Your DNA is a twisted ladder. When it copies itself, it doesn’t build a whole new one from scratch. That would be chaos. Instead, it unzips down the middle.
Each old strand acts as a template. A perfect, reliable blueprint. The new strand builds right onto it. This is the “semi” part: half the old ladder stays, half is brand new.
This keeps mutations rare. Why? Because the old strand is a proofreader. It’s like having a friend who remembers the correct lyrics to a song you keep botching.
The Proofreading Party
Imagine you’re copying a recipe. If you wrote the whole thing from memory, you’d mess up. But if you keep the original next to you, you check every word.
That’s exactly what your cells do. An enzyme called DNA polymerase matches new building blocks (called bases) to the old strand. It’s a stickler for detail. If it pairs the wrong letter—say, a C where a G belongs—it gets fired.
Literally. It cuts the wrong piece out and tries again. This accuracy is mind-blowing. Your cells make only one mistake for every ten billion letters they copy.
