Biology · Molecular Basis of Inheritance · NEET
It proved that DNA replication is semiconservative. This means each new DNA molecule keeps one old (parental) strand and makes one new strand. It does not prove conservative or dispersive replication. For NEET, remember: first experimental proof of semiconservative replication, shown in E. coli.
Nitrogen is present in the nitrogen bases of DNA. 15N is a heavy isotope and 14N is the normal light isotope. Growing E. coli in 15N makes the DNA heavy; shifting to 14N makes new strands light. This weight difference lets them tell old DNA from new DNA. Note: 15N is not radioactive, it is just heavier.
After one round of replication in 14N medium, every DNA molecule has one heavy 15N old strand and one light 14N new strand. Its density is exactly between heavy and light, so it is called hybrid or intermediate DNA. Seeing a single hybrid band after one generation is the key proof of semiconservative replication.
They used density gradient centrifugation in a cesium chloride (CsCl) solution. Heavier DNA settles lower and lighter DNA stays higher, forming bands at different positions. The position of the band tells you whether the DNA is heavy, light, or hybrid.
It was first done in a bacterium, Escherichia coli (E. coli). Only later was semiconservative replication shown in higher organisms such as plants (Taylor's work on Vicia faba). NEET has directly asked this: the answer is Bacterium, not Plant.
After one generation (20 min): all DNA is hybrid (one band, intermediate density). After two generations (40 min): half the DNA is hybrid and half is light, giving two bands. The heavy 15N strands are never destroyed, they are just diluted into more molecules.
The experimental proof for semiconservative replication of DNA was first shown in a
Ten E. coli cells with 15N-dsDNA are incubated in a medium containing 14N nucleotides. After 60 minutes, how many E. coli cells will have DNA totally free from 15N?
Match List I with List II. A. Frederick Griffith B. Francois Jacob and Jacque Monod C. Har Gobind Khorana D. Meselson and Stahl — I. Genetic code II. Semi-conservative mode of DNA replication III. Transformation IV. Lac operon
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Matthew Meselson and Franklin Stahl performed it in 1958 using the bacterium Escherichia coli (E. coli).
No. 15N is a stable heavy isotope of nitrogen, not a radioactive one. It only makes the DNA heavier so it can be separated by density.
Density gradient centrifugation using cesium chloride (CsCl). DNA forms bands based on its weight (density).
A single hybrid band rules out conservative replication (which would give two separate bands, heavy and light). It confirms each daughter DNA has one old and one new strand, which is semiconservative.
Meselson and Stahl used 15N/14N in E. coli DNA. Taylor used radioactive tritiated (3H) thymidine in the plant Vicia faba to prove chromosomes replicate semiconservatively.
