It sounds like you're asking about the process of chromosome formation. To answer that question accurately, we need to clarify what you mean by "become."

Here's a breakdown of how chromosomes form:

* Chromosomes are made of DNA. DNA is the genetic material that carries all the instructions for building and maintaining an organism. It exists within the nucleus of every cell.

* During cell division (specifically, during the S phase of interphase), the DNA replicates. This means that each chromosome is duplicated, creating two identical copies called sister chromatids.

* Sister chromatids are held together by a structure called a centromere. This gives the chromosome its characteristic "X" shape.

* During mitosis or meiosis, the replicated chromosomes condense and become visible under a microscope. This is when they are most easily observed and studied.

So, to answer your question:

* Chromosomes aren't "created" from scratch. They are formed from the existing DNA within the nucleus.

* They "become" visible as distinct structures during the condensation process in cell division.

Let me know if you have any further questions about chromosomes or cell division!