Share Presentation:   http://NeoK12.com/pres/ZCELDIV2
Meiosis
   
Meiosis 15 by GreenFlames09 on Flickr

The first and longest phase of meiosis, Prophase I, occurs as the nuclear envelope disappears, centrosomes (#5) begin to migrate to opposite poles of cell, and chromatin condenses to form visibly thick chromosomes.
Slide Options : Move UP,   DOWN,   DELETE,   Insert BLANK Slide
Meiosis 4 by GreenFlames09 on Flickr

In phase II, Metaphase I, homologous chromosomes, attached to spindle fibers, are pulled to the center of the cell.  

  • Karyotypes are constructed as pictographs of this stage.
  • Chromosomes appear as tetrads, composed of 4 sister chromotids.  
  • Overlapping pieces can swap chromosomes (crossing over), even between nonsister chromosomes (chiasmata).  
  • Crossing over can occur multiple times in the same chromosomes.

Slide Options : Move UP,   DOWN,   DELETE,   Insert BLANK Slide
Meiosis 7 by GreenFlames09 on Flickr

In the 3rd phase, Anaphase I, the sister chromosomes are pulled toward the opposite poles.
Slide Options : Move UP,   DOWN,   DELETE,   Insert BLANK Slide
Meiosis 1 by GreenFlames09 on Flickr


Telophase I & cytokinesis = end of meiosis I

-->2 diploid cells = 2 copies of their DNA  (2n).

  • Meiosis II begins.  Each of these cells repeats Prophase to Telophase -->4 haploid cells =  only 1 copy of DNA (1 n).
  • The  production of egg cells is called oogenesis and results in 1 ovum & 3 polar bodies. 
  • The production of sperm cells is called spermatogenesis and results in 4 spermatids/ sperm.
  • In humans, each egg then has 1 X sex chromosome + 22 autosomes.  
  • Each sperm carries either an X or a Y sex chromosome + 22 corresponding autosomes. 
  • When fertilization occurs, the sperm's DNA is transferred to the ovum, resulting in the formation of a zygote with a full set of 46 chromosomes (23 pairs).

Slide Options : Move UP,   DOWN,   DELETE,   Insert BLANK Slide
Meiosis 14 by GreenFlames09 on Flickr


Note in Meiosis II only sister chromatids are joined together.  Crossing over has likely occured in Metaphase I.
Slide Options : Move UP,   DOWN,   DELETE,   Insert BLANK Slide

 Share this Presentation:  http://NeoK12.com/pres/ZCELDIV2NeoK12.com - Educational Videos, Lessons, Quizzes & Presentations