Title Cell Cycle Handout.pdf ✅

Unit 1: Introduction to Genetics and Cell Biology 2 cells. It also has the ability to give rise to new stem cells. Embryonic stem cells are found in developing embryos while adult stem cells are found in adult organisms. While embryonic stem cells are for giving rise to a new organism, adult stem cells act to maintain the cell numbers throughout an organism’s lifetime. Cells also have a limited lifespan so there is a need to replace old cells in the body. Examples of adult stem cells are hematopoietic stem cells which produce blood cells indefinitely, another are stem cells found in hair follicles which stimulate growth of hair and lastly the stem cells in the skin which produce the skin cells before they become keratinized. Figure 3.1 Overview of the cell cycle. The cell cycle is composed of 4 total phases and is essential for survival of different species. (Source: Alberts et al., 2007) Interphase is essentially the stage where the cell prepares for cell division. The time for interphase takes up majority of the time for cell division. Usually, the mitotic phase in humans last around an hour while interphase could last for several, depending on the state of the cell. The order of events during interphase is G1, S, and then G2. During the gap phases, the cell produces enough material for 2 daughter cells. This includes enough cell organelles, enough cytoplasm, enough lipids and enough proteins for 2 cells. Gap phases allow the cell to make these necessary molecules. In between the gap phases is the S phase. It is during the S phase that DNA is replicated. After the S phase, the cell contains twice the amount of DNA it normally would have. G1 focuses a lot on preparing the machinery for the S phase in addition to expanding the cells and creating the cell material for the daughter cells. G2 focuses more of the proteins and enzymes that are needed for the M phase of the cell cycle. The cell has many checkpoints during interphase and cells are not allowed to go into the next phase without first ensuring that everything that needs to be done in one phase is completed. Chromosomes during cell division Chromosomes are not generally visible as dense masses under the microscope. DNA exists as chromatin in the nucleus. This chromatin is a complex of DNA and proteins. If you think back on hematoxylin-eosin stained samples you have seen, the nucleus, and the DNA inside it, appear as one solid block of color. Nucleoli may be more darkly stained but in general, chromatin is loosely packed