Meiosis

Meiosis is the process by which reproductive cells are formed.  The cell will divide twice before stopping. Each new cell will contain only half of the number of chromosomes as the parent cell and will be genetically different from all of the other daughter cells.

Stages of Meiosis

Interphase: G1 phase- Before DNA Replication

            The reproductive cell grows in size.  It is also performs all of the day-to-day tasks of a healthy cell. This phase is called the G1 phase, because G stands for gap. 

Interphase: S phase- After DNA Replication

            The chromosomes have undergone DNA replication (or synthesis) to make an extra copy of the DNA.   The rod- shaped chromosomes become X-shaped; each half of the X is called a ‘sister chromatid.’

Phophase I

            The condensed chromosomes undergo a process called ‘crossing over’ where they exchange DNA.  The spindle fiber network begins to appear and centrioles move to the poles of the cell.  

Metaphase I

            The chromosomes move to the center of the cell and line up randomly.  Each chromosome has spindle fibers attached to the center which help move it around the cell.

Anaphase I

            The spindle fibers shrink, pulling entire chromosomes to the either side of the cell.  At the end of anaphase, each side of the cell will have half of the chromosomes from the cell.

Telophase I

            Each side of the cell is now haploid, it has half of the genetic material of the original cell.  The cytoplasm begins to divide into two and two daughter cells are formed.

Prophase II

            The nuclear membrane breaks up again as the spindle network appears.  The chromosomes do not replicate and there is no crossing over in Prophase II.

Metaphase II

            The chromosomes line up in the center of the cell.  The spindle fibers connect the centrioles to the centromere of each chromosome.

Anaphase II

            Sister chromatids separate and move to opposite poles of the cell.  Each sister chromatid is now considered to be a full chromosome.

 Telophase II

            The nuclei reform and the cell divides into two.  At the end of the telophase II there will be a total of 4 cells, each with half of the original number of chromosomes.