2.9: Cell Reproduction- Mitosis and Meiosis

Mitosis or Meiosis?

Figure \(\PageIndex{1}\) shows a tiny embryo just beginning to form. Once an egg is fertilized, the resulting single cell must divide many times to develop a fetus. Both mitosis and meiosis involve cell division; is this type of cell division an example of mitosis or meiosis? The answer is mitosis. With each division, you are making a genetically exact copy of the parent cell, which only happens through mitosis.

Figure \(\PageIndex{1}\): 4-celled embryo

Cell Reproduction

You have probably have heard of the importance and prevalence of cell division. While there are a few cells in the body that do not undergo cell division (such as gametes, red blood cells, most neurons, and some muscle cells), most somatic cells divide regularly. A somatic cell is a general term for a body cell, and all human cells, except for the cells that produce eggs and sperm (which are referred to as germ cells), are somatic cells. Somatic cells contain two copies of each of their chromosomes (one copy received from each parent). A homologous pair of chromosomes is the two copies of a single chromosome found in each somatic cell. The human is a diploid organism, having 23 homologous pairs of chromosomes in each of the somatic cells. The condition of having pairs of chromosomes is known as diploidy.

Cells in the body replace themselves over the lifetime of a person. For example, the cells lining the gastrointestinal tract must be frequently replaced when constantly “worn off” by the movement of food through the gut. But what triggers a cell to divide, and how does it prepare for and complete cell division? The cell cycle is the sequence of events in the life of the cell from the moment it is created at the end of a previous cycle of cell division until it then divides itself, generating two new cells.

The Cell Cycle

Cell division is the process in which one cell, called the parent cell, divides to form two new cells, referred to as daughter cells. How this happens depends on whether the cell is prokaryotic or eukaryotic. Cell division is simpler in prokaryotes than eukaryotes because prokaryotic cells themselves are simpler. Prokaryotic cells, like bacteria cells, have a single circular chromosome, no nucleus, and few other organelles. Eukaryotic cells, including human cells, in contrast, have multiple chromosomes contained within a nucleus and many other organelles. All of these cell parts must be duplicated and then separated when the cell divides. Cell division is just one of several stages that a cell goes through during its lifetime. The cell cycle is a repeating series of events that include growth, DNA synthesis, and cell division. The cell cycle in prokaryotes is quite simple: the cell grows, its DNA replicates, and the cell divides. This form of division in prokaryotes is called asexual reproduction. In eukaryotes, the cell cycle is more complicated.

Mitosis vs. Meiosis

Most human cells divide their nucleus by a process called mitosis that results in two cells that are genetically identical to the parent cell and to each other. In mitosis the parent cell and the daughter cells all have two copies of each type of chromosome (they are dipliod). Somatic cells reproduce by mitosis.

Meiosis is a specialized cell division that only occurs in germ cells that divide to produce sperm or eggs. It produces cells that have only one copy of each type of chromosome (are haploid)

Both mitosis and meiosis result in eukaryotic cell division. The primary difference between these divisions is the differing goals of each process. The goal of mitosis is to produce two daughter cells that are genetically identical to the parent cell. Mitosis happens when you grow, and to replace injured and worn out cells. You want all your new cells to have the same DNA as the previous cells. The goal of meiosis is to produce sperm or eggs, also known as gametes. The resulting gametes are not genetically identical to the parent cell. Gametes are haploid cells, with only half the DNA present in the diploid parent cell. This is necessary so that when a sperm and an egg combine at fertilization, the resulting zygote (the fertilized egg) has the correct amount of DNA—not twice as much as the parents. The zygote then begins to divide through mitosis.

Table \(\PageIndex{1}\): comparison of mitosis and meiosis

	Mitosis	Meiosis
Purpose	To produce new cells	To produce gametes
Number of Cells Produced	2	4
Rounds of Cell Division	1	2
Haploid or Diploid	Diploid	Haploid
Are daughter cells identical to parent cells?	Yes	No
Are daughter cells identical to each other?	Yes	No

Figure \(\PageIndex{2}\) shows a comparison of mitosis, meiosis, and binary fission.

• Binary fission occurs in bacteria. Note that bacterial cells have a single loop of DNA. The DNA of the cell is replicated. Each loop of DNA moves to the opposite side of the cell and the cell splits in half.
• Mitosis and Meiosis both occur in eukaryotic cells. In the example below the cell has 4 total chromosomes. These are replicated during the S phase (the DNA synthesis phase).
  • In mitosis, the chromosomes line up in the center of the cell. Then, sister chromatids separate and move to the opposite poles of the cell. The cell divides, producing two cells with 4 total chromosomes
  • In meiosis, the homologous chromosomes line up in the center of the cell. Then each chromosome moves to opposite poles and the cell divides.
    • Next, the chromosomes line up in the center of the cell. Then, sister chromatids separate and move to the opposite poles of the cell. This produces four cells with 2 chromosomes each. These are the gametes.
    • Two gametes combine to form a zygote with 4 total chromosomes.

Figure \(\PageIndex{2}\): A comparison between binary fission, mitosis, and meiosis.

Review

1. What is the goal of mitosis? Or meiosis?
2. How many cells are created from cytokinesis following mitosis? Following meiosis?
3. Which process, mitosis or meiosis, creates genetically identical cells?
4. "Gametes are haploid cells." What does this sentence mean?

   Explore More

   https://bio.libretexts.org/link?17046#Explore_More

   Explore More I

   • Mitosis and Meiosis Simulation in the following video:

   1. What are homologous chromosomes?
   2. How does the location of specific genes compare between homologous chromosomes?
   3. What is the outcome of mitosis?
   4. What is a tetrad? Why are they an important feature of meiosis?
   5. How does meiosis differ between females and males?

   Explore More II

   Explore How Cells Divide and answer the following questions:

   1. How many daughter cells arise from mitosis? How many daughter cells are produced in meiosis?

Attributions

1. 4 cell embryo by Nina Sesina, CC BY-SA 4.0 via Wikimedia Commons
2. Three growth types by domdomegg, CC BY-SA 4.0 via Wikimedia Commons
3. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0