Cell Division and Human Reproduction Processes

Understanding Mitosis and Cell Division

Mitosis is a way of producing new cells. It takes place when organisms are growing or replacing damaged cells. Asexual reproduction is a type of reproduction that involves only one parent. During mitosis, the nucleus of the cell divides so that the daughter cell has an identical set of genes as the parent cell. Genes are carried in the nucleus in long strings called chromosomes. Nearly all cells contain 46 chromosomes in each nucleus. The newly formed cells are called clones, which are exact replicas of the parent cell.

Meiosis and the Production of Gametes

Meiosis is another way of producing new cells. It results in gametes, which have half the number of chromosomes as the parent cell. It happens in the production of sex cells. After meiosis, a human sex cell contains only 23 chromosomes instead of 46. This means it has half the usual number, so when a sperm cell joins with an ovum, the result is a new cell with 46 chromosomes—a new individual. This cell is called a zygote, and it then starts to divide by mitosis.

The Menstrual Cycle and Fertilization

Close to the time of ovulation, the uterus must prepare itself for the ovum. Ovulation is the release of an egg cell from an ovary in women. The lining of the uterus starts to thicken as a network of blood capillaries grows inside it. If the ovum is fertilized by a sperm cell, it develops into a baby. If it remains unfertilized, it dies, and the soft lining breaks up; both then exit the body through the vagina. This process is called menstruation. A period typically lasts between three and seven days. The replacement of the lining of the uterus and the production of an egg cell once a month is called the menstrual cycle.

After menstruation, the uterus grows a fresh lining for each new ovum. If the egg is fertilized by a sperm cell by day 14 (which means that a sperm cell joins with an egg cell), the uterus remains and the placenta starts to develop—partly from the fertilized egg cell and partly from the mother—to feed and protect the developing embryo. When the fertilized egg cell reaches the uterus, which is rich in blood, it sinks into the soft lining. This is called implantation.

How a New Life Begins

When a sperm enters, the surface membranes of the ovum change to stop any more sperm cells from getting into it. The nucleus of the sperm cell, which contains genetic material, fuses with the nucleus of the ovum. The fusion of these two gametes is called fertilization. At this point, a new life starts to develop. The fertilized egg cell contains two sets of genetic material. One set comes from each parent, ensuring the new individual has characteristics of both parents and is unique.