[Music] cell division introduction cell division is a process by which a parent cell divides into two or more daughter cells a typical human cell contains 46 chromosomes two copies of the 22 homologous chromosomes plus two copies of the sex chromosome when the cell divides it roughly doubles all of its contents including organel and the cytoskeleton and duplicates each chromosome into two sister chromatids the cell then splits the large double cell into two identical daughter cells each with the same 46 chromosomes as the mother cell cell cycle it is the ordered sequence of events that occur in a Cell in the preparation of cell division to produce two daughter cells the total cell cycle time for a typical rapidly dividing human cell is 24 hours and the most minimum time is 12 hours the cell cycle can be divided into four phases G1 Gap 1 phase s or DNA synthesis phase G2 Gap 2 phase M mitosis phase the cell involves two phases interphase and mitosis interphase further involves the G1 phase or Gap One S phase synthesis and G2 phase Gap 2 which prepare the cell for division G not phase is not a part of interphase the time taken for S G2 and M are similar for most cell types G1 phase lasts several hours to months S phase lasts for 6 to 8 hours G2 phase lasts approximately 2 to 5 hours and M phase for 1 to 2 hours in contrast the duration of G1 phase shows a vast variation in a rapidly dividing phase G1 phase can be less than 2 hours in a resting cell G1 phase can be Beyond 100 hours or more in ovaries primary us sites remain in ditin stage of meosis from fifth month of intrauterine life until the period before ovulation begins let's focus on events which takes place during each phase G1 phase this phase lasts for several hours to months and occurs after mitosis this phase is critical For the synthesis of RNA proteins and cell organel the synthesis of proteins that are required for mitosis such as the mitotic spindle is particularly important important during this phase there is one chromatid present per chromosome the cell grows during this phase nucleotide excision repair takes place at the end of this phase there is a G1 checkpoint before entering S phase G1 checkpoint or restriction checkpoint is a cell division checkpoint that restricts entry into the synthesis or S phase without growth signals that lift this checkpoint cells will not divide after this checkpoint cells become committed to division loss of this checkpoint for example from loss of p-53 function leads to unregulated cell division S phase this phase lasts for approximately 8 hours during the S phase the cell replicates its DNA so that by the end each chromosome has two copies the copies are attached to each other by cohesion proteins in a structure called a centromere and together they're referred to as sister chromatids each pair of sister chromatids is considered to be one chromosome because they are just copies of the same genetic material and still attached to one another this phase of the cell cycle is where histones the proteins that bind and organized DNA are duplicated at this point the cell has double the amount of genetic material most mismatch repair takes place during the S phase once the S phase is initiated the cell must be completed G2 phase this phase lasts for approximately 2 to 5 hours during G2 the cell continues to grow and prepares for division by reorganizing its organel and cytoskeleton this phase is characterized by further synthesis of proteins required for mitosis at the end of G2 phase and before entering the the mitosis there is a checkpoint known as G2 checkpoint G2 checkpoint is a cell division checkpoint that occurs during the G2 phase which checks for DNA damage and completeness of DNA replication it facilitates the initiation of mitosis through phosphorilation of various proteins for example histones regulat synchronized cell division mediated by mitosis promoting factor that cycl B cdk1 complex M phase in the M phase the DNA condenses to form visible pairs of sister chromatids attached at the centrr they're subsequently separated and moved to opposite poles of the cell after the M phase the cell splits into two daughter cells in a process called cyto Kinesis karyotypes are obtained from cells in the M phase mitosis is the final step in cellular division prior to the two daughter cells separating from one another and it has four stages stage one prophase profase is a stage during which the DNA condenses into 46 pairs of sister chromatids linked together at the center by a centrr centrr keep the sister chromatids together until it is time to separate and ensure that each daughter cell gets the right sister chromatids without any duplicates or loss centrosome separation the centrosome is the point of origin of the mitotic spindle it consists of two centrioles and a surrounding Matrix from which the microtubules emerge prophase is a stage of formation of the mitotic spindle stage two metaphase metaphase is a stage during which all the sister chromatid pairs align at the center of the cell and prepare for separation this is a crucial step in which checks are made to make sure that all the chromosomes are attached to the mitotic Centrum mirors in such a way that when they are pulled apart the sister chromatids are evenly split between the two new cells clinical link Vin Christen a chemotherapy drug prevents the formation of microtubules that attach to the centrom in mitosis M checkpoint spindle checkpoint it is a third checkpoint between metaphase and anaphase in mitosis this checkpoint ensures correct alignment of the chromosomes and sister chromatids at the equatorial plane before the separation of sister chromatids stage three anaphase in anaphase the sister chromatid start separating and moving to the opposite poles of the cell stage four tase in tase the cell starts physically dividing into two it is characterized by DEC condensation of the chromosomes disintegration of the mitotic spindle formation of a new nuclear membrane cell bodies division at the equatorial plane ribosomal RNA synthesis clinical link proper functioning of the mitotic spindle is a prerequisite for chromosome Transportation inhibition with spindle poisons leads to arrest of mitosis and cation of cell division spindle poisons includ cesin as well as vinka alkaloids and taxanes which inhibits microtubular polymerization cytokinesis it is usually marked by formation of a Clift in the cell membrane as the mother cell starts dividing into two daughter cells coming to the G not phase it is also known as the resting phase during the G not phase the cell exits the cell cycle and becomes quiescent until it receives external stimuli in the form of growth factors to start replicating again at which time it will enter back into the cell cycle at the G1 stage growth factors are of different forms including hormones and proximity signals from nearby cells these ultimately initiate a signaling Cascade inside the cell that activates the proteins necessary for initiation of cell division the amount of time a cell spends in G not phase is dependent on the cell type mature neurons spend most if not all of their lifespan in G not phase while cells that produce the intestinal lining May rarely or never enter G not phase