The fusion of sperm and oocyte membranes causes cortical reaction to occur. Cortical granules inside the secondary oocyte fuse with the plasma membrane of the cell, causing enzymes inside these granules to be expelled by exocytosis to the zona pellucida. This in turn causes the glycoproteins in the zona pellucida to cross-link with each other — i.e. the enzymes cause the ZP2 to hydrolyse into ZP2f — making the whole matrix hard and impermeable to sperm. This prevents fertilization of an egg by more than one sperm. Fusion of genetic material Preparation In preparation for the fusion of their genetic material both the oocyte and the sperm undergo transformations as a reaction to the fusion of cell membranes. The oocyte completes its second meiotic division. This results in a mature haploid ovum and the release of a polar body. The nucleus of the oocyte is called a pronucleus in this process, to distinguish it from the nuclei that are the result of fertilization. The sperm's tail and mitochondria degenerate with the formation of the male pronucleus. This is why all mitochondria in humans are of maternal origin. Still, a considerable amount of RNA from the sperm is delivered to the resulting embryo and likely influences embryo development and the phenotype of the offspring. Fusion The sperm nucleus then fuses with the ovum, enabling fusion of their genetic material. Blocks of polyspermy When the sperm enters the perivitelline space, a sperm-specific protein Izumo on the head binds to Juno receptors on the oocyte membrane. Once it’s bound, two blocks to polyspermy then occur. After approximately 40 minutes, the other Juno receptors on the oocyte are lost from the membrane, causing it to no longer be fusogenic. Additionally, the cortical reaction will happen which is caused by ovastacin binding and cleaving ZP2 receptors on the zona pellucida. These two blocks of polyspermy are what prevent the zygote from having too much DNA. Replication The pronuclei migrate toward the center of the oocyte, rapidly replicating their DNA as they do so to prepare the zygote for its first mitotic division. Mitosis Usually 23 chromosomes from spermatozoon and 23 chromosomes from egg cell fuse (approximately half of spermatozoons carry X chromosome and the other half Y chromosome). Their membranes dissolve, leaving no barriers between the male and female chromosomes. During this dissolution, a mitotic spindle forms between them. The spindle captures the chromosomes before