04 Cell Cycle PDF

Topic 4: The Cell Cycle Topic 4: The Cell Cycle (Corresponding Textbook Material: Chapter 9) Objectives: Introduction to the Cell Cycle (Outline Location: I and II) Describe wh...

Topic 4: The Cell Cycle Topic 4: The Cell Cycle (Corresponding Textbook Material: Chapter 9) Objectives: Introduction to the Cell Cycle (Outline Location: I and II) Describe why the cell cycle is important to life. Interphase (Outline Location: III) For each of the stages of Interphase (G1, Synthesis, G2), describe what is happening within the cell and why it is important to the cell cycle. Cell Division (Outline Location: IV) For each of the phases of cell division, describe what is happening within the cell and how it contributes to cell division. Cell Cycle Regulation (Outline Location: V) Describe the purpose of each of the major checkpoints (G1, G2, and M) that regulate the cell cycle. Describe the internal and external factors that play a role in regulating the cell cycle. Describe how the loss of cell cycle regulation affects the body.  Topic 4: The Cell Cycle I) Review A) According to Cell Theory, cells are the fundamental structural and functional unit of life 1) The activity of an organism depends on the individual and collective activity of its cells 2) The activity of the cell is dictated by subcellular structures 3) The continuity of life has a cellular basis (i.e. all cells originate from other cells) B) The number of cell in an organism can vary from 1 to trillions depending on the size of the organism II) Cells undergo a cycle of growth, development, and reproduction known as the cell cycle A) The cell cycle is important because it allows organisms to: 1) Grow and develop 2) Repair themselves 3) Reproduce asexually (in some organisms) B) The cell cycle is composed of two basic parts: 1) A period of growth and development called interphase 2) A period of cell division III) Interphase—a period of growth and development. A) A cell in interphase performs all its normal functions in the body and its DNA is being transcribed and translated to produce proteins  Topic 4: The Cell Cycle B) Interphase is composed of three parts: 1) The first Gap phase (G1) a) A new cell just entering G1 will have a diploid number (2n) of chromosomes (Figure 1) b) Chromosomes that were condensed for purposes of cell division are uncoiled so that the DNA can be used c) The duration of G1 can vary tremendously—G1 may be as short as a few hours or a cell may stay in G1 indefinitely, in which case phase is often called the G0 phase 2) The Synthesis phase (S) a) Period during which the DNA is replicated (Figure 2) i) Results in duplicate copies of all the chromosomes ii) The duplicate copies are called sister chromatids iii) The duplicate copies are connected to each other by a centromere Note: Based on the example in figure 1, there are still just six chromosomes after DNA replication. However, each chromosome now contains its genetic information in duplicate copy. b) Synthesis may never occur if the cell stays permanently in G0  Topic 4: The Cell Cycle 3) The second Gap phase (G2) a) During G2, a cell is accumulating the resources and cellular materials needed for cell division b) During G2, the centrosome duplicates to form two centrosomes. The centrosomes will play an important role during cell division IV) Cell division A) The process of cell division has two parts: 1) Mitosis 2) Cytokinesis B) Mitosis 1) Mitosis is the process of dividing the nucleus of a cell (and the genetic information it contains) into two equal nuclei that each have a complete copy of the genetic information 2) The process of mitosis is divided into several steps (Figure 3) a) Prophase i) Early prophase Chromatin coils (condenses) into chromosomes The nucleoli disappear Spindles form and extend out from the centrosomes Centrosomes move toward opposite ends of the cell ii) Late prophase (also called Prometaphase) Nuclear envelope fragments Some spindles attach to specialized protein structures on the centromeres known as kinetochores Some spindles from opposite poles interact with each other and do not attach to kinetochores  Topic 4: The Cell Cycle b) Metaphase i) Chromosomes line up on plane called the metaphase plate ii) Movement of the chromosomes to the metaphase plate due to the actions of spindles extending from the centrosomes at opposite poles of the cell c) Anaphase i) Spindles contract separating the sister chromatids ii) Each sister chromatid is now a chromosome iii) The cell elongates as the spindles that are not attached to the kinetochores elongate d) Telophase i) Nuclear envelope forms around each set of chromosomes ii) Nucleoli reappear iii) Chromosomes begin to uncoil so that DNA can be used for protein synthesis iv) Remaining spindles are dismantled C) Cytokinesis 1) Cytokinesis is the process of dividing the cell (other than the nucleus that already divided during mitosis) into two daughter cells. 2) Cytokinesis normally occurs nearly simultaneous with telophase 3) Cytokinesis does not always occur, which can result in cells with multiple nuclei (e.g. muscle cells) V) Cell cycle regulation A) Proper control and timing of the cell cycle is crucial to normal maintenance, growth, and development. B) Three major checkpoints that regulate the cycle 1) G1 checkpoint—determines whether a cell will proceed toward cell division or enter the G0 phase 2) G2 checkpoint—makes sure that all DNA is accurately replicated and/or repaired before allowing mitosis to begin 3) M (Metaphase) checkpoint—makes sure that all chromosomes are properly connected to a spindle before allowing mitosis to proceed into anaphase  Topic 4: The Cell Cycle C) The cell cycle is subject to both internal and external control 1) External regulators include a) Presence or absence of essential nutrients b) Growth factors/hormones from other cells in the body c) Cell density (density-dependent inhibition) d) Anchorage to a substrate (Anchorage dependence) 2) Internal regulators are generally regulatory proteins (such as cyclins and kinase enzymes) that activate or inactivate other proteins within the cell D) Loss of cell cycle control can result in cancer 1) Cancer is an uncontrolled proliferation of cells 2) Cancers may be caused by any change in cell cycle regulation that a) abnormally increases the rate, or changes the location, of cell proliferation b) abnormally decreases the rate of programmed cell death (apoptosis)