Mitosis & Meiosis Defined

Two processes necessary for all animals.

Mitosis and Meiosis are the processes by which an organism prepares cells to participate in either asexual or sexual reproduction to create an entire organism.

Examples of Mitosis and Meiosis

Mitosis is the “Asexual” reproduction, in which an organism or cell reproduces itself. There are many single-celled organisms which reproduce in this manner including paramecium, bacteria, protozoa, etc… In multi-cellular organisms, mitosis takes place at the cellular level with the reproduction of skin, organ cells, hair, blood cells, etc. These cells are referred to as “Autosomal” cells.

Meiosis is the production of sperm and egg cells. These cells are “Gamete” or “Sex” cells and the cells must combine to begin the division process. As the cells divide each new cell receives half the number of chromosomes from the original combination of sperm and egg. This is “Sexual” reproduction, in which one organism reproduces by crossing with another organism. There are many types of organisms that reproduce sexually including plants, animals, and insects.

If meiosis did not occur, animals would be unable to reproduce. If mitosis did not occur animals would have very short life spans and there would be no single-celled organisms in nature. For these reasons, meiosis and mitosis are both vital to the survival of all species.

Compare and Contrast of Mitosis and Meiosis

Mitosis is used by single-celled organisms to reproduce. Meiosis is found in the sexual reproduction of organisms. The male and female sex cells, egg and sperm, combine to create new, genetically different offspring. Mitosis, during cell division, retains their number of chromosomes while meiosis reduces their number of chromosomes during cell divisions by half.

Differences between mitosis and meiosis using the example of plant versus animal cells.

In animal cells in telophase, there are cleavage furrow and spindle fibers that pull the chromosomes apart and in plant cells, there are growing cell plates. In animal mitosis, there is no distinct or similar appearance in the outcome of the cells. Although they also undergo similar phases, they do not have a rigid appearance, but rather, they adapt to their surroundings. Furthermore, after mitosis, animal cells have smaller vacuoles, that contain water and do not perform photosynthesis. In plant cells, photosynthesis is an important process for energy and food production for plants. During mitosis, most plant cells show uniformity in structure and appearance, owing to their membranes being made up of cellulose, and plant cells have large vacuoles that hold a lot of water necessary for their own survival.

Disease Related with Meiosis and Mitosis

Cancer is related to uncontrolled cell division in which cells grow rapidly due to mutating somatic cells that escape the restraints that normally control them. One cause is the mutation of cells during meiosis which causes changes in the nucleotide sequence of genomic DNA. A second cause is when the normal process of programmed cell death that exists in normal cells ceases to happen causing extreme cell growth. Any drug would work within the cell to insure cell death thereby stopping cells from being able to reproduce at the DNA level. This drug would need to contain tumor suppressor genes which inhibit cell division of the mutated cells.

~Advanced Discussion on Mitosis~

The Cell Cycle

Interphase describes the period between cell divisions. Although it may seem as though the cell is inactive during interphase, quite the opposite is true. During this phase (the longest phase of the cell cycle) the cell grows, DNA is replicated and the centrioles divide. The cell increases in size and makes a copy of its DNA which is made during the S phase. There are three phases of interphase:

G1 phase or “first gap” is a growth phase for the cell.
S phase or “synthesis” is when the cell copies its chromosomes.
G2 phase or “second gap” is a second growth phase where further growth and preparations for division occur.

Mitosis (M Phase) is the process of nuclear division and is divided into the following phases: 

Prophase is the first mitotic phase. During prophase, the nucleoli disappear and the chromatin (DNA and associated proteins) condenses into discrete chromosomes that are observable with a light microscope. Chromosomes are made up of chromatin which is composed of DNA and its associated proteins. Each replicated chromosome is composed of two sister chromatids, both containing the same genetic information. The sister chromatids are joined together at their centromeres. The mitotic spindle forms from the centrioles and begins to elongate. As the centrioles reach opposite ends of the cell the spindle fibers from each of the centrioles attach to each chromosome at a specialized protein structure called the kinetochore. The kinetochore is located at the centromere of each chromosome. Other spindle fibers elongate, but instead of attaching to chromosomes, they interact with the spindle from the opposite pole.

Metaphase is the longest stage of mitosis. It is during this stage that the tension applied by the mitotic spindle fibers aligns all of the chromosomes along with the metaphase plate, an imaginary line the divides the cell in two. This organization is necessary to ensure that in the next phase, when the chromosomes are separated, each new nucleus will receive one copy of each chromosome. 

Anaphase is the shortest stage of mitosis. The spindle fibers shorten during anaphase, pulling the sister chromatids apart towards opposite ends of the cell.

Telophase marks the stage where the daughter chromosomes arrive at the poles and the spindle fibers begin to disperse. Two daughter nuclei form, nuclear envelopes are constructed and the chromosomes become less condensed. Mitosis, which describes the division of the nucleus, is now complete.

Cytokinesis: The end of the Cell Cycle

Cytokinesis describes the division of the cytoplasm and while it is not a stage of mitosis (nuclear division) but does result in the completion of cell division and the end of the cell cycle. In animal cells, cytokinesis involves the formation of a cleavage furrow, which pinches the cell into two distinct daughter cells.

Advanced Discussion on Meiosis

Meiosis is a division of a germ cell involving the fission of two nuclei, creating four gametes (sex cells), each possessing half of the number of chromosomes from the original cell. This process occurs in the sexual reproduction of organisms when the male and female sex cells (egg and sperm) combine to create new, genetically different offspring. Meiosis occurs in two phases: I and II with Meiosis I identical to a mitotic division. This division results in a copy of a diploid cell. In Meiosis II, cells split making 4 haploid daughter cells.

Meiosis I

This representation shows the chromosomal DNA movement through Meiosis. Meiosis I reduces the number of chromosomes in the two cells that it forms. They go from diploid to haploid.

Prophase I- The chromosomes coil up, the nuclear membrane begins to disintegrate, and the centrosomes begin moving apart.

Metaphase I- Bivalents (tetrads) become aligned in the center of the cell and are attached to spindle fibers. 

Anaphase I- begins when homologous chromosomes separate.

Telophase I- The nuclear envelope reforms and nucleoli reappear. This stage is absent in some species.

The end of meiosis I result in two haploid daughter cells.

Meiosis II

Meiosis II functions much the same way as mitosis but regulates the number of chromosomes in order to keep the number the same in the two cells that it forms.

Prophase II

Metaphase II 

Anaphase II

Telophase II

Cytokinesis

The end of meiosis II results in four diploid daughter cells with 2 nuclei present and 46 chromosomes (23 from each parent).

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Photo by National Cancer Institute on Unsplash