Genetic recombination/crossing over is a intricate process that involves alignment of two homologous DNA strands (Homologous chromosomes are chromosome pairs, one from each parent, that are similar in length, gene position and centromere location.), precise breakage of each strand, equal exchange of DNA segments between the two strands, and sealing of the resultant recombined DNA molecules through the action of enzymes called ligases. Despite the complexity of this process, recombination events occur with remarkable accuracy and precision in the vast majority of instances.

During meiosis, when recombination occurs, the cell's homologous chromosomes line up extremely close to one another. Then, the DNA strand within each chromosome breaks in the exact same location, leaving two free ends. Each end then crosses over into the other chromosome and forms a connection called a chiasma. During this process, it is common for large sections of DNA containing many different genes to cross from one chromosome to another. Finally, as prophase I draws to a close and metaphase I begins, the crossing-over process concludes, and the homologous chromosomes prepare to separate. When the homologous chromosomes are later pulled apart during anaphase I, each chromosome carries new, unique allele combinations that are a direct result of recombination.

Crossing Over is an awe-inspiring process. When homologous chromosomes are lined up during meiosis, they can, in a very precise way, exchange genetic material. There are five steps in achieving this:

• Enzymes open the double helix of DNA in the aligned chromosomes to permit intermolecular base pairing.
• One strand of each helix is cut at equivalent positions.
• The enzyme ligase joins them to form a half-chromatid chiasma(because only one strand of each chromatid cross over), resulting in a cross-shaped molecule.
• The cross-shaped molecule is cut in half by an enzyme, leaving a break in one strand of each recombinant.
• The break is sealed by ligase.

The process has to be extremely precise. If even one nucleotide is transferred incorrectly, the genetic message becomes useless. A normal crossover is really a miraculous process. Somehow the genetic material from one parental chromosome and the genetic material from the other parental chromosome are cut up and pasted together during each meiosis, and this is done with complete reciprocity. In other words, neither chromosome gains or loses any genes in the process. In fact, it is probably correct to say that neither chromosome gains or loses even one nucleotide in the exchange. How is this remarkable precision attained?