What defines a chromatid?

A chromatid is defined as an identical copy of a chromosome that is formed during cell division. Following DNA replication, each chromosome consists of two sister chromatids, which are joined together at a region called the centromere. This structure is critical during mitosis and meiosis, as the chromatids are separated and distributed to the daughter cells, ensuring that each cell receives an accurate copy of the genetic material.

When discussing the other possible options, a variation of a gene typically refers to different alleles or forms of a gene, which does not define a chromatid. A type of RNA molecule, such as mRNA or tRNA, is involved in the process of transcription and translation but is not a chromatid. Finally, while a section of DNA could refer to many structures within a chromosome, it does not specifically describe the relationship of a chromatid, which emphasizes its status as an identical copy of a chromosome made evident during the stages of cell division.