How does fertilization occur in human reproduction?

Fertilisation in human reproduction occurs when a sperm cell penetrates an egg cell in the female's fallopian tube.

In more detail, the process of fertilisation begins with sexual intercourse, during which millions of sperm are ejaculated into the female's vagina. These sperm then travel up through the cervix and uterus, propelled by their whip-like tails and the contractions of the female's reproductive tract. The majority of these sperm will not survive this journey, but a few will reach the fallopian tubes, where fertilisation takes place.

The egg, or ovum, is released from one of the female's ovaries during ovulation, which typically occurs in the middle of the menstrual cycle. The egg then travels down the fallopian tube, where it may encounter a sperm. If a sperm successfully penetrates the egg's outer layer, the two cells fuse together, combining their genetic material. This fusion results in a single cell called a zygote, which begins to divide and grow into an embryo.

The fertilisation process is highly complex and involves a number of chemical reactions. For example, when a sperm first encounters an egg, it releases enzymes that help it to penetrate the egg's protective outer layer. Once inside, the sperm's nucleus fuses with the egg's nucleus, combining their DNA. This process is facilitated by calcium ions, which trigger the release of secondary messengers inside the egg. These messengers activate enzymes that lead to the hardening of the egg's outer layer, preventing other sperm from entering and ensuring that the genetic material of the egg and sperm remains intact.

After fertilisation, the zygote travels down the fallopian tube and into the uterus, where it implants into the uterine wall and begins to develop into a foetus. This marks the beginning of pregnancy. The entire process, from ejaculation to implantation, takes about a week.