Cell Division Directions: Read and Highlight then answer the questions. What has to happen for a cell to divide? Maybe some addition and subtraction? Just kidding. When we talk about the division of a cell, we are talking about how a cell reproduces. For cells to divide they have to recreate themselves much like a copier recreates all of the homework I hand out. Ok, maybe it isn’t that simple, but it does follow a similar idea. When a cell copies itself, it is an exact copy. But that is just one step in the process. If you look at the picture to the right, you can see cells dividing. In order to divide, one cell goes through a process of copying the organelles inside with one very specific organelle making certain the copying goes just right (chromosomes). What happens if the copying goes wrong though? Or worse yet, what if our cells stop copying all together? Imagine the first stages of life. In humans, an egg is fertilized (birds and bees stuff ☺) forming the first cell. But humans are made up of trillions of cells, so where do the new cells come from? Remember that according to cell theory, all cells must come from existing cells. From that one cell, an entire baby will develop. And that’s not just for humans. Every living species (eukaryotic at least) on earth goes through a similar process of copying itself. How does a new life go from one cell to so many? Simply put, the cell divides in half, creating two cells. Then those two cells divide, for a total of four cells. The new cells continue to divide and divide. One cell becomes two, then four, then eight, and so on. Each division can take anywhere from 20 minutes to 24 hours to occur. Besides the development of a baby, there are many other reasons that cell division is necessary for life: a. To grow and develop, you must form new cells. Imagine how often your cells must divide during a growth spurt. Growing just an inch requires countless cell divisions. b. Cell division is also necessary to repair damaged cells. Imagine you cut your finger. After the scab forms, it will eventually disappear and new skin cells will grow to repair the wound. Where do these cells come from? Some of your existing skin cells divide and produce new cells.
Developed by Scott DelliGatti c. Your cells can also simply wear out. Over time you must replace old and worn‐out cells. Cell division is essential to this process. Every 20 years, your body recreates itself making an entire new you.
The Cell Cycle The process of cell division in eukaryotic cells (remember eukaryotic cells have a nucleus) is carefully controlled. The cell cycle is the lifecycle of a cell, with cell division happening at the end of the cycle. This cycle has a series of steps that lead to cell division. The cell cycle can be divided into two main components, interphase and mitosis. Interphase is when cells perform their everyday functions just like we did in the cell factory. During interphase, cells spend most of their time creating a 2nd set of organelles. Mitosis is when the cell prepares to become two cells. This life cycle is like a clock as cells spend most of their time in interphase and only a small amount of time in mitosis. As a clock, interphase would be go from midnight all the way to 11:00 pm (23 hours). Mitosis would then go from 11:00 PM to midnight (1 hour). Once the cell finishes mitosis, it goes back into interphase getting ready to repeat the process again. Mitosis and Chromosomes At the end of interphase, the cell has made duplicates of everything in preparation for dividing. As the cell begins to divide, it goes through a process called mitosis. In mitosis, the nucleus divides followed by the cytoplasm dividing, resulting in two cells. After the cytoplasm divides, cell division is complete. Scientists say that one parent cell, or the dividing cell, forms two identical daughter cells (kind of like twins). Each cell has an identical set of DNA (chromosomes), and this DNA is also identical to that of the parent cell. If the cell cycle is not carefully controlled, it can cause a disease called cancer, which causes cell division to happen too fast. A tumor can result from this kind of growth. The genetic information of the cell, or DNA, is stored in the nucleus specifically in the organelle called chromosomes. The creation of an exact copy of DNA by the chromosomes insures that the cells are identical. Each organism has a unique number of chromosomes. In human cells, our DNA is divided up into 23 pairs of chromosomes. The picture to the right has a DNA molecule on the left and a chromosome on the right (shaped like an “X”).
The Four Phases of Mitosis Mitosis is divided into four phases: 1. Prophase: This is the beginning phase. The membrane that surrounds the nucleus disappears so everything inside the nucleus is just floating in the cytoplasm. The chromosomes that usually look like loose strings tighten up and pull together. 2. Metaphase: The chromosomes line up in the center of the cell. The chromosomes line up in a row, one on top of the next. It almost looks like a dance is getting ready to happen in the center of the cell. 3. Anaphase: The copies of each chromosome separate moving to opposite sides of the cell resulting in two sets of identical chromosomes. The separate like opposite ends of a magnet pushing as far apart as possible. 4. Telophase: A nuclear membrane forms around the chromosomes in both cells. Each new nucleus contains the exact same number and type of chromosomes as the original cell. Mitosis is simply the creation of two new nuclei (more than one nucleus). At the end of mitosis, the cell actually separates in a process called cytokinesis or division of the cytoplasm. It’s kind of weird, but this part of the process happens after mitosis. When the cells separate, they produce two identical cells, each with its own nucleus. That sounds confusing so let’s repeat that. Mitosis is everything just before the cell splits in half. Cytokinesis is what we call the cell separating into two cells (the division of the cytoplasm). If you look at the picture to the left, this is a representation of dividing plant cells. Each box represents one cell. When the cell has 1 or 2 nuclei, the cell is in interphase. These cells look like they have little black dots in them. This is when all of the organelles are duplicating. You can see that most of the cells in the picture are in interphase. The cells that have squiggly lines (chromosome) in them are going through mitosis. The squiggly lines look like spaghetti.
Review Questions 1. In what phase of mitosis are chromosomes moving toward opposite sides of the cell?
2. What step of the cell cycle is the longest?
3. What is the term for the division of the cytoplasm?
4. Interphase used to be considered the “resting” stage of the cell cycle. Why is this not correct?
5. What are some reasons that cells divide?
6. Why must your cells be perfect when making new cells?
7. What would happen if the cells in your liver stopped going through the process of mitosis?
8. What do you think might happen if mitosis could NOT stop happening to the cells in your brain?