system
Given the following post, respond as if you're commenting on Reddit:
human
If the DNA is the same in every cell, how does the body know to build an arm here, run a blood vessel there, put a femur in the thigh, and put 4 chambers in the heart?

I think I understand that the DNA in cells command the cell to make certain types of amino acids which then form into proteins, and that's difference between an epithelial or goblet cell or connective or muscular neuron cell.  But if the DNA is the same, how do the cells end up making different proteins?  And how do the cells end up physically organizing into just the way we need to create the human body, all the way down to super smart Circle of Willis blood vessel in the brain if every human being?
gpt
This is kind of the root of embryology/development biology, which is a huge field with lots still to be discovered.  At the adult level, differentiated cells have different levels of DNA expression - just because the DNA is there does not mean it is expressed the same way everywhere.  There's a lot of regulation at both the chromosomal level (eg histone methylation, chromatin condensation) and the nucleotide level (eg transcription factors, mRNA splicing, etc).  This is further mediated by signalling via chemicals (eg cytokines/growth factors/hormones etc), as well as physical parameters (eg the stiffness of the underlying substrate, or contact inhibition when cells become confluent).  

In the fetus, one of the key players is the HOX gene, which regulates development along the anterior-posterior axis. Various defects in this signalling pathway may result in things like duplication of structures (eg with synpolydactyly). There are many others (if you're interested, check out Shh and wnt-7a), and while we have a general idea of what each one generally regulates, there are many details that are yet to be perfectly described.

To address your last line, note that 'normal' anatomy is highly variable.  The circle of Willis is actually one of the sites where the exact connections and locations of various vessels can vary a lot between individuals.  So it's not like our DNA encodes every single connection in the body - often times, it's more a matter of regulation at the cellular level (eg cells sense lack of oxygenation, and release signals like hypoxia inducible factors to stimulate angiogenesis).