Crossing over is a fundamental genetic process that occurs during meiosis [type of cell division that produces gametes (sperm and egg cells)]. It involves the exchange of genetic material between homologous chromosomes, which results in the recombination of genes. 

Crossing over is an important process as it contributes to genetic diversity in sexually reproducing organisms by creating new combinations of alleles and this exchange process occurs during prophase I of meiosis, leading to recombinant chromosomes with new combinations of alleles.

The Process of Crossing Over:

Crossing over occurs during prophase I of meiosis when homologous chromosomes pair up in a process called synapsis. Here are the key steps:

1. Synapsis: During prophase I of meiosis, homologous chromosomes (one from each parent) pair up, aligning closely to form a structure known as a bivalent or tetrad.

2. Formation of the Synaptonemal Complex: A protein structure called the synaptonemal complex forms between homologous chromosomes, holding them together and facilitating the exchange of genetic material.

3. Chromatid Crossing: Non-sister chromatids of homologous chromosomes (one from each parent) physically cross over at points called chiasmata. This involves the breakage and rejoining of DNA strands, allowing the exchange of genetic material.

4. Exchange of Genetic Material: The physical crossing over results in the exchange of segments between the chromatids. This creates recombinant chromatids, which have a new combination of maternal and paternal alleles.

5. Resolution: The chiasmata hold the homologous chromosomes together until anaphase I, when the chromosomes separate. The resolution of chiasmata results in chromosomes that have exchanged genetic information.

6. Completion of Meiosis: After crossing over, meiosis continues with the separation of homologous chromosomes in anaphase I and the subsequent separation of sister chromatids in meiosis II, resulting in four genetically diverse gametes.