After meiosis, you have either sperm cells or egg cells. Eventually, one of those sperm or egg cells may combine and form a zygote, or the beginning of a new life. As soon as the sperm donates DNA from the father to the the DNA of the mother (that’s in the egg), the cell begins to divide. In other words, two haploid cells (n) combined to form one diploid cell (2n). Now, this zygote is undergoing mitosis.
Soon, this bag of cells will begin to differentiate, or become different types of cells. Different types of cells are called tissues, such as muscle tissue, cardiac tissue (that makes up your heart) and nervous tissue (making up nerves, brain and spinal cord). Tissues can be organized into organs (such as the heart, skin, brain and liver) and then organs can be grouped into organ systems (such as the nervous system).
If your body didn’t start to produce different types of cells, then you would just be a 140-or-so-pound ball of living cells that couldn’t really do much of anything. It would be just about as boring as it sounds. But because the purpose of life is to make more life, we need to have a reproductive system that is made up of the organs like ovaries and testicles. Also, we need to be able to eat so that we can get nutrients for our reproductive system; therefore, we need a digestive system that includes organs like the mouth, stomach and anus.
You can see that for every need, there is a system of organs that is responsible for taking care of that need. Since we have a skeletal system, we can walk around; our nervous system allows us to communicate among all the different parts of our body. What is most interesting, however, is the fact that all of these systems come from two simple cells that combine to form a zygote: a sperm and an egg.
Cell differentiation means that all of these cells – heart, skin, bones, nerves, ovaries – essentially come from one zygote. This zygote has to divide enough times and make enough changes each time so that all of these different cells can be made. Even though mitosis produces two cells that have the same DNA, these two cells may not look or act the same!
Students in a school can all be presented with the same lunch, not every student will do the same thing with the lunch. Some students will say, “Eww, this looks disgusting,” and choose to eat chips instead. Other students will say, “Well, I guess I could eat this,” and still others will say, “Wow, this looks great!” In other words, different people had different reactions to the same lunch. In the same way, the different cells that your body produces have different reactions to the same DNA. Over time, all of the different cells in your body will be produced. After a cell has been differentiated, it usually can’t turn into any other type of cell. For example, a nerve cell will always be a nerve cell; a heart cell will always be a heart cell.