The structure that holds each chromosome to its exact copy is called the centromere.

Here's a breakdown:

* Chromosomes are thread-like structures made of DNA that contain genetic information.

* Centromeres are specialized regions of DNA that hold sister chromatids (identical copies of a chromosome) together.

* Sister chromatids are the two identical copies of a chromosome that are created during DNA replication.

During cell division (mitosis and meiosis), the centromere plays a crucial role:

1. Holding the sister chromatids together: The centromere acts like a glue, keeping the two identical copies of the chromosome attached.

2. Serving as an attachment point for spindle fibers: Spindle fibers are protein structures that attach to the centromere during cell division and pull the sister chromatids apart, ensuring that each daughter cell receives a complete set of chromosomes.

So, the centromere is the key structure that ensures accurate chromosome segregation during cell division, preventing genetic errors.