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: