Why is mitosis important for a healthy organism?

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By Deborah Meister

Mitosis and meiosis both involve cells dividing to make new cells. This makes them both vital processes for the existence of living things that reproduce sexually. Meiosis makes the cells needed for sexual reproduction to occur, and mitosis replicates non-sex cells needed for growth and development. Together, they provide the cellular basis for healthy growth and sexual reproduction.

Meiosis is the process by which sexually reproducing organisms make their sex cells, sperm and eggs. During meiosis, a specialized cell called a germ cell splits to make four new sex cells, each with half the number of chromosomes as the original germ cell. Each of the four sex cells has a unique combination of DNA, meaning no two sperm or eggs are genetically identical.

Chromosomes are the cell's way of neatly arranging long strands of DNA. Non-sex cells have two sets of chromosomes, one set from each parent. Meiosis makes sex cells with only one set of chromosomes. For example, human cells have 46 chromosomes, with the exception of sperm and eggs, which contain only 23 chromosomes each. When a sperm cell fertilizes an egg, the 23 chromosomes from each sex cell combine to make a zygote, a new cell with 46 chromosomes. The zygote is the first cell in a new individual.

One of the benefits of sexual reproduction is the diversity it produces within a population. That variety is a direct product of meiosis. Every sex cell made from meiosis has a unique combination of chromosomes. This means that no two sperm or egg cells are genetically identical. Every fertilization event produces new combinations of traits. This is why siblings share DNA with parents and each other, but are not identical to one another.

Mitosis is the process by which non-sex cells divide to make new cells. After a fertilization event, mitosis kicks in to begin making copies of the newly formed zygote. The first cell will duplicate itself through mitosis. The two resulting cells will also be duplicated. And the process will continue exponentially throughout the organism's lifetime. This is how individuals grow body parts, develop, repair damaged tissues, replace dead cells and change at a cellular level as they mature. Mitosis always produces two cells that are genetically identical to each other and the original cell.

Mitosis is important for sexual reproduction indirectly. It allows the sexually reproducing organism to grow and develop from a single cell into a sexually mature individual. This allows organisms to continue to reproduce through the generations.

Cell division is key to life: from the moment we are first conceived, we are continually changing and growing. In order for our bodies to grow and develop, they must produce new cells—and allow for the death of old cells. Cell division is also an essential component of injury repair. If our cells couldn’t divide and create new cells, our bodies could never produce new skin cells to heal road rash, or grow a fingernail back. However, when cell division goes awry, dramatic results may occur. Without sufficient cellular oversight, repeated rounds of unregulated cell division can lead to a minor condition like psoriasis or a life-threatening disease like cancer. Cell division takes occurs by a strict cycle, with multiple stages and checkpoints to ensure things don’t go awry.

Perhaps most importantly, without cell division, no species would be able to reproduce—life would simply end (or would have ended a long time ago). Every human, as well as every sexually reproducing organism, begins life as a fertilized egg (embryo) or zygote. Trillions of cell divisions subsequently occur in a controlled manner to produce a complex, multicellular human. In other words, that original single cell is the ancestor of every other cell in the body. Single-celled organisms use cell division as their method of reproduction.

Figure 1. A sea urchin begins life as a single cell that (a) divides to form two cells, visible by scanning electron microscopy. After four rounds of cell division, (b) there are 16 cells, as seen in this SEM image. After many rounds of cell division, the individual develops into a complex, multicellular organism, as seen in this (c) mature sea urchin. (credit a: modification of work by Evelyn Spiegel, Louisa Howard; credit b: modification of work by Evelyn Spiegel, Louisa Howard; credit c: modification of work by Marco Busdraghi; scale-bar data from Matt Russell)

Learning Outcomes

• Understand chromosome structure and organization in eukaryotic cells
• Identify the stages of the cell cycle, by picture and by description of major milestones
• Identify and explain the important checkpoints that a cell passes through during the cell cycle
• Identify the stages of meiosis by picture and by description of major milestones; explain why meiosis involves two rounds of nuclear division
• Describe and explain a range of mechanisms for generating genetic diversity
• Examine karyotypes and identify the effects of significant changes in chromosome number

All living organisms are capable of growing and producing offspring. All eukaryotic organisms—including aquatic plants and algae—grow through the process of mitosis. Mitosis is a process where one cell divides into two cells (Fig. 2.46). Chromosomes in the original cell are duplicated to ensure that the two new cells have full copies of the necessary genetic information.

The process of mitosis generates new cells that are genetically identical to each other. Mitosis helps organisms grow in size and repair damaged tissue. Some species of algae are capable of growing very quickly. The giant kelp Macrocystis pyrifera can grow as much as 30 centimeters (cm) in length in a single day.

Some organisms can use mitosis to reproduce asexually. The offspring of asexual reproduction are genetically identical to each other and to their parent. Most single-celled, microorganisms reproduce asexually by duplicating their genetic material and dividing in half. For example, phytoplankton reproduce primarily through asexual reproduction. Some single-celled eukaryotes, including some plants and animals, reproduce asexually in a processes called fragmentation or budding.

Sexual reproduction is the production of offspring through the combination of sex cells or gametes. Meiosis is the process of producing gametes, each of which has half of the genetic material needed to create a new organism (Fig. 2.47).

1. Chromosomes are duplicated. Meiosis begins in a fashion similar to mitosis with chromosome replication.
    
2. Matched sets of chromosomes pair together.
    
3. Genes are swapped between matched chromosomes. The process of crossing over, or recombination, exchanges genetic information between chromosomes in a cell. The resulting chromosomes are brand new, unique combinations of genetic information.
    
4. First division separates one of each chromosome pair. The parent cell divides in half as in mitosis, producing two cells with a complete amount of DNA (although they are not identical because of crossing over).
    
5. Second division separates each chromosome, leaving one copy of each chromosome per cell. The two new cells divide a second time to produce four new gametes. These gametes contain one-half of the genetic information needed to form a new individual.
    
6. Each parent provides one gamete to the process of fertilization, which results in a cell called a zygote with a full compliment of chromosomes.
    
7. Offspring produced through sexual reproduction are genetically distinct from both parents, since each of their gametes has a unique combination of chromosomes.
    

In summary, mitosis produces two identical cells, each with the full amount of DNA. Meiosis produces four genetically unique cells, each with half the amount of DNA. See Table 2.10 for a comparison of mitosis and meiosis.

Many species of algae have complex life histories and can reproduce through both sexual and asexual means. It is common for algae to have an alternation of generation, where one generation is made through mitotic cell division and the other is made from cells created through meiotic cell division.