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

Unit 1: Introduction to Genetics and Cell Biology 4 immunohistochemistry last chapter and how antibodies are used to locate specific targets. The antibodies recognize their antigens which are proteins. When we want to locate specific nucleic acid sequences, we use their complementary sequence to locate them. This is called hybridization. More on this will be discussed later on in the module. Once cells enter the M phase, the chromosomes begin to condense. They become the chromosomes which are most familiar to us. During the S phase, DNA is replicated. Let’s look at humans which have a total 46 chromosomes, 44 of which are autosomal chromosomes and 2 sex chromosomes (1 paternal and 1 maternal). During the S phase, you actually have a total of 92 chromosomes. Each autosomal chromosome will have a total of 4 copies and each sex chromosome will have 2 copies. This is because at the end of mitosis, you want to have 2 daughter cells with 46 chromosomes each, exactly the same as the parent cell. When a chromosome is replicated, it joins with its newly replicated copy. Those two identical copies are what we call sister chromatids (Figure 3.2.B). Other terms used to describe the sister chromatids are duplicated chromosomes or replicated chromosomes. Whatever term used, they refer to the same thing. These sister chromatids are attached at the centromere and the arms are also glued together by proteins called cohesins. These sister chromatids move through the cell during cell division. At the end of the mitotic phase, the chromosomes uncoil and become loosely packed once more. Review of mitosis The process of mitosis, also called cell division, is designated as the M phase of the cell cycle. Mitosis results in 2 daughter cells which are identical to the parent cell. Mitosis is used by different species throughout their life for reproduction, growth and tissue or cell repair. The details of each phase of mitosis are summarized in the next section. Stage I: Prophase The chromatin fibers begin to condense while nucleoli disappear and the duplicated chromosomes join together at their centromere. Microtubules begin to form from the centrioles inside the centrosome. We call this the mitotic spindle. The centrioles act as the microtubule organizing center throughout mitosis. The nuclear envelope remains intact while lengthening microtubules push the centrosomes to opposite poles of the cell. Stage II: Prometaphase The nuclear envelope starts to break down. The microtubules begin to invade the nuclear area and attach themselves to the kinetochores of