Consider how we all start out

Humans and other sexually producing organisms start out as fertilized eggs. This single cell divides over and over again to produce the billions of cells that make up a human. The process of mitosis explains how organisms grow larger, how organisms heal, and how organisms replace old cells. However, mitosis does not explain how an organism can be made up of so many different types of specialized cells. If mitosis was the only process in play after fertilization, humans would be a mass of cells all identical to one another in form and function. Instead, humans are made of hundreds of different cell types ranging from skin, blood, and muscle to brain and cardiac cells. These specialized cell types are organized into tissues, organs, and organ systems.

To generate such a wide diversity of cell types, something besides mitosis occurs. There must be a process that creates cells that are unique from one another in form and function, like a nerve cell vs a stomach cell.

“Like most other bodily processes, [this process] is controlled by genes, the genetic instructions encoded in the DNA of every cell. Genes instruct each cell how and when to build the proteins that allow it to create the structures, and ultimately perform the functions, specific to its type of cell.”

“Surprisingly, every nucleus of every cell has the same set of genes. A heart cell nucleus contains skin cell genes, as well as the genes that instruct stomach cells how to absorb nutrients. This suggests that in order for cells [...] to become different from one another, certain genes must somehow be activated, while others remain inactive.”

From: PBS Learning Media

In other words, chemical signals inside your body tell your blood stem cells when you need more red blood cells or white blood cells. When the blood stem cell receives a signal to specialize, specific genes inside the cell turn on and off changing how and what the cell develops into.

Read the questions and record your answers on a separate document to be turned in later.

1. Based on information provided from previous lessons, explain why cells need to divide? In other words, what is the purpose of mitosis?

2. Humans start out as a single cell (a fertilized egg). Based on information from the reading, explain why cell division alone (through mitosis) cannot be the only process involved in human development from a single cell to a fully functioning being.

3. What controls how a stem cell specializes into a more specific cell type. In other words, how can two cells with identical DNA go on to be completely different cell types?

4. (a) Define stem cells. (b) How are embryonic stem cells different from adult stem cells?

5. (a) Define and describe hematopoietic stem cells (HSC’s). (b) What types of cells do HSC’s mature into and where does this process occur?

6. Using the model of blood development, compare the two symmetric cell division situations (1) and (3). In what types of situations would an organism's HSCs produce only more stem cells (1)? And when would an organism’s HSCs produce only differentiated cells such as red blood cells (3)?