Dyad
A bivalent is a pair of sister chromatids. This occurs in prophase I of meiosis. After DNA replication, the two sister chromatids align side-by-side and appear to have an undivided centromere, in contrast to mitosis, in which each chromatid appears to have its own separate centromere.
The term bivalent can also be used to describe protein morphology.
Read more about this topic: Chromatid