What is a chromatid?

• A. One of two copies of a replicated chromosome, joined at the centromere
• B. A single, unreplicated chromosome
• C. A protein that binds DNA
• D. The full set of chromosomes in a cell

Answer: (A)

Chromatids are one of the two identical copies that make up a replicated chromosome. After DNA replication, a chromosome consists of two sister chromatids held together at the centromere, each containing an identical DNA molecule. When the cell divides, those chromatids separate and become separate chromosomes in the daughter cells. So the correct statement is that a chromatid is one of two copies of a replicated chromosome, joined at the centromere. The other descriptions fit other concepts: a single, unreplicated chromosome is not a chromatid, a protein that binds DNA is not a chromatid, and the full set of chromosomes in a cell describes the genome or karyotype, not a single chromatid.