Meiosis

• Meiosis


  -type of division that produces haploid (N) sex cells or gametes

  -allows both parents to contribute genetic material to the offspring

  -involves recombining genetic material
• Sperm cells are haploid cells that resulted from meiosis
• Two rounds


  Meiosis I: reductional

  Meiosis II: equational
• Haploid (N)


  a cell that contains a single set of chromosomes
• Chromosome


  A threadlike structure found inside the nucleus of a cell. A chromosome is made up of proteins and DNA organized into genes. Each cell normally contains 23 pairs of chromosomes.
• Meiosis I


  -Synapsis is a unique event in meiosis I that involves close association between homologs.

  -For diploid organisms, synapsis involves the formation of tetrads.

  -Crossing over or genetic recombination also takes place during meiosis I.
• Synapsis


  pairing of homologous chromosomes
• Crossing Over


  exchange of genetic material between the members homologous pair.
• Homologous chromosomes


  paired chromosomes within a cell(maternal and paternal) which has similar characteristics.
• Prophase I


  -Nuclear membrane and nucleolus disintegrates.

  -Chromosomes condense.

  -Synapsis and crossing over occurs.

  
• Metaphase I


  -Homologous pairs at the metaphase plate.

  -Homologues are oriented from each other at opposite poles, and spindle fibers are attached to their kinetochores.

  
• Spindle Fibers


  protein structure that divides the genetic material in a cell
• Kinetochore


  connect chromosomes to microtubules of the mitotic and meiotic spindles in order to distribute the replicated genome from a mother cell to its daughters.

  
• Anaphase I


  -Spindle fibers pull the homologous chromosomes (instead of the sister chromatids) toward the opposite poles.

  -Chromosomes are still duplicated.

  
• Telophase I


  -Members of the duplicated homologous chromosomes reaching the opposite poles.

  -Cytokinesis occurs alongside with telophase I.

  -Haploid daughter cells are

  
• Telophase I


  -Members of the duplicated homologous chromosomes reaching the opposite poles.

  -Cytokinesis occurs alongside with telophase I.

  -Haploid daughter cells are formed.

  
• Meiosis II


  -Second meiotic division is equational, i.e., there is no further reduction in chromosome number.

  -It is not preceded by a DNA replication.

  -It begins with the two haploid daughter cells of meiosis I.

  -The process is similar to mitosis which involves the separation of the sister chromatids.

  -ends up producing four daughter cells
• Prophase II


  a new spindle forms around the chromosomes

  
• Metaphase II


  Metaphase II chromosomes line up at the equator

  
• Anaphase II


  Centromeres divide. Chromatids move to the opposite poles of the cells.

  
• Telophase and Cytokinesis


  A nuclear envelope forms around each set of chromosomes. The cytoplasm divides

  
• Mitosis vs. Meiosis