Build a DNA double helix
"Build a DNA double helix" is a free, interactive Class 12 Biology simulation for NEET, built on the NCERT section "The double helix and central dogma" in Molecular Basis of Inheritance. Build a duplex: antiparallel strands, A=T with 2 H-bonds, G≡C with 3, 10 bp per 3.4 nm turn; Chargaff and length readouts. NEET has asked about this idea 5 times in 10 years of papers.
From the lesson The double helix and central dogma in Molecular Basis of Inheritance. Change the values and watch what happens.
What it shows
NCERT § The DNA
Build a duplex: antiparallel strands, A=T with 2 H-bonds, G≡C with 3, 10 bp per 3.4 nm turn; Chargaff and length readouts.
Controls
4 to try
- Edittop-strand sequence (up to 20 bp)
- Slider%A in a genome (Chargaff mode)
- Togglerotate helix / flat ladder
- Readoutcomplementary strand, H-bond total, length in nm, %G/%C/%T
Try this
Given 20% adenine, set the genome so the readout shows the correct %G, then build a 10-bp turn with exactly 25 hydrogen bonds.
Common mistake
Saying G–C has two hydrogen bonds and A–T three.
In NEET
10 years of papers
Asked 5 times in 10 years of NEET.
See the past questions: DNA Double Helix, Molecular Basis of Inheritance (chapter)
The idea behind it
NCERT § "The DNA"
- X-ray diffraction data from Maurice Wilkins and Rosalind Franklin, together with base-pairing, led James Watson and Francis Crick to propose the double-helix model in 1953.
- Erwin Chargaff had shown that in double-stranded DNA the amounts of A and T are equal, and so are G and C, which fits with complementary pairing.
- The two chains run antiparallel: one is 5′→3′ and the other 3′→5′. The sugar-phosphate backbones lie outside and the bases stack inside.
- A pairs with T through two hydrogen bonds; G pairs with C through three. A purine always faces a pyrimidine, keeping the distance between the two strands uniform.
- The chains coil in a right-handed manner. Each turn of the helix is 3.4 nm long (the pitch) and holds roughly 10 bp, so neighbouring base pairs are about 0.34 nm apart.
- Stacking of one base-pair plane over the next adds stability to the helix on top of the hydrogen bonds.
- Francis Crick proposed the central dogma: information flows DNA → RNA → protein. In some viruses information flows in reverse, from RNA to DNA.
Questions
4 answers
- What does the "Build a DNA double helix" simulation show?
- Build a duplex: antiparallel strands, A=T with 2 H-bonds, G≡C with 3, 10 bp per 3.4 nm turn; Chargaff and length readouts.
- What mistake does the "Build a DNA double helix" simulation fix?
- A common mistake it clears up: saying G–C has two hydrogen bonds and A–T three.
- Has NEET asked about this idea?
- Yes. NEET has asked about this idea 5 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 "The double helix and central dogma" in Molecular Basis of Inheritance (Class 12 Biology).
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