A bivalent, sometimes referred to as a tetrad, is the association of a pair of homologous chromosomes physically held together by at least one DNA crossover. This physical attachment allows for alignment and segregation of the homologous chromosomes in the first meiotic division. read more
The chromosome and chromatid count during meiosis works a bit differently. Recall that there are two divisions during meiosis: meiosis I and meiosis II. The genetic material of the cell is duplicated during S phase of interphase just as it was with mitosis resulting in 46 chromosomes and 92 chromatids during Prophase I and Metaphase I. read more
A) During prophase I there is one diploid cell; during prophase II there are two haploid cells. B) During prophase I chromosomes line up single file in the middle of the cell; during prophase II the chromosomes line up in double file in the middle of the cell. read more
The formation of a bivalent occurs during the first division of meiosis (in the pachynema stage of meiotic prophase 1). In most organisms, each replicated chromosome (composed of two identical sister chromatids) elicits formation of DNA double-strand breaks during the leptotene phase. read more