All complex organisms originate from a single fertilized egg. That one cell divides and becomes two. Those two cells divide to become four, and so on. Cells then specialize and change into their various roles like nerve cells and muscle cells.

Do we have the same number of cells throughout our lifetime? No! Do we have the same number of cells throughout our lifetime? We get more cells through a process called cell division.

Organisms grow. Multicellular organisms, including you, grow because cell division increases the total number of cells in an organism. Damaged or old cells need to be replaced. This also requires new cells.

Every day, billions of red blood cells in your body wear out and are replaced. When you get a cut or scrape, you need to replace those cells. Cell division is important to unicellular organisms, too—it’s how they reproduce themselves.

cell division—the formation of new cells from cells that have a nucleus; they divide, then divide again mitosis—the process in cell division by which the cell’s nucleus divides to form two nuclei, each with the same exact chromosome and DNA as the original cell

When mitosis happens, the two new nuclei are identical—they have the same number and type of chromosomes as the original cell. Each of the trillions of cells in your body has a copy of the same DNA--except sex cells.

The Process of Making Sex Cells Meiosis The Process of Making Sex Cells

Sexual Reproduction Most organisms reproduce sexually. This means two parents are needed to produce offspring. This requires a sex cell from each parent.

What are sex cells? Sex cells, or gametes, are the cells that produce offspring. The sex cell produced by males is sperm. The sex cell produced by females is the egg.

What are chromosomes? They are found in the nucleus of cells and are made of DNA. They determine all of the physical characteristics of organisms. Alleles, or genes, are on the chromosomes.

What are chromosomes? Different organisms have different numbers of chromosomes. Humans have 23 pairs for a total of 46.

Sutton & Grasshoppers Walter Sutton used a microscope to count the chromosomes in grasshopper cells He counted 24 chromosomes in every cell, except the sex cells. Those cells only had 12 chromosomes.

What did Sutton learn? He figured out that grasshoppers must get half their chromosomes (and alleles) from the female parent and half from the male parent. 12 chromosomes in the egg + 12 chromosomes in the sperm 24 chromosomes in the zygote (fertilized egg)

What is meiosis? The process where the number of chromosomes in body cells is reduced by half in sex cells. This happens through a series of stages. You may have your mother’s eyes and your father’s nose because of the DNA you received from each parent through your father’s sperm and your mother’s egg.

Reminders: Your body makes body cells and sex cells. Body cells form your brain, skin, bones, and other tissues and organs. A sex cell has half of the number of chromosomes found in a body cell. Think about it—if each sex cell had 23 pairs of chromosomes (46 total), then when two combined you’d have a total of 46 chromosomes (92 total)!

Asexual Reproduction Asexual reproduction is where offspring are produced by only one parent. Since offspring receive all of their genes from that one parent, they are identical to that parent. Two examples of asexual reproduction are budding and binary fission.

Budding These are yeast cells reproducing by budding.

Binary Fission These are bacteria reproducing by binary fission.

Mr. Lee Now that you have learned more, listen to the Mr. Lee song one more time. YouTube - Mr. Lee - Genetics rap