Let me try to give you an idea of how much work is needed to “fully” understand that thing we call our “brain”, even trying to answering your question is going to truly boggle your mind, but here goes anyway:
Our body is made up of many different cell types, current estimates are around 200 different types. A discovery of something in one cell type will probably be investigated in all cell types
Each cell contains lipids, nucleic acids (the various DNAs and RNAs) and proteins - let’s just focus on proteins for now.
A “typical” cell (if there is such a thing) is calculated to contain the genetic material (genes) for about 200 different proteins, but each one of these genes could produce as many as 100 different actual functioning proteins, so potentially 20,000 proteins. Too much I know, lets try to keep it simple.
Focussing just on what we know about the brain: There are 13 distinct areas of our brain, each with a different composition, scientists have determined that there are 15,331 genes in our brain tissues which code for protein manufacture, even if each of those 15,331 proteins did not exist in 100 different variations of each protein, we’ve got enough to go on there.
Brains contain basically neurones (brain cells) and non-neurone cells (mostly glia, the support network for the neurones). Lets make it easy and just focus on the calculated 86 billion neurones in the brain (there about as many non-neuronal cells)
Here comes the maths…
the 86 billion neurones (8,600,000,000) can each connect with every other neurone, giving potentially (lets round it up for ease of maths) about 100 trillion neuronal connections at the bare minimum (100,000,000,000,000)
Now change just one of those proteins into one of it’s potential 100 variants, that’s 100 trillion connections that might be changed so would need to be investigated again - just for that one neurone with one changed protein connecting 100 trillion times to every other neurone
Now repeat ad infinitum for every protein variant for every neurone with every connection to every other neurone 
Then… factor in single variant neurone + non-variant neurone and do it all again, and so on and so on
Quantum computing will allow massive advances in calculations such as this, but that is still something being developed. When I was at Uni in the early 1990’s, we were told “there are more potential connections in the human brain than there are particles in the known Universe”, so that is the level of knowledge we would need to fully understand the human brain.