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2. Meiosis and its Phases

Division I –Reduction division

The chromosome number is reduced to half the parent cell chromosome number. End result of division one is two cells.

Prophase I—Main features

  • Chromosomes coil, becoming shorter and thicker, the two-stranded nature becomes apparent, two strands are called a chromatid and chromosomes are aligned in pairs. Each pair of chromosomes has four chromatids and they have a centromere attached in the center holding the four strands together.
  • Nucleolus disassociates and nuclear envelope dissolves.
  • Segments of the closely associated pairs of chromatids may be exchanged with each other (between the pair members) this is called crossing-over. Each chromatid contains the original amount of DNA but now may have “traded” genetic material.
  • The chromosomes separate. Some spindle fibers are forming and some are attaching to the centromeres of the chromosomes. The fibers extend from each pole of the cell.

Metaphase I—Main features

  • In pairs, the chromosomes align at the equator of the cell, with the centromeres and spindle fibers apparent.
  • The two chromatids, from each chromosome, function as a single unit.

Anaphase I—Main features

  • One entire chromosome, consisting of two chromatids, migrates from the equator to a pole. The chromosomes do not separate from each other and retain both chromatids when the reach their pole. At each pole, there will be half the chromosome number. If crossing over occurred in prophase then the chromosomes will consist of original DNA and DNA from a homologous chromosome—now at the opposite pole.
  • The centromere remains intact in each pair of chromatids.

Telophase I—Main features

  • What occurs in this step, depends on the species involved, as they may revert to interphase or proceed directly to division II.
  • If they revert to interphase, they will only do so partially and the chromosomes will become longer and thinner.
  • Nuclear envelopes will not form, but the nucleoli will generally recluster.
  • Telophase is over when the original cell becomes two cells or two nuclei.

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