Understand how the one of the DNA segments separates at the center. Before crossing over can occur, one of the DNA segments is cut at the center by an enzyme. You can refer to this reciprocal segment as strand 1.
Distinguish between the 5' end and the 3' end of the DNA strands. The 5' end of the DNA has a terminal phosphate group, whereas the 3' end of the DNA has a terminal hydroxyl group. After the DNA strand splits, the enzyme digests the interior 5' end of each of the strands.
Analyze what happens when the broken DNA strand comes in contact with the reciprocal segment, or strand 2. The broken DNA, strand 1, inserts itself into the intact reciprocal strand 2. If a complementary sequence of bases is found, then a bond is formed. For example, if the invading sequence is ATGGACG, then the complementary sequence is TACCTGC because adenine, A, pairs with thymine, T, and cytosine, C, pairs with guanine, G.
Understand the process that occurs after the complementary sequences are bonded. Regular DNA replication is what occurs in order to fill the gaps that have now formed as a result of the pairing with the reciprocal segments of the DNA.
Examine how strand 1 incorporates the strand 2 through cutting and ligation. Where strand 1 and 2 meet, a cutting occurs so that the segments can be exchanged. The end result is strand 1 with portions of strand 2, and strand 2 with portions of strand 1.