One Old Strand in Every Copy

You'll learnwhy replication is semiconservative and why only one new strand can be built continuously.

A cell copies its DNA by pulling the helix apart and using each old strand as the pattern for a new one — so every copy keeps half the original, and the two new strands cannot be built the same way. Twelve short steps; press Next for each one.

12 STEPS · 6 QUICK CHECKS · SEMICONSERVATIVE · POLYMERASE ADDS TO THE 3′ END ONLY · LEADING vs LAGGING · v5

the fork opens · one strand runs smooth, one comes out in pieces

Before you start

What you're looking at

A flat DNA ladder about to be copied, and the Y-shaped fork that copying opens in it.

The question

How does a cell copy DNA, and why can't both new strands be built the same way?

Step 1 / 12