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Cellular reproduction is the way in which the cells of organisms multiply or the manner in which they reproduce. It consists of the generation of two daughter cells from the division of a mother cell. There are two processes that generate cell division: mitosis and meiosis. In the case of meiosis, there are two successive divisions that generate four daughter cells at the completion of the process. In sexually reproducing organisms, meiosis is the mechanism that generates gametes, that is, sexual cells, spermatozoa, and oocytes. Mitosis is the mechanism of cell division involved in the growth and repair of tissues, and in asexual reproduction: mitosis in the form of reproduction of genetically identical cells.
mitosis
Mitosis is the stage of the cell cycle that involves the division of the cell nucleus and the separation of the chromosomes . The cell division process is completed with cytokinesis, when the cell’s cytoplasm divides, completing the formation of two differentiated daughter cells.
Before starting mitosis , the cell prepares for division by increasing its mass and duplicating all the structures that will later make up the two daughter cells; DNA is replicated, duplicating the chromosomes, and the number of organelles is also doubled. The stage of the cell cycle prior to mitosis is called interphase . After DNA replication, the cell will have two identical sets of DNA that will make up the genetic information of the two daughter cells, which will separate during mitosis. For this, another important process also occurs at this stage: the transformation of chromatin into chromosomes.
Mitosis occurs in five stages. The first is prophase , the stage in which the centrosomes are duplicated, migrating to opposite ends of the cell, around which microtubules begin to develop, as shown in the previous figure. In this stage the nucleolus of the cell disappears. Prometaphase is the second stage of mitosis, although it is sometimes considered part of prophase; at this stage the microtubules expand from the two centrosomes.
During the metaphase of mitosis, the chromosomes align on the metaphase plate or equatorial plane, as shown in the third diagram of the previous figure. The next stage, anaphase , is crucial in mitosis; It consists of the separation of the chromosomes forming two identical copies of the genetic material of the mother cell. Mitosis is completed in telophase : the envelope of the cell nuclei reforms around the new chromosomes, which unfold to form chromatin.
In this way, diploid daughter cells are generated, which contain two identical sets of chromosomes, genetically identical to the mother, with the same number and type of chromosomes. Somatic cells are examples of cells that use the mechanism of mitosis for their multiplication. Somatic cells are all types of cells in the human body, excluding sex cells . The number of chromosomes of the somatic cells of humans is 46 while the number of chromosomes of the sex cells is 23.
meiosis
Meiosis is the form of multiplication of sexual cells, sperm and oocytes, in sexually reproducing organisms. Meiosis involves two cell divisions, called meiosis I and meiosis II, as shown in the following scheme.
The two processes of cell division develop in the stages described for mitosis. In meiosis I, the pairs of homologous chromosomes formed from the 2n chromosomes of the parent cell pair up in prophase, forming a protein structure that allows recombination of homologous chromosomes. Chromosome clumping at the equatorial plate during metaphase results in the migration of n chromosomes to each of the centrosomes. In meiosis II, the homologous chromatids of each chromosome divide and form the nuclei of the daughter cells. Between meiosis I and II there is no DNA replication.
At the completion of meiosis, four haploid cells, which have a single set of n chromosomes, were produced from a diploid mother cell, which contains two identical sets of 2n chromosomes. Haploid daughter cells are not genetically identical to the parent cell. In sexual reproduction, the haploid gametes will unite during fertilization to produce a diploid zygote. Then the zygote will divide by mitosis, a division that will continue with subsequent cells until a new individual develops.
Daughter cells and the division of chromosomes
How are daughter cells guaranteed to have the proper number of chromosomes after cell division? To answer this question, it is necessary to delve into the processes of cell division, particularly in the so-called spindle apparatus , achromatic spindle, meiotic spindle or mitotic spindle.. This is the set of microtubules that will be mentioned that begin to develop in prophase that, together with specific proteins, manipulate the chromosomes during cell division. Spindle fibers attach to replicated chromosomes by moving them apart at the appropriate time. The microtubules move the chromosomes toward the centrosomes, ensuring that each daughter cell has the correct number of chromosomes. These structures also determine the location of the metaphase plate or equatorial plane, that is, the plane in which the cell divides.
cytokinesis
As can be seen in the previous diagrams, the process of cell division is completed with cytokinesis. This process begins during anaphase of mitosis and ends after telophase. In cytokinesis, the division of the mother cell into two daughter cells is completed, with the participation of microtubules.
The spindle apparatus has differentiated characteristics in cytokinesis depending on whether it is animal or plant cells. In animal cells, the spindle apparatus determines the location of an important structure in the cell division process called the contractile ring. The contractile ring is made up of proteins and actin microtubule filaments together with the motor protein myosin. Myosin contracts the ring of actin filaments, forming a deep groove called the cleavage furrow. As the contractile ring continues to contract, it divides the cytoplasm and collapses the cell, dividing it in two along the cleavage furrow.
In plant cells, a cleavage furrow is not formed in cytokinesis. Instead, daughter cells are spread across a cell plate made up of vesicles that are released from organelles in the Golgi apparatus. The cell plate expands laterally and fuses with the cell wall, forming a partition between the daughter cells that have formed. As the cell plate matures, it becomes a cell wall.
Cancer
The mitotic division of cells is tightly regulated to ensure that errors are corrected and that cells divide with the correct number of chromosomes. If errors occur in the verification system, the resulting daughter cells may be different from each other. While normal cells produce two identical cells in mitosis, cancer cells can produce more than two daughter cells; Three or more daughter cells can develop from dividing cancer cells, while these cells are produced at a higher rate than normal cells. Because cancer cells divide abnormally, the daughter cells they generate may have a different number of chromosomes than normal.
Cancer cells are often the result of mutations in genes that control cell growth, or genes that kill cancer cells. These cells grow out of control, depleting nutrients from their environment. Some cancer cells travel to other parts of the body through the circulatory system or the lymphatic system, where they continue to reproduce uncontrollably.
Sources
Introduction to cell biology . Panamerican Medical Editorial, 2011.
Neil A. Campbell, Jane B. Reece. Biology Campbell. Ninth edition. Pearson/Benjamin Cummings, 2011.
