Question

what biological event triggers crossing over?
chromosome separation
synapse formation
aging of chromosomes
centromere duplication

Explanation:

Crossing - over occurs during meiosis. Synapse formation (also known as synapsis) is the process where homologous chromosomes pair up closely. This close pairing (synapsis) allows for the physical exchange of genetic material between non - sister chromatids of homologous chromosomes (crossing - over). Chromosome separation occurs later in meiosis (anaphase I and anaphase II) and is not the trigger for crossing - over. The concept of "aging of chromosomes" is not a recognized biological event related to crossing - over. Centromere duplication occurs during the S - phase of the cell cycle (in preparation for cell division) and is not related to the triggering of crossing - over.

Answer:

synapse formation