Mitosis and Meiosis

Mitosis and Meiosis

  • When a eucaryotic microorganism reproduces asexually, its genetic material must be duplicated and then separated so that each new nucleus possesses a complete set of chromosomes.
  • This process of nuclear division and chromosome distribution in eucaryotic cells is called mitosis.
  • Mitosis actually occupies only a small portion of a microorganism’s life as can be seen by examining the cell cycle.
  • The cell cycle is the total sequence of events in the growth-division cycle between the end of one division and the end of the next.
  • Cell growth takes place in the interphase, that portion of the cycle between periods of mitosis.

The Eucaryotic Cell Cycle.

  • Interphase is composed of three parts. The G1 period (gap 1 period) is a time of active synthesis of RNA, ribosomes, and other cytoplasmic constituents accompanied by considerable cell
  • This is followed by the S period (synthesis period) in which DNA is replicated and doubles in quantity. Finally, there is a second gap, the G2 period, when the cell prepares for mitosis, the M period, by activities such as the synthesis of special division proteins.
  • The total length of the cycle differs considerably between microorganisms, usually due to variations in the length of G1.
  • Mitotic events are summarized in. During mitosis, the genetic material duplicated during the S period is distributed equally to the two new nuclei by cytoskeletal elements so that each has a full set of genes.
  • There are four phases in mitosis:
  • In prophase, the chromosomes-each with two chromatids—become visible and move toward the equator of the cell.
  • The mitotic spindle forms, the nucleolus disappears, and the nuclear envelope begins to dissolve.
  • The chromosomes are arranged in the center of the spindle during metaphase and the nuclear envelope disappears.
  • During anaphase the chromatids in each chromosome separate and move toward the opposite poles of the spindle.

Generalized Eucaryotic Life Cycle

  • Finally during telophase, the chromatids become less visible, the nucleolus reappears, and a nuclear envelope reassembles around each set of chromatids to form two new nuclei.
  • The resulting progeny cells have the same number of chromosomes as the parent.
  • Thus after mitosis, a diploid organism will remain diploid.
  • For example, the nuclear envelope does not disappear in many fungi and some protists.
  • Frequently cytokinesis, the division of the parental cell’s cytoplasm to form new cells, begins during anaphase and finishes by the end of telophase.
  • However, mitosis can take place without cytokinesis to generate multinucleate or coenocytic cells.
  • Many microorganisms have a sexual phase in their life cycles.
  • In this phase, they must reduce their chromosome number by half, from the diploid state to the haploid or 1N (a single copy of each chromosome).
  • Haploid cells may immediately act as gametes and fuse to reform diploid organisms or may form gametes only after a considerable delay.
  • The process by which the number of chromosomes is reduced in half with each daughter cell receiving one complete set of chromosomes is called meiosis.
  • Meiosis is quite complex and involves two stages.
  • The first stage differs markedly from mitosis.
  • During prophase, homologous chromosomes come together and lie side-by-side, a process known as synapsis.
  • The homologues move to opposite poles in anaphase, thus reducing the number of chromosomes by half.
  • The second stage of meiosis is similar to mitosis in terms of mechanics, and chromatids of each chromosome are separated.
  • After completion of meiosis I and meiosis II, the original diploid cell has been transformed into four haploid cells.

Mitosis and Meiosis

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