Final answer:
Synapsis is the process where chromosomes align along their length during prophase I of meiosis, leading to genetic recombination through crossing over.
Step-by-step explanation:
When there are pairs of chromosomes aligning tightly along their entire length during cell division, this process is known as synapsis. It occurs during prophase I of meiosis, where homologous chromosomes pair up and align precisely with each other. This phase facilitates the exchange of genetic material between the homologous chromosomes, an event called crossing over or genetic recombination. The structure responsible for holding the chromosomes together during this exchange is called the synaptonemal complex. Ultimately, crossing over leads to genetic variation in gametes, laying the foundation for one aspect of Mendel's law of independent assortment.