DNA double helix: interactive simulation
DNA is two strands wound into a right-handed double helix. A always pairs with T using two hydrogen bonds, and G with C using three.
Useful for: NEET, CBSE Boards
Try this
Type your own sequence. The partner strand, hydrogen bond count, GC content, length and mRNA are worked out instantly using the Watson–Crick numbers in NCERT: 0.34 nm per base pair and about 10 base pairs per turn.
- 1
Compare AAAATTTT with GGGGCCCC.
What you should see
Both are 8 bp long, but the first has 16 hydrogen bonds and the second has 24. GC-rich DNA is harder to separate.
- 2
Type a 20-base sequence.
What you should see
Length = 20 × 0.34 = 6.8 nm, and the helix makes 2 full turns.
- 3
Look at the mRNA line.
What you should see
It is complementary to the template strand, with U in place of T.
- 4
Drag to turn the helix.
What you should see
The two backbones are not evenly spaced — you can see a wide major groove and a narrow minor groove.
Formulas behind it
Chargaff’s rule
Hydrogen bonds
Length of DNA
Common mistakes
- Writing U in DNA. Uracil is found in RNA; DNA has thymine.
- Giving A–T three hydrogen bonds. It is two; G–C has three.
- Forgetting that the strands are antiparallel (one runs 5′→3′, the other 3′→5′).
FAQ
How long is the DNA in one human cell?
About 6.6 × 10⁹ bp × 0.34 nm ≈ 2.2 m in a diploid cell, as NCERT calculates.
What holds the two strands together?
Hydrogen bonds between paired bases, plus stacking of the bases on top of each other.
Who proposed the double helix?
James Watson and Francis Crick in 1953, using X-ray diffraction data from Rosalind Franklin and Maurice Wilkins.
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