Bi-valent is a term that is used to refer two homologous chromosomes at prophase I of meiosis specifically the zygotene and pachytene stages. A chromosome pair consists of 4 chromatids are called bivalents. In this stage recombination takes place between homologous chromosomes.
Each bivalent consists of 4 chromatids and 2 centromeres and is also called as tetrad.
The number of chromosome = The number of centromeres.
Here two centromeres, therefore 2 chromosomes. Each chromosome with 2 chromatids each. In total 4 chromatids, therefore also called as tetrad.
The number of DNA molecule = Count the number of chromatids.
The process of crossing over takes place during this bi valent stage. Crossing over is the exchange of segments between non-sister chromatids of homologous chromosomes during pachytene stage of prophase I of meiosis I. This process is the major reason for genetic variation between individuals in a population.