Of the approximately 300 million sperm eliminated in ejaculation, only about 200 reach the fallopian tube, and only one fertilizes oocyte II.

When released from the ovary, the oocyte is encased in the zona pellucida, formed by a network of glycoprotein filaments. Externally the zona pellucida there is the corona radiata, formed by follicular cells (ovarian-derived cells).

At fertilization, the sperm passes through the corona radiata and upon reaching the zona pellucida undergoes changes forming the fertilization membrane, which prevents the penetration of other sperm in the oocyte.

At the same time, meiosis is finalized giving rise to the egg and forming the second polar corpuscle.

At fertilization, sperm provide the zygote with the nucleus and centriole. Sperm mitochondria disintegrate in the cytoplasm of the egg. Like this, all mitochondria of the new individual's body are of maternal origin.

It is now known that there are many diseases caused by mitochondrial DNA mutations and that they are transmitted directly from mothers to their offspring. In addition, mitochondrial DNA analysis has been used in maternity tests to verify who is the mother of a child.

The haploid nucleus of the egg and the nucleus of the sperm are respectively named female pro-core and male pro-core. With the union of these nuclei (anfimixia), we have the formation of the egg cell or zygote and the beginning of embryonic development.

Each cell in the human body has hundreds of mitochondria. Within a single mitochondria are several circular DNA molecules, each of which includes 37 genes related to the synthesis of proteins involved in the stages of respiration.

Mutations in mitochondrial DNA have been linked to aging and a host of degenerative diseases, especially the brain, muscles, kidneys and hormone-producing glands. These mutations alter the functioning of mitochondria so that they no longer produce energy for cells to continue performing their normal functions.

The following table summarizes some of the human diseases that may be caused by mitochondrial DNA mutations. Some of them are also caused by mutations in chromosome DNA.

Disease Feature
AlzheimerProgressive loss of cognitive ability.
Progressive chronic ophthalmoplegia Paralysis of the eye muscles.
Diabetes Mellitus High blood glucose levels leading to complications such as blindness, renal dysfunction and lower limb gangrene.
DystoniaAbnormal movements involving muscle stiffness.
Leigh's Syndrome Progressive loss of verbal and motor skills is potentially lethal in childhood.
Leber Optic Atrophy Temporary or permanent loss of vision due to damage to the optic nerve.