What is a chromatid?

• A. One of two copies of a replicated chromosome, joined at the centromere
• B. A single gene
• C. The centromere
• D. A protein that binds DNA

Answer: (A)

During DNA replication, a chromosome is copied to form two identical copies called sister chromatids. Each chromatid is one copy, and they remain attached at the centromere to make a duplicated chromosome. When cell division proceeds, the chromatids separate at the centromere so that each chromatid becomes its own chromosome in the daughter cells. A gene is a unit of heredity found on DNA, not a copy of a chromosome. The centromere is the constricted region where the chromatids stay joined and where spindle fibers attach during division. A chromatid is not a protein that binds DNA.