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The DNA Ladder
Age 15 · Biology
DNA is shaped like a long, twisted ladder made of two strands winding around each other. The rungs that connect the strands are made of paired building blocks: adenine always pairs with thymine, and cytosine always pairs with guanine.
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Age 15: open this week
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In the picture
- One of the two twisted strands that spiral around each other
- A rung connecting the two strands, made of two joined blocks
- The long orange block joined to its matching short blue block
- The long green block joined to its matching short yellow block
Examples
- Like a spiral staircase with two handrails winding around a central column
- Like a zipper where each tooth only closes with one matching tooth from the other side
Key takeaways
- DNA’s two strands are held together by matched pairs of building blocks, never random ones
- Adenine only pairs with thymine, and cytosine only pairs with guanine - this pairing rule lets DNA copy itself accurately
Quiz: The DNA Ladder (Age 15)
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What joins the two strands of DNA to each other?
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Which of these pairs could never form a rung of the DNA ladder?
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Why does it matter that each building block has only one possible partner?
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