Lecture 3 Notes.docx

Biology Lecture 3 Notes (Cell Division)  Mitosis serves as a way to divide DNA equally and precisely; Mitosis occurs due to three systems that work together: o One ‘system’ is the timing and programming to ensure that DNA can divide; ‘an elaborate master program of molecular checks and balances that ensures an orderly and timely progression through the cell cycle’. o The second ‘system’ divides the DNA into two copies. o The final system uses the cytoskeleton to separate the DNA into the two daughter cells.  Depending on the age, some cells would undergo a different type of division called meiosis, creating daughter cells that only have half the normal amount of chromosomes.  Replication of DNA of each chromosome creates two molecules called sister chromatids. o The equal distribution is called chromosomal segregation.  Steps of Mitosis: o Interphase:  Interphase is the longest phase is cell division as it prepares the cell of mitosis (M cycle).  Within the G 1ycle, the cell creates RNA, proteins and other types of cellular molecules but not DNA.  G 1ycle stands for gap, where DNA is not being replicated.  After the G1cycle, the S cycle begins; duplicating chromosomal proteins as well as DNA and continues synthesis of other molecules as well.  S cycle stands for synthesis, where DNA is being (duh) synthesized.  Once S cycle is completed, the cell enters2G cycle, where DNA synthesis stops, but other molecules contain to be replicated.  After the end of the G2cycle, actual cell division beings. o Prophase:  During prophase, the chromatin condenses into chromosomes.  The nucleolus disappears in most species of cells, shutting own RNA synthesis; rRNA is made in the nucleolus.  The mitotic spindle forms in between the two centrosomes as they start heading towards the opposite ends of the cell to form the spindle poles. o The spindles develop as microtubules come out from the spindle poles. o Prometaphase:  During prometaphase, the nuclear envelope starts to break down.  Kinetochores form on either side of the chromatid on the sides of the centromere.  Since microtubules attach to the kinetochores, the connections are important as they divide the chromatids from each other.  Microtubules attach to the kinetochores, and the ones that don’t attach overlap the opposing side’s tubules. o Metaphase:  The sister chromatids become aligned on the spindle midpoint (also known as