What is a condensed chromosome?

• A. Can be seen under a light microscope; can be duplicated
• B. Cannot be seen under light microscope
• C. Not made of DNA
• D. Always contains two centromeres

Answer: (A)

Condensed chromosomes are the highly compact form chromosomes take during mitosis, after DNA replication. Once chromosomes have been duplicated, each chromosome consists of two sister chromatids held together at a single centromere. When mitosis begins, the chromatin condenses enough to be visible under a light microscope as distinct, X-shaped structures. That combination—visible under light microscopy and consisting of replicated chromatids—is exactly what the statement describes. Chromosomes are still made of DNA (with associated proteins), so saying they are not made of DNA isn’t correct, and a chromosome typically has one centromere, not two, with the two chromatids sharing that centromere until they separate later.