3Three reasons why cells divide : (multi-cellular organisms) 1. Growth 2. Repair 3. Replacement GRR…SOME single-celled organisms use cell division to reproduce ASEXUALLY! Cell Division = Asexual Reproduction = Natural CloningALL organisms have cells that go through MITOSIS. Only SOME cells in SOME organisms undergo MEIOSIS!
4The Cell Cycle Interphase: G1 (and G0), S, G2 Mitotic Phase: Division of the cellMitosis: Division of the nucleus (1st Stage of Mitotic Phase)ProphasePro-MetaphaseMetaphaseAnaphaseTelophaseCytokinesis: Division of cytoplasm (2nd Stage of Mitotic Phase)Cell Cycle ≠ Mitosis Cell Cycle = Interphase + Mitotic PhaseCell Division ≠ Mitosis Cell Division = Mitosis + CytokinesisCell Division = Asexual Reproduction = Natural Cloning(for SOME single-celled organisms)
6Interphase 1st Phase of Cell Cycle During INTERPHASE, the cell is engaged in normal metabolic activity. This is when the cell is most active, and genes are transcribed to make proteins.G1 is the main growth stage when a cell grows from a small daughter cell into a full-sized cell.S-Phase starts if/when the cell receives a signal that it may need to divide at some point in the future.During G1 and S-phases, chromosomes appear as threadlike or spaghetti-like strands called chromatin.During the S-phase, the DNA in the nucleus is replicated (to produce sister chromatids.The two identical sister chromatids joined at the centromere.By the END of G2, each chromosome and its copy (the sister chromatids) will become condensed and appear as thick, worm-like chromosomes!During G2, the mitochondria in plant and animal cells and the chloroplasts in plant cells replicate.CELL MEMBRANENucleusCytoplasm
7G0 phase is viewed as either an extended G1 phase, where the cell is neither dividing nor preparing to divide, or a distinct “quiet” stage that occurs outside of the cell cycle.On occasion, a distinction in terms is made between a G0 cell that is only temporarily “resting” (eventually may re-enter the cell cycle) versus a cell that has become “quiet” and will never re-enter the cell cycle (e.g., heart muscle cells and neurons).Some types of cells, such as nerve and heart muscle cells, become “quiet” when they reach maturity (i.e., when they are fully differentiated).Quiet cells continue to perform their main functions for the rest of the organism's life, but do not divide and cannot be replaced.Multi-nucleated muscle cells are basically muscle cells that never fully finished cell division (did not complete cytokinesis) and therefore have multiple nuclei and continuous cytoplasm.Because these cells do not undergo cytokinesis, they are also often considered to be in the G0 stage.
9Prophase 1st step in Mitosis (1st Step in Mitotic Phase) Mitosis begins (cell begins to divide)DNA appears as condensed, worm-like chromosomes (not spaghetti-like chromatin seen in Interphase).Chromosomes consist of two identical sister chromatids joined at the centromere.Two pairs of centrioles appear and begin to move to opposite ends (poles) of the cell.Centrioles occur in pairs; each centriole in a pair is oriented at a right angle to its partner.Spindle fibers form between the two pairs of centrioles.CentriolesSister chromatidsSpindle fibers
11Pro-MetaphaseThe NEW step in the cycle that occurs between Prophase and Metaphase.Activities formerly considered to mark the last part of Prophase:Nuclear envelope disintegrates.Centrioles arrive at the poles of the cell.Activities formerly considered to indicate the beginning of Metaphase:Spindle fibers attach to the centromeres linking two sister chromatids together.Chromosomes (sister chromatids) begin migrating/moving towards center of cell.
12Metaphase 2nd (or 3rd) step in Mitosis Spindle Fibers attach to the chromatids (sister pairs of chromosomes) at the centromere.Spindle fibers are usually visible under a microscope in this stage.Sister Chromatids migrate until they are all lined up at the cell “equator” along the metaphase plate.The location of the equator or metaphase plate is determined by WHERE the chromosomes line up.Meta = MiddleOrganization of sister chromatids along metaphase plate helps ensure that in the next phase, when the chromosomes are separated, each new nucleus will receive one copy of each chromosome.CentriolesSpindle fibers
14Comparing Prophase, Prometaphase, and Metaphase Animal Cell: comparison of prophase, prometaphase, and metaphase.Plant Cell:comparison of prophase,prometaphase,and metaphase.
15Anaphase 3rd (or 4th) step in Mitosis The paired chromosomes (sister chromatids) separate and begin to move/migrate towards the opposite ends (poles) of the cell.“Ana” = oppositeThe migrating chromatids may appear V-like as they are pulled towards opposite poles.CentriolesSpindle fibers
17Telophase 4th (or 5th) step in Mitosis (Last step in mitosis, but NOT the last step in the Mitotic Phase)The chromatids arrive at the poles of the cell.A new nuclear envelope begins to form around each of the two clumps of chromatids, eventually forming two new nuclei.Chromosomes gradually become uncondensed and appear as chromatin (threads rather than rods).A cleavage furrow or cell plate may become visible towards the end of telophase.This is the end of Mitosis.NucleiNucleiChromatin
19Cytokinesis 2nd “Stage” in the Mitotic Phase Cytokinesis: the division of the cytoplasm (and everything in it).Cytokinesis usually begins during Telophase (occurs concurrently).In animal cells, the cleavage furrow pinches inward.In plant cells, a cell plate forms along the metaphase plate region and grows towards the cell walls.Cytokinesis results in two daughter cells – each with its own nucleus with identical chromosomes.Each daughter cell will end up with half of the cytoplasm and about half of the organelles—there are usually enough in the original cell that each daughter cell will end up with some.
25Interesting People Meet And Talk Constantly REMEMBER!InterphaseProphaseMetaphaseAnaphaseTelophaseCytokinesisIPMATCA mnemonic to help remember the stages of mitosis.Interesting People Meet And Talk Constantly
29From double helix… to chromatin… to chromosome…. The Many Faces of DNA During interphase, DNA that is actively being transcribed to make proteins is generally loosely wrapped around special proteins called histones.The DNA + histone structure is called a nucleosome.A strand of loosely wrapped, spaced out nucleosomes is called “beads on a string.”Chromatin is actually a spaghetti-like fiber made of hundreds of thousands of tightly wrapped and closely packed nucleosomes.Chromatin is the way DNA is stored during interphase if it is NOT being actively transcribed.
30At the end of G2 and into the beginning of prophase (the mitotic phase), the chromatin fiber will begin folding up along a protein scaffold (chromosome loops), which twist up and wrap in on itself again (forming condensed chromosome loops), and then coils up on itself to create the highly condensed chromosomes that we observe during the mitotic phase.
31Beads on a String: Active DNA being transcribed to make proteins during INTERPHASE. Chromatin Fiber: Inactive DNA being stored/not transcribed during INTERPHASE.Condensed Chromatin: Chromatin begins to fold up at the end of G2.Condensed Chromosome: In PROPHASE, condensed chromatin coils in on itself to form the highly condensed chromosome.
33CLICK HERE for animation of DNA double helix wrapping up around histones to form nucleosomes and the “beads on a string” structure,nucleosomes packing up tight to create chromatin,and chromatin coiling up and condensing to form chromosomes.
34The process of asexual reproduction begins after a sperm fertilizes an egg.