# DNA Upgrades

How DNA copies itself, how cells repair a mismatched letter, and how rare mutations spread through generations or are edited on purpose with CRISPR\.

By Kunal Kapoor

Topic: Nature & Biology

[Open the interactive Jig](https://ask.jig.com/j/sGnwSUdfIggA)

## Transcript

### 1. A recipe in every cell

This twisted ladder is DNA, the instructions for building and running a living thing\. Almost every cell in your body carries a copy\. Drag anywhere to turn it around\.

Your DNA holds about 3 billion pairs of letters\.

### 2. Four letters that pair up

Each rung of the ladder is a pair of chemical letters called bases: A, T, G and C\. A always pairs with T, and G always pairs with C\. The order of the letters spells out the instructions\.

The whole ladder is only about 2 nanometres wide, around 50,000 times thinner than a hair\.

### 3. Unzip and copy

Before a cell divides, it copies its DNA\. The ladder unzips down the middle, and each half becomes a pattern for a new matching half\. Because A only fits T and G only fits C, both copies come out the same\.

Each new ladder keeps one old strand and gets one new strand\.

### 4. A tiny typo

Copying billions of letters is fast work, and now and then the wrong letter slips in\. Look for the glowing rung: a T has landed opposite a G, so the pair does not fit properly\.

The copying machine makes roughly one slip in every 100,000 letters\.

### 5. Spell\-check

Cells check their work\. Repair helpers spot the badly fitting pair, remove the wrong letter and put in the right one\. Press the button to fix the typo\.

After all the checking, only about one mistake in a billion letters is left\.

### 6. When a change sticks

A few changes slip past every check\. Once copied, the new letter pair is passed on like any other\. This is called a mutation\. Most do nothing at all, a few cause problems, and a very few turn out to be useful\.

Each baby is born with about 60 to 70 brand\-new letter changes\.

### 7. Nature's slow upgrades

A useful change can spread through a family, and then a whole population, over many generations\. Watch the golden rung pass from one generation to the next\. This is how living things slowly adapt\.

Example: a change that lets adults digest milk spread in some groups over the last 10,000 years or so\.

### 8. Upgrades on purpose

Scientists can now edit DNA with a tool called CRISPR\. A protein named Cas9 carries a short guide that matches one spot\. It cuts both strands there, and the cell's own repair kit can mend the gap with a new letter\.

In 2023, a CRISPR treatment for sickle cell disease was approved for the first time\.

### 9. Your own story

Copying, tiny mistakes, careful repair and the rare useful change: that is how DNA gets upgraded\. Each change is just one letter among billions, yet over time it has shaped every living thing\.

Turn the ladder and look for the golden letters that changed\.