Mitosis and meiosis


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Mitosis and meiosis

  2. 2. CELL CYCLE The cell cycle, or cell-division cycle, is the series of events thattake place in a cell leading to its division and duplication(replication). In cells without a nucleus (prokaryotic), the cellcycle occurs via a process termed binary fission. In cells with anucleus (eukaryotes), the cell cycle can be divided in twoperiods: interphase—during which the cell grows, accumulatingnutrients needed for mitosis and duplicating its DNA—andthemitosis (M) phase, during which the cell splits itself into twodistinct cells, often called "daughter cells" and the finalphase, cytokinesis, where the new cell is completely divided. Thecell-division cycle is a vital process by which a single-celled fertilized egg develops into a mature organism, as well asthe process by which hair, skin, blood cells, and some internalorgans are renewed.
  3. 3. So, the cell cycle consists of:*Interphase*Mitosis• Chromosomes are not visible because they are uncoiled.
  4. 4. INTERPHASE Interphase is the phase of the cell cycle in which the cell spends the majority of its time and performs the majority of its purposesincluding preparation for cell division. The stages of interphase are:• G1 (Gap 1), in which the cell grows and functions normally. During this time, much protein synthesis occurs and the cell grows (to about double its original size) - more organelles are produced, increasing the volume of the cytoplasm. If the cell is not to divide again, it will remain in this phase.• Synthesis (S), in which the cell duplicates its DNA (via semiconservative replication). This is also known as the Swanson phase.• G2 (Gap 2), in which the cell resumes its growth in preparation for mitosis.
  5. 5. MITOSIS Mitosis is the process by which a eukaryotic cellseparates the chromosomes in its cell nucleus intotwo identical sets, in two separate nuclei. It isgenerally followed immediatelyby cytokinesis, which divides thenuclei, cytoplasm, organelles and cellmembrane into two cells containing roughly equalshares of these cellular components. Mitosis andcytokinesis together define the mitotic (M)phase of the cell cycle—the division of themother cell into two daughter cells, geneticallyidentical to each other and to their parent cell.
  6. 6. PROPHASE • The chromosomes coil.• The nuclear membrane disintegrates. • Spindle fibers form.
  7. 7. METAPHASE• The nuclear membrane is completely gone. • The chromosomes become aligned.
  8. 8. ANAPHASE • The chromatids separate; the number of chromosomes doubles.• Pushed and pulled toward opposite poles by the spindle fibers.
  9. 9. TELOPHASE • The cell divides into two. • The chromosomes uncoil. • The nucleus reforms.• The spindle apparatus disassembles.
  10. 10. MEIOSISMeiosis is a special type of cell divisionnecessary for sexualreproduction in eukaryotes. The cellsproduced by meiosis are gametes or spores.In many organisms, includingall animals and land plants, gametes arecalled sperm and egg cells.
  11. 11. Meiosis I : Reductional DivisionMeiosis I separates homologouschromosomes, producing two haploid cells (Nchromosomes, 23 in humans), so meiosis I is referredto as a reductional division. A regular diploid humancell contains 46 chromosomes and is considered 2Nbecause it contains 23 pairs of homologouschromosomes. However, after meiosis I, although thecell contains 46 chromatids, it is only considered asbeing N, with 23 chromosomes. This is becauselater, in Anaphase I, the sister chromatids will remaintogether as the spindle fibres pull the pair towardthe pole of the new cell. In meiosis II, an equationaldivision similar to mitosis will occur whereby thesister chromatids are finally split, creating a total of4 haploid cells (23 chromosomes, N) - two from eachdaughter cell from the first division.
  12. 12. Prophase IDivided into 5 substages: Leptonema During this stage, the chromosomes begin to condense and become visible. Researchers also believe that homologous pair searching begins also at this stage. Zygonema The chromosomes continue to become denser. The homologous pairs have also found each other and begin to initially align with one another, referred to as rough pairing. Lateral elements also form between the two homologous pairs, forming a synaptonemal complex. Pachynema Coiling and shortening continues as the chromosomes become more condense. A synapsis forms between the pairs, forming a tetrad.
  13. 13. DiplonemaThe sister chromatids begin to separate slightly, revealingpoints of the chiasma. This is where genetic exchangeoccurs between two non-sister chromatids, a processknown as crossing over.DiakinesisThe chromosomes continue to pull apart, but non-sister chromatids are still loosely associated via thechiasma. The chiasma begin to move toward the endsof the tetrad as separation continues. This process isknown as terminalization. Also during diakinesis, thenuclear envelope breaks down and the spindle fibersbegin to interact with the tetrad.
  14. 14. METAPHASE I • The centrioles are at opposite poles of the cell.• The pairs of homologous chromosomes (the bivalents), now as tightly coiled and condensed as they will be in meiosis, become arranged on a plane equidistant from the poles called the metaphase plate.• Spindle fibers from one pole of the cell attach to one chromosome of each pair (seen as sister chromatids), and spindle fibers from the opposite pole attach to the homologous chromosome (again, seen as sister chromatids).
  15. 15. ANAPHASE I• Anaphase I begins when the two chromosomes of each bivalent (tetrad) separate and start moving toward opposite poles of the cell as a result of the action of the spindle. • Notice that in anaphase I the sister chromatids remain attached at their centromeres and move together toward the poles. A key difference between mitosis and meiosis is that sister chromatids remain joined after metaphase in meiosis I, whereas in mitosis they separate.
  16. 16. TELOPHASE I• The homologous chromosome pairs complete their migration to the two poles as a result of the action of the spindle. Now a haploid set of chromosomes is at each pole, with each chromosome still having two chromatids. • A nuclear envelope reforms around each chromosome set, the spindle disappears, and cytokinesis follows. In animal cells, cytokinesis involves the formation of a cleavage furrow, resulting in the pinching of the cell into two cells. After cytokinesis, each of the two progeny cells has a nucleus with a haploid set of replicated chromosomes. • Many cells that undergo rapid meiosis do not decondense the chromosomes at the end of telophase I. Other cells do exhibit chromosome decondensation at this time; the chromosomes recondense in prophase II.
  17. 17. PROPHASE II• While chromosome duplication took place prior to meiosis I, no new chromosome replication occurs before meiosis II. • The centrioles duplicate. This occurs by separation of the two members of the pair, and then the formation of a daughter centriole perpendicular to each original centriole. The two pairs of centrioles separate into two centrosomes. • The nuclear envelope breaks down, and the spindle apparatus forms.
  18. 18. METAPHASE II• Each of the daughter cells completes the formation of a spindle apparatus. • Single chromosomes align on the metaphase plate, much as chromosomes do in mitosis. This is in contrast to metaphase I, in which homologous pairs of chromosomes align on the metaphase plate. • For each chromosome, the kinetochores of the sister chromatids face the opposite poles, and each is attached to a kinetochore microtubule coming from that pole.
  19. 19. ANAPHASE II• The centromeres separate, and the two chromatids of each chromosome move to opposite poles on the spindle. The separated chromatids are now called chromosomes in their own right.
  20. 20. TELOPHASE II• A nuclear envelope forms around each set of chromosomes. • Cytokinesis takes place, producing four daughter cells (gametes, in animals), each with a haploid set of chromosomes.• Because of crossing-over, some chromosomes are seen to have recombined segments of the original parental chromosomes.