Question

Meiotic nondisjunction usually occurs during meiosis I. What is not separating properly: bivalents or sister chromatids? What is not separating properly during mitotic nondisjunction?

Short answer

During meiotic nondisjunction, it is mainly the bivalents or tetrads- pairs of homologous chromosomes- that do not separate correctly. Conversely, in a process known as mitotic nondisjunction, it's the failure in the separation of the identical sister chromatids.

Step-by-step solution

Understanding Meiotic Nondisjunction

This process usually occurs during meiosis I, the stage at which homologous chromosomes, each composed of two sister chromatids, separate. Technically, the structures not separating correctly during this process are called bivalents or tetrads, which are pairs of homologous chromosomes.

Understanding Mitotic Nondisjunction

During mitosis, however, it is the sister chromatids that do not separate properly in the case of nondisjunction. These sister chromatids are essentially identical copies of a single chromosome that are supposed to be separated into two different daughter cells during mitotic division.