Question

What is synapsis?

Answer 1

Synapsis or syndesis is defined as the process of pairing homologous chromosomes during the process of meiosis.

Complete answer:
Synapsis allows matching up of the homologous chromosome pairs prior to being subjected to the process of segregation and possible chromosomal crossover process between them. This stage usually takes place during the prophase I of the meiosis.
When these chromosomes synapse, their ends first get attached to the nuclear envelope. These end-membrane complexes later migrate with the assistance of the extranuclear cytoskeleton unless the matching ends undergo pairing.
Then the chromosomes intervening region come together by a protein complex along the entire length of the chromosome which is known as the synaptonemal complex.
Even the sex chromosomes undergo synapsis. However, in this case, the synaptonemal protein complex responsible for holding the homologous chromosomes together is present only at one of the ends of each of the sex chromosome.
This process is not to be confused with the process of mitosis as even mitosis shows prophase but need not ordinarily show the homologous chromosome pairing.
Additional information: Meiosis is the process by which the cell divides to produce four daughter cells which have half the genetic information of the parent cell. The various stages in meiosis are,

Prophase
Metaphase
anaphase
Telophase and cytokinesis
Meiosis occurs in two rounds, i.e. meiosis I and meiosis II.
Note: Synapsis is followed by a recombination reaction referred to as the synthesis dependent strand annealing or SDSA reaction. The result of a synapsis is a tetrad.