Mitosis and Meiosis

 
                                               
       
       
       
       
       
       
       
       
                                       
  Introduction

           According to the Cell Theory, all living cells arise from preexisting cells. The process whereby new cells are formed is called cell division or mitosis, which involves the division of the cell's nucleus (karyokinesis) and the cytoplasm (cytokinesis). Mitosis results in the production of two genetically identical daughter cells. Formation of an adult organism from a fertilized egg, regeneration, and the repair of tissues are all accomplished by mitosis.

In animals that reproduce sexually, the production of sex cells or gametes (i.e., eggs and sperm), requires that each parent's chromosomes be reduced by one half the normal number. Reducing the parent's chromosome number from the diploid (2n) number (46 in humans) to the haploid (n) number (23 in humans) is accomplished by a type of cell division known as meiosis.

Prior to the initiation of mitosis or meiosis, the cell must duplicate its chromosomes and manufacture the materials needed for cell division. This is accomplished in a phase that precedes cell division known as interphase. Interphase can be divided into three phases. In the first, called the Gl phase, the cell carries out its basic functions and performs specialized activities. In the second period of interphase, the S phase, each chromosome in the nucleus is replicated so that there are now two identical copies of each chromosome. Identical chromosomes are attached to each other by way of a centromere. When attached by their centromeres, chromosomes are referred to as chromatids. In plants, the mitotic spindle is also formed during the S phase. In animal cells, a pair of centrioles duplicates to form two pairs of centrioles. Like the mitotic spindle in plants, the centrioles will form spindle fibers. Spindle fibers are composed of microtubules and are used to pull the duplicated chromosomes apart during cell division. In the third period of interphase, the G2 phase, proteins and structures that will be used during mitosis or meiosis, are synthesized. After the G2 phase has been completed, the cell can then undergo mitosis or meiosis.

MITOSIS

Although mitosis is a continuous process, it is divided into four phases for ease of understanding. The phases are: prophase, metaphase, anaphase, and telophase. Interphase, plus the four phases of mitosis, are referred to as the cell cycle. The following is a brief synopsis of the events of the four phases of mitosis (the events of interphase are described above):

 

Prophase -During prophase, the first phase of mitosis, the chromosomes condense (shorten) and become visible when stained and viewed under a microscope. During the latter part of prophase, the nuclear membrane disappears and the newly formed spindle fibers attach to a region of the centromere called the kinetochore.

Metaphase -During metaphase, the spindle fibers move the replicated chromosomes to the middle of
                                      the cell where they line up in single file along the equatorial plane.

Anaphase -During anaphase, the sister chromatids are "pulled" apart (separated) and move toward
                                      opposite poles of the cell. The separated chromatids are now called daughter
                                      chromosomes
.

Telophase -During telophase, the last phase of mitosis, the spindle apparatus disappears and nuclei reform around each set of daughter chromosomes. Cytokinesis also occurs during telophase. In animal cells, a cleavage furrow forms and eventually divides the cytoplasm in half among the two daughter cells. In plant cells, cytokinesis is accomplished with the formation of the cell plate.

 

 

MEIOSIS

Meiosis is used for sexual reproductions.  It consists of two nuclear divisions {meiosis I and meiosis II) and results in the production of haploid cells called gametes. Because the chromosome number is reduced by one half, meiosis is sometimes referred to as reduction division. Before meiosis can proceed, however, homologous chromosomes must first pair together, a process known as synapsis. This is the time that crossing over can occur. Crossing over results in the exchange of genetic material between homologous chromatids. As in mitosis, meiosis is a continuous process.