This is The Guardian.

Hi, Madeline here. We're on our summer break at Science Weekly. We'll be back with some new

episodes for you next week. But for now, we're going back to some of our favourites from the

year so far. This episode explores the alarming concept of mirror

life and why scientists are trying to make sure it never moves from idea to reality. It's a fascinating

and at times terrifying window into a part of biology that was totally new to me. Enjoy.

Imagine you're playing a game of Tetris.

You have a hole you want to fill, two squares up and one to the right.

Out from the Tetris heavens emerges a suitable looking piece.

Two up, but one to the left. It's the mirror image of the piece you need. And no matter how

many times you flip it, it just won't fit in the hole like you'd planned.

This picture actually plays out in nature.

There are molecules that can exist in one form and its mirror image too.

But for some mysterious reason, when it comes to life, nature prefers to pick. It likes to build, say, DNA,

proteins, cells, using one orientation of a molecule or its mirror, but not both. This choice defines our whole biology.

Scientists, of course, don't like to leave nature to have all the fun.

And so a pursuit had begun to create mirror life,

constructing those same building blocks but using the mirror image molecules.

Eventually, the idea would be to make a whole mirror cell or microbe. It was a thrilling prospect, with plenty of potential benefits. But recently,

38 leading scientists in the field signed a letter warning that mirror life should never be created.

There's a new warning from a group of over two dozen biologists.

They are calling for a ban on mirror cell research.

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