Cellular Division & Reproduction

Why is cell division important? Your body is made up of cells- trillions of cells. Many organisms start as just one cell. That cell divides and becomes two, two becomes four, four becomes eight, and so on.

Many-celled organisms, including you, grow because cell division increases the total number of cells in an organism. Even after growth stops, cell division is still important.

Every day, billions of red blood cells in your body wear out and are replaced. During the few seconds it takes you to read this sentence, your bone marrow produces about six million red blood cells. Cell division isn’t as simple as just cutting the cell in half, so how do cells divide?

Red blood cells shown from a SEM

The Cell Cycle A living organism has a life cycle. A life cycle begins with the organisms formation followed by growth and development, and finally ending in death.

Right now you are in the stage of your life called adolescence, which is a period of active growth and development.

Length of Cycle The cell cycle is a series of events that takes place from one cell division to the next. The time it takes to complete the cell cycle is NOT the same in all cells. In some human cells, the cell cycle takes about 16 hours.

Types of Reproduction Reproduction is the process by which an organism produces others of its same kind. Among living organisms, there are two types of reproduction- sexual and asexual. Sexual reproduction usually requires two organisms.

In asexual reproduction, a new organism is produced from one organism. The new organism will have hereditary material of the parent organism.

Asexual Reproduction Organisms with eukaryotic cells asexually reproduce by mitosis and cell division. New strawberry plants can be reproduced asexually from horizontal stems called runners.

Bacteria can reproduce asexually by the process called fission.

During fission, the one-celled bacterium without a nucleus copies its genetic material and then divides into two identical cells.

Potatoes reproduce by budding. Budding is a type of asexual reproduction made possible because of mitosis and cell division. When the bud becomes large enough, it breaks away to live on its own.

Potatoes Budding

Budding Hydra

Some organisms can regrow damaged or lost body parts. Regeneration is the process that uses mitosis and cell division to regrow body parts. Sponges, planaria, and sea stars use regeneration.

Budding of sea star

Cloning is another type of asexual reproduction. Cloning describes a process that can be used to produce genetically identical copies of a biological entity. The copied material which has the same genetic makeup of the original, is referred to as a clone.

Dolly (first cloned sheep)

Sexual Reproduction During sexual reproduction, two sex cells, sometimes called an egg and sperm, come together. Sperm are formed in male reproductive organs, and eggs are formed in female reproductive organs.

The joining of an egg and sperm is called fertilization, and the cell that forms is called a zygote. Following fertilization, mitosis and cell division begin. A new organism with a unique identity develops.

Your body forms two types of cells- body cells and sex cells. Body cells far outnumber sex cells. A typical human body has 46 chromosomes. Human body cells have 23 pairs of chromosomes. When cells have similar pairs of chromosomes, they are said to be diploid.

Sex cells do not have pairs of chromosomes, they are called haploid. Sex cells do not have pairs of chromosomes, they are called haploid. They have only half the number of chromosomes as body cells. Haploid means “single form”. They have only half the number of chromosomes as body cells. Haploid means “single form”. Human sex cells have only 23 chromosomes- one from each of the 23 pairs of similar chromosomes. Human sex cells have only 23 chromosomes- one from each of the 23 pairs of similar chromosomes.