Meselson and Stahl: how does DNA copy itself?
"Meselson and Stahl: how does DNA copy itself?" is a free, interactive Class 12 Biology simulation for NEET, built on the NCERT section "Replication" in Molecular Basis of Inheritance. ¹⁵N cells moved to ¹⁴N: CsCl band positions and shares each generation under semiconservative, conservative and dispersive models. NEET has asked about this idea 3 times in 10 years of papers.
From the lesson Replication in Molecular Basis of Inheritance. Change the values and watch what happens.
What it shows
NCERT § Replication
¹⁵N cells moved to ¹⁴N: CsCl band positions and shares each generation under semiconservative, conservative and dispersive models.
Controls
3 to try
- Slidergenerations in ¹⁴N 0–4 (20 min each)
- Togglemodel — semiconservative / conservative / dispersive
- Readoutheavy/hybrid/light band intensities
Try this
Find the generation where light:hybrid DNA is 3:1 under the semiconservative model, and the model that never gives a hybrid band.
Common mistake
Expecting a heavy band to persist after one generation.
In NEET
10 years of papers
Asked 3 times in 10 years of NEET.
See the past questions: DNA Replication, Molecular Basis of Inheritance (chapter)
The idea behind it
NCERT § "Replication"
- Watson and Crick proposed semiconservative replication: the strands separate and each acts as a template, so every new molecule has one parental and one newly made strand.
- Meselson and Stahl (1958) grew E. coli for many generations in ¹⁵NH₄Cl so their DNA became heavy, then moved them to ¹⁴NH₄Cl. DNA was separated by density in a caesium chloride (CsCl) gradient.
- E. coli divides in 20 minutes. After one generation (20 min) all DNA had hybrid density. After two generations (40 min) there were equal amounts of hybrid and light DNA, as predicted by the semiconservative model.
- Taylor and colleagues (1958) used radioactive thymidine on Vicia faba (faba beans) and showed that DNA in chromosomes also replicates semiconservatively.
- The main enzyme is DNA-dependent DNA polymerase. E. coli, with 4.6 × 10⁶ bp, finishes replication in about 18 minutes, an average of about 2000 bp per second, with high accuracy.
- Deoxyribonucleoside triphosphates act both as substrates and as the energy source, from their two terminal high-energy phosphates.
- The helix cannot open fully at once, so replication happens in a small opened region called the replication fork. It begins at a defined site, the origin of replication.
- DNA polymerase adds nucleotides only in the 5′→3′ direction. So on the template with 3′→5′ polarity synthesis is continuous, while on the template with 5′→3′ polarity it is discontinuous, and the short pieces made there are later sealed together by DNA ligase.
- In eukaryotes DNA replicates in the S phase of the cell cycle. If replication is not followed by cell division, polyploidy results.
Questions
4 answers
- What does the "Meselson and Stahl: how does DNA copy itself?" simulation show?
- ¹⁵N cells moved to ¹⁴N: CsCl band positions and shares each generation under semiconservative, conservative and dispersive models.
- What mistake does the "Meselson and Stahl: how does DNA copy itself?" simulation fix?
- A common mistake it clears up: expecting a heavy band to persist after one generation.
- Has NEET asked about this idea?
- Yes. NEET has asked about this idea 3 times in 10 years of papers.
- Is this simulation free?
- Yes. It is free and runs in the browser on a phone or a laptop. It belongs to the lesson "Replication" in Molecular Basis of Inheritance (Class 12 Biology).
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