Yolanda Ridge - CRISPR: A Powerful Way to Change DNA

Jumping into

Genetics

Knowing its possible to gene-edit our way to better health, better nutrition, and a world without extinctions is one thing, but understanding how is something else altogether. How can we edit out a persons chance of inheriting a certain disease, for example, or edit in a particular crops resistance to a certain pest? In order to understand how gene editing works, we first need to understand how genes themselves go about their business.

The Genome: Your Personal Instruction Manual

Lets start with the big picture. Every living beingfrom a bacteria to a monkeyhas a genome. The genome is like a really detailed instruction manual. Not only does it tell the body how to function, but it also makes sure this information gets passed down from generation to generation. Its the instructions in your genome that told your body how to do everything from growing a toe to building a brain (with added information about how to keep it running). The genome gave your parents a similar set of instructions, and it will make sure that any kids you have will know how to do this stuff too.

Since every species is different, the genome of every species is different (although the genome of a mosquito and the genome of an elephant are more similar than you might expect). For now, lets focus on the being that matters to us mostthe human being.

Most cells in the human body keep a copy of the entire instruction manual in their command centerotherwise known as the nucleus. Its estimated that the average human body is made up of 37.2 trillion cellsyes, you read that right: 37,200,000,000,000. Thats a lot of instruction manuals!

DNA

Instead of being written in words, like the instruction manuals were used to, the genome is written with DNA. If you were to unravel the genome and zoom in with a really powerful microscope, youd eventually see that DNA is the core of it all.

Inside the nucleus of every cell, DNA is bundled into chromosomes that make up the genome.

So, what is DNA? The letters stand for deoxyribonucleic acid, which is a long way of saying that DNA is made up of two long strings of molecules called nucleotides. These stringsor strandsspiral together to create whats known as a double helix. If you think of DNA as a twisted ladder, each strand of nucleotides is one side of the ladder. The rungs that attach the sides together are formed by the nitrogen base of each nucleotide binding with its mate on the other side. This is why nucleotide pairs are often referred to as base pairs.

Our alphabet has 26 letters; DNA has 4A, G, T, and Cwhich stand for the nucleotides adenine, guanine, thymine, and cytosine. In DNA, adenine pairs with thymine, and cytosine pairs with guanine.

The workers inside the cell read sequences of three nucleotides as three-letter words, or codons. These three-letter words then string together to form a sentence. And these sentences make up the instruction manual.

A nucleotide (or base) is a molecule with a nitrogen base that binds (or pairs) with the nitrogen base of the nucleotide on the opposite strand.

To get a sense of how this works, lets imagine that our genome is an instruction manual on how to build things out of toy bricks.

Of course, the human genome isnt instructing the body on how to build a rainbow. Instead, its telling our 37 trillion cells how to do everything from thinking to digesting foodall of the essential instructions we need to function.

We look through the instruction manual to find what we want to buildsay, a rainbow.

At the beginning of the step-by-step instructions on how to build a rainbow, there will be a three-letter word (or codon) that means START.

Right after that will be a three-letter word that tells us which brick to get first.

The three-letter word that comes next is the code for the next size and shape of brick you need to add.

And so on... until you get to a three-letter word that means STOP.

Your rainbow is complete!

Genes

So, where do genes fit in? If the genome is the cells entire instruction manual, the gene is like a specific sentence. It tells the cell how to make one specific thinga protein. When making a protein, the two strands of DNA split apart so they can be copied by something called messenger RNA, or mRNA. (RNA, which stands for ribonucleic acid, is just a single-stranded version of DNA with U standing in for T.) The three-letter words in the mRNA are then used to build a protein using amino acids. Like the toy bricks used to make our rainbow, amino acids are building blocks made of elements from the periodic table that come together in different combinations to form a protein.