Cell TypeProcessTime
fly embryo 8 minutes
bacteria 20 minutes
human skin 20 - 24 hours
human liver 1 year or more
human nerve never, once mature
Notice that nerve cells are the only type of cell that DO NOT undergo mitosis. This is why it is so very important that we protect our heads and spinal cord from injury. If these nerve cells are damaged, they will never repair and thus, cannot send the signals to and from the brain for us to think, move, and react.
When our cells replicate, the DNA that is found in the nucleus also replicates. This DNA is a molecule that stands for: Deoxyribonucleic Acid. It is a double stranded helix, like a twisted ladder, in shape. Our DNA bunches up in our nucleus to form Chromosomes. Humans have 46 chromosomes: 23 from our moms, 23 from our dads. When our cells replicate, these chromosomes have to replicate too. Here is a picture of our chromosomes. They look like chubby little X's.
Below is an actual image of the chromosome, likely using an electron microscope to get this 3-D picture. Technology is truly amazing!
The process of Mitosis takes several steps. Below is a detailed diagram of these steps. Do not worry about learning all the "phases" and names for the phases. Your focus in Grade 8 is to recognize that when a cell undergoes Mitosis, the chromosomes in the nucleus lines up, divides into two, and the one cell, becomes two "daughter" cells that are identical. In high school you will learn more about this process, and learn about Meiosis which is similar, but different.