Like, how do the genes actually change, and how do you make sure you are only changing the right thing?
In: Technology
It’s very hard to ELI5 this topic.
So the basic principle is editing to insert some DNA (micro injection) or virally deliver it or just deliver some machinery for genomic editing into the zygote or the embryonic stem cells. You would get a chimera because some of these initial cells of the animal have the Edit and some don’t. And then it’s a gamble for whether the edit makes it to the germ line (sperms and eggs). If they are in the germ line, you breed the mouse with another and you keep breeding till you have a homozygous or heterozygous mouse for the gene you edited.
There are many many ways to edit the actual genome, I’ll list a few. Most modern approach is CRISPR, where you can literally use a customized enzyme to go and edit the DNA. You can also use gene targeting methods to generate a mouse with a knocked out gene that can be bred to make a mouse line with basic strategies. These could either be the good old homologous recombination techniques that are not so efficient because they take quite some effort to get to work, or they use designer enzymes like Zinc Finger Proteases or TALENS (this is good but takes some effort too, so weeks to a few months). You can also use the amazing Cre LoxP system, where you use transgenic techniques to insert two sequences called LoxP around the gene you wanna remove. Then in another mouse, insert the gene coding for Cre recombinase (an enzyme) with some promoter differences and/or an estrogen receptor fused to make it expressed in whichever tissue you want and at whatever developmental stage you want and then breed the two mice. The pups will have cre and LoxP, and whenever cre is expressed in a cell, it’ll find the LoxP sequences and just remove the gene between them. If you have an estrogen receptor on the Cre, you can also not allow this to happen unless you give the mouse tamoxifen, synthetic estrogen, to allow the enzyme to do its job. I mean there’s literally dozens of methods, but CRISPR beats them all.
Now to address the specificity issue.. Well if you want to just overexpress a gene (like add more copies of it) that’s easy, you don’t care where the gene is as long as it’s not disturbing other genes and is in a good spot that is accessible for transcription (lots of trial and error here). But if you want to delete or edit that’s hard, and that’s where you use things like CRISPR, where you embed a sequence homoglous (the guide part) to where you want to insert your gene, and so it sort of finds right place.
I know for a fact my explanation was a bit technical, so I apologize in advance. But I am ready to clarify anything you don’t understand.
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