How many cells do you begin with? -1 How many do you have now? -Billions How does one cell become a billion different cells with different roles to play? -Differentiation and specialization. We start with fertilization. The egg and sperm come together and create a zygote. The zygote will divide and (we will skip a few) become a blastula. A blastula is a group of cells that is not differentiated yet. It is a hollow sphere of cells. All the cells are stem cells so they can become anything. This is the time that the blastula changes into a gastrula. The blastula begins to fold inward and form a three germ levels. This will be the forming of the mouth. "It is not birth, marriage, or death, but gastrulation, which is truly the most important time in your life." Lewis Wolpert (1986) This is when the cells begin to specialize. There are three levels. - Ectoderm -> This is the outer layer. This will form the skin. -Mesoderm -> This is the middle layer. This will form the organs and nervous system. -Endoderm -> This is the inner layer. This will become the digestive tract. Why do we want cells to be specialized? - Taste with feet. - Smell with hands. We need our cells to know what they are doing. How does the cell specialize? - The DNA controls it. Is DNA the same in all of our cells? - Yes So how do different cells do different things? - DNA will only send out the RNA for certain proteins. - The proteins will tell the cell what they need to do.