Final answer:
Chromosomes are condensed chromatin, which is DNA wound tightly with proteins, and become visible only during cell division.
Step-by-step explanation:
Chromosomes are condensed chromatin. Within the nucleus, DNA is usually in the form of chromatin, a loosely arranged structure combined with histone proteins. When a cell prepares to divide, the chromatin coils tightly and condenses into the familiar X-shaped chromosomes. At this stage, chromosomes are most compact, and they consist of DNA wrapped around histone proteins and further structured with the help of scaffold proteins. Chromosomes become readily visible and distinguishable under a microscope only during the cell division phases.