false
- (d) A is false, but R is true
Answer(c) A is true, but R is false
The last milestone in this chronology, examined for how it was done. Dolly was made by transferring an adult cell nucleus into an emptied egg, not by fertilisation, and that distinction is what makes her the landmark she is.
- practice — not a real PYQ
Dolly the sheep was produced by which technique?
- (a)In-vitro fertilisation
- (b)Somatic-cell nuclear transfer
- (c)Artificial insemination
- (d)Embryo splitting
Answer(b) Somatic-cell nuclear transfer — an adult udder cell nucleus was placed in an enucleated egg.
- practice — not a real PYQ
The double-helical structure of DNA was published in 1953 by
- (a)Mendel and Morgan
- (b)Watson and Crick
- (c)Jeffreys and Sanger
- (d)Lederberg and Tatum
Answer(b) Watson and Crick — using X-ray diffraction data from Franklin and Wilkins.