Lesson 2 ·Class 10 · Structure of Chromosomes, Cell Cycle and Cell Division · Label Lab
The DNA Double Helix
A twisted ladder, and what each rung is.
Tap any part of the diagram to learn its name and its job. Try: Follow one backbone all the way down, then find a single rung and see that it is made of two halves meeting in the middle.
Free play — explore the diagram, or switch to "Label it yourself" and place every label. Switch to when you're ready to test yourself.
Think of a twisted rope ladder. The two ropes are sugar-phosphate backbones; each rung is a pair of nitrogenous bases held together by hydrogen bonds. A always pairs with T, and G always with C.
What this lesson covers
What you do
- Follow one backbone all the way down, then find a single rung and see that it is made of two halves meeting in the middle.
The parts you label
- Sugar-phosphate backbone — One of the two side rails, made of alternating deoxyribose sugar and phosphate.
- Nitrogenous base — Adenine, thymine, guanine or cytosine — one half of a rung.
- Base pair — A complete rung. A pairs only with T, and G only with C.
- Hydrogen bond — The weak bond holding the two bases of a pair together. Weak enough for the ladder to unzip when the DNA is copied.
- Nucleotide — The repeating unit: one phosphate, one sugar and one base.
- Double helix — The whole twisted-ladder shape, described by Watson and Crick.
Challenges to clear
- Tap the repeating unit that the whole molecule is built from.
- Tap the weak bond that lets the ladder unzip for copying.
- Label the whole molecule.
Hint
Think of a twisted rope ladder. The two ropes are sugar-phosphate backbones; each rung is a pair of nitrogenous bases held together by hydrogen bonds. A always pairs with T, and G always with C.