Fertilization:
Human fertilization, known as conception, is the fusion of genetic material from the haploid sperm cell and the secondary oocyte, to form the zygote. The process takes approximately 12-24 hr and normally occur in the ampulla of the uterine tube.
           
Fertilization is a series of complex, coordinated molecular event that begins with the sperm and egg meeting and ends with mixing of maternal and paternal chromosomes during metaphase 1 division of zygote. Following ovulation, the oocyte, which is about 0.15mm in diameter, passes into the uterine tube. The oocyte, having no power of locomotion, is wafted along by the cilia and by the peristaltic muscular contraction of the uterine tube. At the same time the cervix, which is under the influence of estrogen, secretes a flow of alkaline mucus that attracts the spermatozoa. In the fertile male at intercourse, approximately 200 million sperm are deposited in the posterior fornix of the vagina. Approximately 2 million reach the loose cervical mucus, survive and propel themselves towards the uterine tubes, while the rest are destroyed by the acid medium of the vagina. Approximately 200 sperm will ultimately reach the oocyte. Sperm swim from the vagina and through the cervical canal using their whip-like tails. Prostaglandins from semen and uterine contractions as a result of intercourse facilitate the passage of the sperm into the uterus and beyond. Once inside the uterine tubes , the sperm undergo a process known as capacitation. This process takes up to 7 h. Influenced by secretions from the uterine tube, the sperm undergo changes to the plasma membrane, resulting in the removal of the glycoprotein coat and increased flagellation. The zona pellucida of the oocyte produces chemicals that attract capacitated sperm only. The acrosomal layer of the capacitated sperm becomes reactive and releases the enzyme hyaluronidase known as the acrosome reaction, which disperses the corona radiata allowing access to the zona pellucid. Many sperm are involved in this process. The proteolytic enzyme acrosin and other enzymes such as esterases and neuraminidase produce an opening in the zona pellucida. The first spermatozoon that reaches the zona pellucida binds to it and subsequently penetrates it.
Upon penetration, the oocyte releases cortical granules; this is known as the cortical reaction. The cortical reaction and depolarization of the oocyte cell membrane makes the zona pellucida impermeable to other sperm. This is important as their are many sperm surrounding the oocyte at this time. Only the head and tail of the penetrating spermatozoon enter the cytoplasm of the oocyte, its plasma membrane and mitochondria are left behind. The spermatozoon and oocyte fuse. The oocyte at this stage completes its second meiotic division and becomes mature, with a haploid number of chromosomes (23) in its pronucleus. On entry to the oocyte, the tail of the sperm degenerates and their is the formation of the male pronucleus, which contains a haploid number of chromosomes (23). The male and female pronuclei fuse to form a new nucleus that is a combination of 23 pairs of chromosomes from both the sperm and oocyte, referred to as a diploid cell. The male and the female gametes each contribute half the complement of chromosomes to make a total of 46. This new cell is called a zygote.
Dizygotic twins are produced from two oocytes released independently but in the same timeframe fusing with two different sperm; they are genetically different from each other.
Monozygotic twins develop from a single zygote for a variety of reasons, where cells separate in to two embryos, usually before 8 days following fertilization. These twins are genetically identical.