1. What is cell differentiation?
Differentiation is the process by which cells become specialized and take on specific roles in an organism.
During human develo pment, a single cell (a fertilized egg) differentiates into every different kind of specialized cell type found in the body, including blood, muscle, and nerve cells.
Cells that are undifferentiated are known as stem cells. Stem cells are found in embryos (embryonic stem cells) and in adults (adult stem cells).
1. Infer What other process, besides cell differentiation, takes place rapidly during human development?
A nerve cell

2. What are the different types of stem cells?
In general, embryonic stem cells can be either totipotent or pluripotent. Adult stem cells are multipotent.
Stem cells that have the ability to differentiate into every type of cell in the body are called totipotent.
Stem cells that can differentiate into most, but not all type of cells are called pluripotent. Cells present within the blastocyst that forms shortly after fertilization in humans are pluripotent.
Stem cells present in an adult organism that can differentiate into a limited number of cell types are called multipotent.
2. Define What does it mean if a cell is multipotent?

3. What is the role of DNA in cell differentiation?
Nearly all cell types in a large organism contain the same DNA, yet they express their genes differently.
Gene expression describes the way in which genetic information is put into action. DNA is first transcribed into mRNA. The codons of mRNA specify the sequence of amino acids in a protein. Proteins, in turn, play a key role in producing an organism’s traits.
3. Define In your own words, define gene expression.

4. How is gene expression controlled?
Genes can be affected in several ways by DNA-binding proteins called transcription factors.
Transcription factors bind to regulatory regions of a gene and affect their expression. Some transcription factors turn genes on, and others help to turn genes off.
Different transcription factors are associated with the differentiation of stem cells into bone, muscle, and nerve cells.
4. Infer What do you think it means if a gene is “turned off”?

5. What is the role of RNA in cell differentiation?
Changes in the pattern of gene expression affect the types and amounts of RNA produced by a cell.
Recall that DNA is transcribed into mRNA. mRNA then provides instructions that direct the production of proteins in the process of translation.
The types and number of proteins a cell generates help to determine how a cell differentiates.
New research suggests that RNA can directly affect cell differentiation by shutting down the expression of targeted genes through a process call RNA interference.
5. Explain What is the relationship between DNA, RNA, and proteins?

6. What is the role of the environment in cell differentiation?
A cell’s internal and external environment can affect its pattern of gene expression and cell differentiation.
For example, during development in many animals, the concentration of certain transcription factors in the cytoplasm of the egg cell determines basic body plan—which part of the embryo will become head, tail, front, and back.
External environment, specifically temperature, during the development of turtles and some other reptiles helps to determine the sex of hatching offspring.
In fruit flies, the expression of genes (colored areas) that control basic body plan formation are affected by the cells’ environment. Specifically, they are turned on or off in response to the relative concentrations of various transcription factors present in the cytoplasm.
6. Apply Concepts Can you think of another example of how the environment affects the way certain traits are expressed?

7. How does the environment affect cell differentiation in plants?
Environmental effects on gene expression are particularly striking in plants.
For example, the environment helps to determine the patterns of gene expression that lead to flowering.
“Short-day” plants will flower only when exposed to extended periods of darkness.
“Long-day” plants will flower only when exposed to short periods of darkness.

8. Can cell differentiation be reversed?
Undifferentiated adult stem cells make it possible to continue to form specialized, differentiated cells. In humans, there are high concentrations of these adult stem cells in the skin, brain, and bone marrow.
Recent research has shown that in many cases, adult skin cells can be induced to reverse their differentiation process and assume many of the properties of embryonic cells. The resultant cells are known as “induced pluripotent stem cells,” or iPS cells.
Induced pluripotency is now the subject of intense research around the world.
7. Infer What might be the advantage of iPS cells over other types of stem cells?