Biology · Molecular Basis of Inheritance · NEET
'Semi' means half and 'conservative' means kept or saved. So semiconservative means each new DNA molecule saves (conserves) half of the old DNA. That half is one full parental strand. The other strand is brand new. So every daughter DNA = 1 old strand + 1 new strand. This is the model NCERT and NEET always test.
There were 3 possible models. Conservative: the old DNA stays fully together and a totally new DNA is made separately (0 old + 2 new in the copy). Semiconservative: each new DNA has 1 old strand + 1 new strand. Dispersive: old and new pieces are mixed in both strands. Meselson and Stahl proved the semiconservative model is correct.
Watson and Crick PROPOSED it in 1953, right after giving the double helix model. They said the two strands separate and each acts as a template for a new strand. Matthew Meselson and Franklin Stahl PROVED it in 1958 using E. coli and heavy nitrogen (15N). Do not mix these two: proposed = Watson-Crick, proved = Meselson-Stahl. NEET asks this every few years.
Because of complementary base pairing (A pairs with T, G pairs with C). If the two strands separate, each old strand already carries the information to build its exact partner. So each strand can act as a template (mould) to make a new complementary strand. This is why the double helix 'suggested a copying mechanism', as Watson and Crick wrote in 1953.
It was first shown in the bacterium Escherichia coli (E. coli) by Meselson and Stahl in 1958. Later it was also shown in higher organisms, such as plants (Taylor used Vicia faba) and human cells. So the FIRST proof was in a bacterium, not a plant. This exact point was a NEET 2018 answer.
Start with 1 DNA molecule = 2 old strands. After one round of replication you get 2 DNA molecules = 4 strands total. Of these, 2 strands are old (one in each daughter DNA) and 2 strands are newly made. The old strands are conserved, just shared out. This counting logic is used in NEET numerical problems with 15N and 14N.
The experimental proof for semiconservative replication of DNA was first shown in a
Taylor conducted the experiments to prove semiconservative mode of chromosome replication on
Match List I with List II. A. Frederick Griffith B. Francois Jacob & Jacque Monod C. Har Gobind Khorana D. Meselson & Stahl with 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.
It is semiconservative. Each daughter DNA has one old strand and one new strand. The conservative model (old DNA fully kept, new DNA fully separate) was disproved by the Meselson-Stahl experiment.
Watson and Crick proposed it in 1953 based on their double helix model. Meselson and Stahl gave the experimental proof in 1958 using E. coli grown in 15N and 14N media.
Because the two strands separate and each old strand acts as a template to build a new complementary strand. So each new double helix keeps one parental strand. This was confirmed by the density (15N/14N) results of Meselson and Stahl.
Each separated parental strand acts as the template. Complementary base pairing (A-T, G-C) lets each old strand direct the exact sequence of its new partner strand.
After 2 rounds you get 4 DNA molecules from 1. The 2 original old strands are still there, one in each of 2 molecules. So 2 out of 4 daughter molecules contain an old (parental) strand; the other 2 are fully new. This logic is used in NEET 15N counting questions.
