Title Cell Cycle Handout.pdf ✅

Unit 1: Introduction to Genetics and Cell Biology 1 Topic 3: The Cell Cycle, Mitosis and Meiosis Cell division is one of the most important features of living things. It serves three main functions: reproduction, growth and development, and tissue renewal. Prokaryotes and simple eukaryotes make use of cell division, specifically binary fission, as a means of asexual reproduction. Multicellular eukaryotes use cell division during growth and development. For example, humans all develop from a single fertilized egg cell. Lastly, whenever there is tissue damage in organisms, cell division is used to repair and renew damaged tissues. Cell division is a process that is under the cell cycle. This chapter focuses on the cell cycle and also includes a review of mitosis and meiosis Learning Objectives At the end of this topic, you will be able to: a. identify the phases of the cell cycle and describe what occurs in each stage. b. identify the stages in mitosis and what occurs in each stage. c. identify the stage in meiosis and what occurs in each stage. d. be able to state the differences between mitosis and meiosis Presentation of Content The cell cycle The cell cycle is a series of events that lead to cell division and the production of two daughter cells, each containing chromosomes identical to the parent cell. Cell division is also called mitosis. There are several events that occur before a cell reaches the mitotic stage. The cell cycle is essentially divided into 2 phases: interphase and the mitotic phase (M phase). Interphase is further divided into gap phases, (G1 and G2) and the synthesis (S phase). The M phase is further divided into mitosis and cytokinesis where mitosis is divided into prophase, prometaphase metaphase, anaphase and telophase while cytokinesis is the physical division of the cell mass. Any cell that is outside the cell cycle is in the G0 phase. A cell remains in G0 unless it receives signals that it needs to divide. Cell division may be triggered by different factors. Injury to tissue is one such factor. The need to replace the damaged cells starts a cascade of signals that push the cell into interphase. In other cases, there are cells that have to be actively dividing almost all the time. Cell division of such cells begin in their stem cells. Stem cells are cells whose daughter cells are able to produce different types of


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