I know that DNA encodes proteins. Truthfully, everything besides that (including ‘what are proteins’) mostly wooshes over my head, but that’s not relevant because whenever I search this question I never even find it addressed anywhere.
The human body has, among other things, two hands each with five fingers, with a very particular bone structure. How are things like that encoded in DNA, and by what mechanisms does that DNA cause these features to be built the way they are? What makes two people have a different nose shape? Nearly everyone in my family has a mole on the left side of their face, how does that come about from DNA?
I’m sure there are many steps involved, but I don’t see how we go from creating proteins to reproducibly building a full organism with all the organs in the right places and the right shapes. Whenever I try to look this up, all of these intermediate steps are missing, so it basically seems like magic.
As I said, any explanation will most likely go over my head and I won’t be able to understand it fully, but I at least want to see an explanation. I’ll do my best to understand it of course.
I recently had a course in developmental biology and this is what I can remember the best:
From the moment of fertilization, the egg has a ‘polarity’, thus a direction like up and down, left and right etc. This can be established by e.g. the entry point of the sperm or protein produced by the mother in one side of the egg. From here on out protein gradients and interactions on cell membranes specify where cells are and thus what kind of tissue they should become
This early polarity leads to a certain pattern of cell division. An important part of this is the folding in of tissue layers called gastrulation, creating the beginning for the gastrointestinal tract.
In vertebrates a streak starts to form in the embryo. This is the start of the vertebrae and the neural tube in it. Also blocks of cells called somites start to form. These blocks are very important for all kinds of tissues in the body.
Hox genes make the cells in somites get an identity like head, arm etc. So in the end Hox genes are probably what you are looking for. If leg Hox genes are expressed where fruitfly normally have antennae, you get this horror:
Thank you! I will read up on that.
So, a core part of my question is what causes certain genes to be expressed in certain places in the body, and specifically how this comes about from genetics alone (i.e. not artificially in lab experiments). In my past searches I did find some info about forcing gene expression in places where it wouldn’t normally happen, which creates horrors similar to the one you shared, but I never found an answer for how this is controlled in natural development. Hox genes seem to be the answer I’m looking for :)
By the way, in case you’d have a guess for the answer:
If scientists really wanted to, throwing all questions of ethics out the window, would it be possible to genetically engineer a person with four arms instead of two, kinda like Goro? Does our current understanding of this go far enough to make deliberate changes like that? And would that baby be able to develop in a normal woman’s pregnancy?
No, and no. My PhD thesis was on gene delivery. We’re barely getting into the simplest modifications for disease treatment. Multigene stuff, and spatiotemporal control is still a ways off.
I would say sort of. They could do it, but the result wouldn’t survive very long. Probably not even to birth.