Biology

The desire to procreate is a basic human drive. DNA plays a role in making the urge for procreation universal. DNA is a molecule that carries genes in a tiny, winded staircase. It is responsible for issuing chemical instructions that make up the body. DNA has the ability to copy itself, in two ways. The first is cloning, in which exact replicas of DNA strands are created. This method is used by bacterium. The second and more advantageous method, sex, is used by majority of the organisms on this planet. It facilitates variety and ensures survival of species.
   
Human body is made of cells. Every cell carries 46 chromosomes, 23 from the mother and the remaining 23 from the father. Genes are carried on long strands called chromosomes. Most cells have two versions of every gene. In sexual reproduction, two individuals contribute DNA, which is placed in sperms and eggs by most species. Sperms and eggs are produced through a process called meiosis wherein a pair of chromosomes replicates, embraces each other and exchanges genetic information. Then the cell divides gradually creating a sperm or egg cell with only half the number of   chromosomes. Every chromosome in the newly created cell contains a new gene combination.

In men, sperms are generated on a daily basis since puberty. However, a woman creates eggs as a fetus. It produces several million eggs in two months, which die with age. Every month, an immature egg nurtures with aid from helpers. The fully grown egg along with its helpers is ejected out of the ovary into the open end of Fallopian tube and then to the uterus, which pulls the egg inside. During intercourse, millions of sperms are released into a fluid from several glands in the male. The fluid passes through a 15-inch tube, followed by the abdomen and is ejected into the vagina through the penis. On entering the vagina, the sperms face acidic conditions which must be escaped to avoid death. Through their journey, womans body and the egg itself control the actions of sperms. To fertilize an egg, a sperm must pass though the cervix past the mucus protective layer, enter the uterus, pass through the chaperones surrounding the egg and break through the eggs protein shield called zona. If the proteins on the eggs surface match those on the sperm cap, the sperm loses its outer coating and releases powerful enzymes creating a hole in the zona. The membranes of the sperm and egg fuse together.

The sperm contents are then pulled into the egg. To create a viable embryo from fertilization, the zona locks out other sperms. The egg and sperm release their 23 chromosomes, which embrace each other followed by cell division. The bundle of cells within the zona is called a blastocyst. It breaks out of zona on the sixth day and releases chemicals to prevent attack from the mothers immune system. For food, it reaches the uterine lining and buries itself into the uterus. Next, it develops a placenta, containing villi on its underside that retrieve oxygen and food from the mothers blood. They are transferred to the fetus, two-month old embryo, through the umbilical cord. Two weeks post conception, the cells of the blastocyst organize themselves into an embryo through gastrulation. The process takes place inside uterine lining in humans. Cells align themselves and organs begin to develop. Cells communicate with each other through chemical messages, which facilitate activation of correct genes and production of right proteins. Proteins are the building blocks of human body.

Heart is one organ that works during early weeks of embryonic development. Other organs and systems develop much later. During early stages of development, the gender of a baby can only be known by examining its chromosomes. By the beginning of third trimester all organs are in place and the fetus main job is growth. In this phase, fat accumulation, even in the brain due to myelin manufacture begins essential. The babys need for fat eventually increases to a point where it must come out of the womb for survival. For birth, the cervix opens up to about four inches. Baby goes through significant contractions before it can come out of the narrow opening of cervix. If the same is not possible a Cesarean section is used.