- Mitosis and Meiosis
- The cell cycle and mitosis
- Phases of mitosis
- The cell cycle and meiosis
- Phases of meiosis
- References
The mechanisms of inheritance are those that control the passage of genes or genetic characteristics from parents to children and occur, through the cell cycle, during the stages corresponding to mitosis and meiosis.
All organisms are made up of cells and the cell theory proposes that each cell is born from another cell that already exists, in the same way that an animal can only be born from another animal, a plant from another plant and so on.
Life cycle of an animal cell outlined (Source: Kelvinsong via Wikimedia Commons)
The steps through which a new cell is born from another cell make up what is known as the cell cycle, which is the most important process for the reproduction of living, unicellular and multicellular beings.
During the cell cycle, cells “copy” all the information inside them, which is in the form of a special molecule called deoxyribonucleic acid or DNA, to pass it on to the new cell that will be formed; So the cell cycle is everything that happens between one division and the next.
Through the cell cycle, unicellular beings when they divide produce a complete individual, while the cells of multicellular organisms have to divide many times to form the tissues, organs and systems that make up, for example, animals and plants.
Mitosis and Meiosis
Multicellular organisms have two types of cells: somatic cells and gametes or sex cells. Somatic cells multiply by mitosis and sexual cells by meiosis.
Prokaryotes and simpler eukaryotic organisms reproduce by mitosis, but "higher" eukaryotes reproduce sexually thanks to meiosis.
The cell cycle and mitosis
Somatic cells are those that divide in an organism to produce the cells that will make up its entire body, therefore, when this happens, it is necessary that all the information inside it is faithfully copied, so that another identical cell can be formed and this It occurs through the cell cycle, which has four phases:
- Phase M
- G1 phase
- S phase
- G2 phase
The M phase (M = mitosis) is the most important of the cell cycle and in it mitosis and cytokinesis occur, which are, respectively, the copy of the genetic material (nuclear division) and the separation or division of the cells that result (the "mother" cell and the daughter cell).
The interface is the period between one M phase and the other. During this time, which comprises all the other phases named above, the cell only grows and develops, but does not divide.
The S phase (S = synthesis) consists of the synthesis and duplication of DNA that is organized in the form of chromosomes within the nucleus (a very important organelle found inside eukaryotic cells).
The G1 phase (G = gap or interval) is the time that elapses between the M phase and the S phase, and the G2 phase is the time between the S phase and the next M phase. In these two stages of the cycle the cells continue growing and preparing to divide.
The cell cycle is regulated mainly at the level of the interval phases (the G1 and G2 phases), since everything must be in good condition for the cell to divide (amount of nutrients, stressors, and others).
Phases of mitosis
So, it is during mitosis that a cell inherits from its daughter everything it needs to "be" a cell, and this is found in the copy of its complete chromosomes. If cytokinesis is counted, mitosis is divided into 6 stages: prophase, prometaphase, metaphase, anaphase, telophase, and cytokinesis.
1-DNA is copied during the S phase of the cell cycle and during prophase these copies condense or become visible within the nucleus as chromosomes. At this stage, the system of "tubes" or "cables" is also formed that will serve to separate the copies of the "original" molecules (the mitotic spindle).
2-The membrane of the nucleus, where the chromosomes are, disintegrates during the prometaphase, and when this happens, then the chromosomes come into contact with the mitotic spindle.
3-Before separating the copy chromosomes from the original ones, they are aligned in the center of the cells in a phase known as the metaphase.
4-In the anaphase is when the duplicated chromosomes separate, some towards one pole of the cell and the other towards the other, and this is known as “segregation” of the chromosomes.
5-After its duplication and its separation, within the cell that is about to divide, two nuclei are formed, each set of chromosomes in a period known as telophase.
6- Cytokinesis is when the cytoplasm and plasma membrane of the "progenitor" cell divide, resulting in two independent cells.
The cell cycle and meiosis
Mitosis is the mechanism by which characteristics are inherited in somatic cells, but meiosis is what forms sex cells, which are responsible for the passage of information from one complete multicellular individual to another through sexual reproduction.
Somatic cells are produced by mitotic divisions of a special cell: the zygote, which is the product of the union between two sex cells (gametes) from the "germ line", produced by meiosis and which come from two different individuals: a mother and father.
Phases of meiosis
In the cell cycle of germ line cells, meiosis consists of two cell divisions, which are called meiosis I (reductional) and meiosis II (similar to mitosis). Each is divided into prophase, metaphase, anaphase, and telophase. The prophase of meiosis I (prophase I) is the most complex and longest.
1-During prophase I, chromosomes condense and mix with each other (recombine) in the cells of each parent that enter meiosis.
2-In metaphase I, the nuclear membrane disappears and the chromosomes line up in the center of the cell.
3-As in mitotic anaphase, during anaphase I of meiosis the chromosomes separate towards the opposite poles of the cell.
4- Telophase I consists, in certain organisms, in the reconstruction of the nuclear membrane and in the formation of a new membrane between the resulting cells, which have half the number of chromosomes than the original cell (haploid).
5-Meiosis II begins immediately and in prophase II the condensed chromosomes are observed. During metaphase II these are located in the middle of the cell, as in mitosis.
6-The chromosomes are separated towards both poles of the cell during anaphase II, thanks to the components of the mitotic spindle, and during telophase II, new nuclei are formed and the 4 daughter cells (gametes) are separated.
Each gamete that is produced by meiosis contains a combination of all the genetic material of the organism from which it came, only in a single copy. When two gametes from different organisms (the parents) fuse, that material mixes and the two copies are restored, but one from one parent and the other from the other.
References
- Alberts, B., Dennis, B., Hopkin, K., Johnson, A., Lewis, J., Raff, M.,… Walter, P. (2004). Essential Cell Biology. Abingdon: Garland Science, Taylor & Francis Group.
- Alberts, B., Johnson, A., Lewis, J., Raff, M., Roberts, K., & Walter, P. (2008). Molecular Biology of The Cell (5th ed.). New York: Garland Science, Taylor & Francis Group.
- Griffiths, A., Wessler, S., Lewontin, R., Gelbart, W., Suzuki, D., & Miller, J. (2005). An Introduction to Genetic Analysis (8th ed.). Freeman, WH & Company.
- Pierce, B. (2012). Genetics: A Conceptual Approach. Freeman, WH & Company.
- Rodden, T. (2010). Genetics For Dummies (2nd ed.). Indianapolis: Wiley Publishing, Inc.