How DNA Copies Itself

In the picture

1. 1The closed double helix: two old strands wound together before copying begins
2. 2The replication fork: hydrogen bonds break and the two old strands separate
3. 3Interlocking base pieces: A pairs only with T and C only with G, so the new strand matches the old one
4. 4A finished molecule: one old strand kept alongside one newly built strand

Examples

• A zip undone down the middle, with each separated half then fitted to a brand-new matching half
• A jigsaw edge that accepts only the one piece cut to fit its bumps and notches

Quiz: How DNA Copies Itself (Age 16)

1. 1

   At what point is a cell’s DNA copied?

   • During S phase, before the cell divides
   • During the division itself, as the cell splits in two
   • After division, separately inside each new cell
2. 2

   As a new strand is built along an old one, what decides which base goes in at each position?

   • Whichever base happens to be closest, so the new strand’s order varies each time
   • The base just before it on the new strand, which fixes what can come next
   • The base opposite it on the old strand, because A pairs only with T and C only with G
3. 3

   Why is DNA replication described as semi-conservative?

   • One finished molecule keeps both old strands while the other is made of two new strands
   • Each finished molecule keeps one old strand alongside one newly built strand
   • Only half of the molecule is copied, and the other half is rebuilt later

The answers, each with its explanation, are in the app. This lesson has left the free week, so its quiz is part of the subscription.