Prokaryotes Reproduce by Binary Fission

Prokaryotes reproduce by binary fission ●​ In prokaryotic cells, genres are located on a single, simple, circular chromosome ●​ This chromosome is precisely copies and the copies are divided during binary fission ●​ Two identical daughter cells are produced from one original parent cell during binary fission The large, complex chromosomes of eukaryotes replicate with each cell division ●​ Throughout most of a cells life its genetic material exists in a form called chromatin; a mix of DNA and proteins ○​ DNA molecules consists of nucleotide monomers (A, C, G, T) ○​ Human DNA contains approximately 3.2x10^9 nucleotides ●​ Each DNA molecule consists of many genes ○​ Genes are sequences of nucleotides in a DNA molecule that code for polypeptides (proteins) ○​ Human cells contain approximately 25,000 genes ●​ Prior to cell division, DNA is replicated and each DNA molecule is copied ●​ At the beginning of cell division, chromosomes form when DNA molecules coil and condense around histone proteins ○​ In its duplicated state, each chromosome now consists of 2 sister chromatids joined at a centromere The cell cycle multiplies cells ●​ Eukaryotic cells that divide undergo a cell cycle ○​ An orderly sequence of events that extends from when the cell is formed until it divides into two, separate, daughter cells ○​ Mitotic cell division produces new cells for: ■​ Replacing worn out or damaged cells in multicellular organisms ■​ Growth and development of multicellular organisms ●​ Millions of cells in our body are dividing every minute in order to maintain a total cell count of approximately 60 trillion cells ○​ Some cells divide daily, others less often, and some not at all (muscle cells, neurons) ●​ The cell cycle consists of two major phases, each divided into sub-phases ○​ Interphase ■​ Gap 0 (non dividing cells are placed in Gap 0), Gap 1, Synthesis, and Gap 2 ○​ Mitotic phase ■​ Mitosis and Cytokinesis ●​ Interphase consists of three sub-stages and accounts for about 9-% of the time required for the cell cycle ○​ Gap 0: non-dividing cells, such as muscle cells and neurons, are permanently in the G0 phase of the cell cycle ○​ Gap 1: Increases protein supply, duplicates organelles, and grows in size. Cell carries out its function in the tissue it is a part of ○​ Synthesis: DNA is replicated ○​ Gap 2: prepared the cell to divide, synthesizes proteins required for cell division ●​ The Mitotic Phase follows interphase: ○​ Chromosomes form and mitosis splits the nucleus and separates each pair of sister chromatids in the duplicated chromosomes ○​ Cytokinesis completes the mitotic phase (and cell cycle) and divides the cytoplasm and organelles of the parent cell into 2 separate daughter cells Cell division is a continuum of dynamic changes ●​ Mitosis is divided into 4 states (PMAT) ○​ Prophase: chromatin coils into chromosomes, the nuclear membrane begins to break up, and the mitotic spindle begins to form and attach to the kinetochores of chromatids ○​ Metaphase: spindle microtubules arrange chromosomes along the metaphase plate ○​ Anaphase: proton motors off the microtubules, powered by ATP, pull the sister chromatids in each chromosome toward the centrosomes (pairs of centrioles) of the cell ○​ Telophase: cell continues to elongate, new nuclear membranes form around each group of chromosomes at the poles of the cell, then the chromosomes uncoil back into chromatin. Cytokinesis begins simultaneously. Anchorage and cell density affect cell division ●​ Normal growth and development in multicellular organisms requires an organism to carefully regulate the timing of cell division ○​ Skin cells are constantly abraded and damaged leading them to wear out frequently and be replaced by the processes of cell division ○​ Liver cells do not divide unless damaged. Neurons don't divide. ●​ Anchorage dependence: plant and animal cells must be anchored to a solid surface begin dividing ○​ Prevents cells that become separated from their normal surroundings from dividing ○​ Animal cells are usually anchored to each other and the extracellular matrix to form tissues ●​ Density-dependent inhibition: cell division will slow down, and eventually stop, as the density of cells increases. If the density of cells diseases, the rate of cell division will increase. ○​ Injuries such as a cut or abrasion lower cell density and lead to increased rates of cell division o replace the dead or damaged cells ○​ Cells require protein-based hormones known as growth factors to begin dividing. As cell density increases the supply of growth factor becomes inadequate o keep cells dividing and cell division stops Growth factor signal the cell cycle control system ●​ The cell cycle control system blocks the progression of the cell cycle at key checkpoints until given a go ahead signal by proteins ○​ G1 Checkpoint: requires the cell to have grown large enough, produce adequate amounts of key protess, and receive a go-ahead signal from growth factors produced by neighboring cells when environmental conditions are right ■​ Growth factors trigger a signal transduction pathway that leads to initiation of the Synthesis (S) phase of interphase by activating enzymes that replicate DNA ○​ G2 Checkpoint: ensures that all chromosomes were replicated add consist of two sister chromatids connected to one another at a centromere before mitosis can begin dividing the nucleus and chromosomes ○​ M Checkpoint: blocks anaphase from beginning unless all sister chromatids are connected to spindle fiber of the mitotic spindle Growing out of control, cancer cells produce malignant tumor ●​ Cancer is a disease of the cell cycle; the cell cycle control system doesn't function properly and cells divide out of control and displace healthy cells ●​ The clump of uncontrollably dividing cells for a tumor ○​ Benign tumors: do not spread to other sites in the body ○​ Malignant tumors: displaces normal tissue cells and can metastasize (spread) ●​ Cancer cells can synthesize their own growth factors in order to bypass the G1 checkpoint and do not respond to density dependent inhibition and often are not anchorage dependent