Mitosis and Meiosis

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Mitosis:

Definition:

• The biological process in which one mother cell is divided into two identical daughter cells having the same number of chromosomes.
• Also known as “Equational Division” because of the equality in the number of chromosomes in mother and daughter cells; “Karyokinesis” relating to the fact of the occurring nuclear division.

Process:

• The principle phases of Mitosis cell division are preceded by a major phase known as Interphase which has a collection of subphases. This subphases are necessary for a cell to prepare beforehand it actually goes under division.

Interphase:

• It is a segment of the cell cycle before Mitosis where a cell prepares itself for division. There are 3 sub-phases which altogether build up the Interphase stage. They are:
  • G1 (first gap) – Cells synthesize a number of materials that are required for completing replication in the following steps including some enzymes, regulators and nutrients.
  • S (synthesis) – In this sub-phase, DNA packed in chromosomes are replicated so that complete set of information are passed on from the mother cell to the daughter cells which will be generated during Mitosis.
  • G2 (second gap) – Immediate phase after DNA Replication where cells synthesize materials required for the formation of spindle fibre during the different phases of Mitosis.

Phases of Mitosis:

• The 5 phases of Mitosis cell division are:
  • Prophase/Early Prophase:
    • Condensation of chromosomes begins.
    • Formation of a spindle shaped apparatus made from microtubules initiates which is often called Mitotic Spindle. The spindle fibres attach to the condensed chromosomes and help in their movement and distribution during the latter phases of cell division.
    • Nucleolus, the part of the cell where ribosomes are produced disappears.

• Prometaphase/Late Prophase:
  • Chromosomes attain a fully condensed and compact structure; the nuclear membrane disappears.
  • A complete spindle apparatus is formed and the microtubules within it gradually start attaching with the condensed chromosomes.
  • Microtubules bind to the chromosomes with the help of a specialized protein present in the centromere of each sister chromatid known as Kinetochore.
  • Microtubules that attach to kinetochores are known as Kinetochore Microtubules and those who are devoid of attachment with Kinetochores help in stabilizing the spindle apparatus by adhering to the microtubules near the poles of the spindle.
  • In animal cells, more microtubules extend from the microtubule organizing centre centrosome towards the edge of the cells forming a structure called Aster.

• Metaphase:
  • The chromosomes attached to the microtubules are aligned in the middle of the spindle apparatus.
  • Each kinetochore from two sister chromatids are attached with the microtubules facing towards the opposite poles.

• Anaphase:
  • The sister chromatids are separated into two individual chromosomes.
  • After separation, the microtubules facilitate the movement of the separated chromatids. The centromere moves ahead facing the poles and the arms follow.
  • Basing on the position of centromere, different shapes of chromosomes are seen in this phase. Some of the notable ones are:
    • Metacentric: Centromere is positioned at the center of the chromosome causing the length of both the arms to be equal. The chromosomes look like the English alphabet “V”.
    • Sub-metacentric: The arms of the chromosomes are of unequal length and the centromere lies nearly at the center. Chromosomes look a lot like “J” and sometimes, “L”.
    • Acrocentric: Centromeres are positioned nearly at one end and the length of the arm beyond the centromere is quite small.
    • Telocentric: Centromere is located at one end resembling a rod like structure.

• Telophase:
  • The nuclear membrane and the nucleolus start regenerating from the left-over parts of the mother cell.
  • The separated chromosomes and other organelles start organizing.
  • This is the stage where the daughter cell formation is “nearly” complete.

• Cytokinesis:
  • It is the process of division of cell’s cytoplasm.
  • In animal cells, two proteins known as Actin and Myosin facilitate the formation of the cell by creating a cleavage furrow.
  • In plant cells, cell plate is formed which separated the almost created cell into two separate daughter cells completing division.

Meiosis:

Definition:

• The process of cell division in which one mother cell divides into four daughter cells having half the number of chromosomes in the mother cell.
• Also known as “Reductional Division” since the number of chromosomes reduce to half, the preparatory steps for meiosis cell division are similar to that of mitosis. Meiosis is rather divided primarily into two phases which are a) Meiosis I and b) Meiosis II both of which have sub-phases.
• In case of meiosis, the chromosomes divide once while the nucleus undergoes division twice.

Meiosis I:

• Similar to Mitosis, before Meiosis I starts, the cell undergoes Interphase where it grows in G1 subphase, chromosomes replicate in S phase and synthesize necessary chemicals for carrying out different sub-phases under Meiosis I and Meiosis II.

• There are 4 sub-phases under Meiosis I which are as follows:
  • Prophase I:
    • This is the longest and most complex stage of Meiosis I and is again divided into 5 sub-stages:
      • Leptotene: This is a comparatively short stage where progressive condensation of chromosome fibers take place.
      • Zygotene: Homologous chromosomes are formed and they pair up with another homologous chromosome by a process called synapsis. The paired chromosomes are called bivalent or tetrad.
      • Pachytene: Two non-sister chromatids form a physical linkage, the site of which is known as Chiasmata. In this region, the non-sister chromatids sometimes exchange their parts in a process called Crossing Over.
      • Diplotene: The synapsis becomes weak and the linkage of the chromatids starts to break causing separation. The chiasmata remain intact until Anaphase I.
      • Diakinesis: Condensation of chromosomes continues; nuclear membrane and nucleolus disappears and formation of spindle apparatus begins. Four parts of the tetrads and the chiasmata are prominently visible in this sub-stage.
  • Metaphase I:
    • Homologous chromosomes arrange in the equatorial region in the completely formed spindle apparatus.
    • Similar to mitosis, kinetochores act as the bridge between chromosomes and the microtubules of the spindle apparatus.
  • Anaphase I:
    • Homologous chromosomes are pulled towards the poles of the spindle apparatus by the shortening of the kinetochore microtubules.
    • The connection between the sister chromatids degrades near the arms but remains unharmed in the centromere.
  • Telophase I:
    • The homologous chromosomes reach the pole of the spindle apparatus.
    • The spindle apparatus starts degrading while the nuclear membrane and nucleolus starts reappearing.
    • Cytokinesis takes place and the animal cells are surrounded by a cell membrane whereas the plant cells are surrounded by a cell plate. Two complete daughter cells are formed.
    • Cells enter a phase known as Interkinesis or Interphase II. No DNA replication occurs in this phase.

Meiosis II:

• Very similar to mitosis cell division, meiosis II can be considered as Mitosis for Haploid Cells.
• Comparatively less complex than Meiosis I, Meiosis II also contains 4 sub-stages. They are:
  • Prophase II:
    • Chromosomes start to condense and nucleolus and nuclear membrane disappears.
    • Spindle fibre formation is seen and the microtubules start attaching to the kinetochores.
  • Metaphase II:
    • Chromosomes arrange in the equatorial plane of the spindle apparatus.
    • The metaphase II equatorial plate is rotated at 90 degree.
  • Anaphase II:
    • Kinetochore microtubules contract and the separated sister chromatids start moving towards the opposite poles.
  • Telophase II:
    • Decondensation of chromosomes occur. The nucleolus and nuclear membrane reappear and the spindle apparatus dissociates.
    • Cytokinesis happens again which ends the meiosis II phase creating four complete haploid daughter cells surrounded by cell membrane in case of animal cells and cell wall/cell plate in case of plant cells.

Reference diagram for both meiosis I and ii:

Extract from HSC Biology Stage 6 Syllabus. © 2017 Board of Studies NSW.