Final answer:
The first step of meiotic recombination or crossing over is synapsis, which occurs during prophase I of meiosis I when homologous chromosomes pair up closely, setting the stage for genetic exchange.
Step-by-step explanation:
The first step of meiotic recombination or crossing over is synapsis. During prophase I of meiosis I, homologous chromosomes come together and align gene-for-gene in a process known as synapsis. This intimate pairing of homologous chromosomes involves the formation of a protein structure called the synaptonemal complex, which supports the exchange of chromosomal segments between non-sister homologous chromatids. Following synapsis, specific DNA breakage occurs near recombination nodules, leading to an exchange of alleles and the rejoining of the DNA strands. This results in novel combinations of alleles, contributing to genetic diversity.
The multistep process of crossing over involves the formation of chiasmata, which are points of contact where genetic material is exchanged between non-sister chromatids. The process culminates in crossover resolution, where the chromosomes separate into distinct non-sister chromatids with a unique combination of genetic material as a result of the crossing-over events. It is important to note that while synapsis is the first step, the entire process is critical for genetic variation and the proper segregation of homologous chromosomes during meiosis.