What are sister chromatids?

• A. Two identical copies of DNA held together by a centromere.
• B. Two different chromosomes joined at the centromere.
• C. Two copies of a gene on homologous chromosomes.
• D. Noncoding regions of a gene.

Answer: (A)

Sister chromatids are two identical copies of a chromosome that are produced during DNA replication. They stay attached to each other at the centromere, so the replicated chromosome appears as a pair of sister chromatids. They are genetically identical until they are separated during cell division, at which point they each become an individual chromosome in the daughter cells. The other statements describe different concepts—two different chromosomes joined at a centromere isn’t what sister chromatids are, two copies of a gene on homologous chromosomes are alleles on homologs, and noncoding regions aren’t relevant to defining sister chromatids.