What directs linear DNA to form chromosomes with perfect X shapes with a centromere?

It's not all that magical. Start with a chromosome - a long string of DNA. It replicates. Now there are two long strings of DNA, held together at the centromere by specific proteins that recognize specific DNA sequences at the centromere. This DNA is wrapped around histones, but is still largely decondensed. Eventually, the chromosome will condense, as other proteins wrap up the histone-bound DNA in various ways. That's what creates the X shape (though, of course, the centromere can be anywhere on the chromosome, so it doesn't always look like a perfect X): two condensed strands of DNA, held together at the centromere.

When DNA is in the shape of an X, that is actually a chromosome that has replicated. (So it is actually 2 chromosomes).

Electron orbitals and their shape are actual wave purposes that fulfill the Schrodinger equation. the type is a complicated function expression of the wave function, and the quantum numbers (n, l, m) that come out of the function take place because of the boundary circumstances placed on the electron around the atom and different electrons. i think that in simple terms the answer to the hydrogen atom is known precisely, for different atoms the interplay of electrons is rather complicated so the orbitals are in simple terms solved numerically or approximately. because of the fact for electrons the Pauli exclusion theory applies each and each pair of electrons needs a various set of quantum numbers and this makes the form of the s and p sub-shells of electrons.