Mitosis

Mitosis is the process by which body cells divide to make new cells.  Each daughter cells will be identical to the parent cell.

Stages of Mitosis

Interphase- Before S Phase

            The cell is performing its day-to-day activities (such as being a liver cell or a stomach cell).  The cell is also preparing for mitosis, or cell division.  Chromosomes will be found as long threads called chromatin.

Interphase- After S Phase

            Chromosomes, which are made of DNA, undergo a process called replication, which creates two copies of the genetic material. The S in S-Phase stands for synthesis, the process by which DNA copies itself.  The cell is now ready to begin mitosis.

Mitosis- Prophase

            Chromatin in the nucleus begins to condense and X-shaped chromosomes appear.  The nuclear membrane dissolves so that chromosomes can move around the cell.  The centrioles move to opposite ends of the cell and spindle fibers start to appear.

Mitosis- Metaphase

            The spindle apparatus fully develops.  Spindle fibers line the chromosomes up in the middle of the cell.   When metaphase is complete, there will be a neat row of chromosomes at the equator of the cell.

Mitosis- Anaphase

            Spindle fibers shrink to pull the chromosomes apart.  One sister chromatid moves to each pole of the cell.  Both sides of the cell have a full copy of the genetic material.

Mitosis- Telophase

            A complete set of chromosomes can be found at each pole of the cell.  The chromosomes uncoil into chromatin.  The cell begins to divide into two daughter cells. The nuclear membrane begins to reappear.

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.