We explain what cellular reproduction, meiosis, mitosis and their phases are. Also, its importance for the diversity of life.
What is cellular reproduction?
It is known as cell reproduction or cell division. the stage of the cell cycle in which each cell divides to form two daughter cells different. It is a process that occurs in all forms of life and that guarantees the perpetuity of their existence, as well as growth, tissue replacement and reproduction in multicellular beings.
The cell is the basic unit of life. Each cell, like living beings, has a lifespan during which it grows, matures, reproduces and dies.
There are various biological mechanisms of cellular reproduction, that is, they allow new cells to be generated, replicating their genetic information and allowing the cycle to begin again.
At a certain point in the life of living beings, their cells stop reproducing (or begin to do so less efficiently) and they begin to age. Until that occurs, cellular reproduction has the purpose of maintaining or increasing the number of cells that exist in an organism.
In single-celled organisms, cellular reproduction creates an entirely new organism. This generally occurs when the cell has reached a certain size and volume, which usually reduces the effectiveness of its nutrient transport processes and, thus, the division of the individual is much more effective.
Types of cell reproduction
In principle, There are three main types of cellular reproduction. The first and simplest is binary fission, in which the cellular genetic material is replicated and the cell proceeds to divide into two identical individuals, just as bacteria do, equipped with a single chromosome and with asexual reproduction processes.
However, more complex beings, such as eukaryotes have more than one chromosome (like human beings, for example, who have a pair of chromosomes from the father and one from the mother).
In eukaryotic organisms more complicated processes are applied cellular reproduction:
- Mitosis. It is the most common form of cell division of eukaryotic cells. In this process, the cell replicates its genetic material completely. To do so, it uses a method of organizing chromosomes in the equatorial region of the cell nucleus, which then proceeds to divide in two, generating two identical chromosome sets. The rest of the cell then proceeds to duplicate and slowly split the cytoplasm, until the plasma membrane ends up dividing the two new daughter cells into two. The resulting cells will be genetically identical to their parent.
- Meiosis. It is a more complex process, which produces haploid cells (with half the genetic load), such as sexual cells or gametes, endowed with genetic variability. This is done in order to provide half of the genomic load during fertilization, and thus obtain genetically unique offspring, avoiding clonal (asexual) reproduction. Through meiosis, a diploid cell (2n) undergoes two consecutive divisions, thus obtaining four haploid daughter cells (n).
Importance of cellular reproduction
Cell division creates colonies of single-celled organisms, but above all allows the existence of multicellular organisms made up of differentiated tissues. Each tissue is damaged, ages, and eventually grows, requiring replacement cells for old or damaged ones, or new cells to add to the growing tissue.
Cell division makes possible both the growth of organisms and the repair of damaged tissues.
On the other hand, disordered cell division can lead to diseases in which this process occurs in an uncontrollable manner, threatening the individual's life (as occurs in people with cancer). That is why in modern medicine the study of cell division is one of the key areas of scientific interest.
Phases of mitosis
In mitosis-type cellular reproduction, we find the following phases:
- Interface. The cell prepares for the reproduction process, duplicating its DNA and taking the relevant internal and external measures to successfully face the process.
- Prophase. The nuclear envelope begins to break down (until it gradually dissolves). All the genetic material (DNA) condenses and forms chromosomes. The centrosome is duplicated and each one moves to one end of the cell, where microtubules are formed.
- Metaphase. The chromosomes line up at the equator of the cell. Each of them has already been duplicated at the interface, so at this point the two copies are separated.
- Anaphase. The two groups of chromosomes (which are identical to each other) move away thanks to the microtubules towards the opposite poles of the cell
- Telophase. Two new nuclear envelopes are formed. Microtubules disappear.
- Cytokinesis. The plasma membrane strangles the cell and divides it in two.
Phases of meiosis
In meiosis-type reproduction, A new bipartition of the daughter cells is then carried out to obtain four haploid cells.
meiosis It involves two distinct phases: meiosis I and meiosis II. Each of them is made up of various stages: prophase, metaphase, anaphase and telophase. Meiosis I is distinguished from meiosis II (and mitosis) because its prophase is very long and during its course the homologous chromosomes (identical because one comes from each parent) pair and recombine to exchange genetic material.
Meiosis I. Known as the reductive phase, it results in two cells with half the genetic load (n).
- Prophase I. It is made up of several stages. In the first stage the DNA is condensed into chromosomes. Then, the homologous chromosomes pair, forming a characteristic structure called the synaptonemal complex, where crossing over and gene recombination occur. Finally, the homologous chromosomes separate and the nuclear envelope disappears.
- Metaphase I. Each chromosome, composed of two chromatids each, aligns on the median plane of the cell and attaches to the microtubules of the achromatic spindle.
- Anaphase I. Paired homologous chromosomes separate and move toward opposite poles. Each pole receives a random combination of maternal and paternal chromosomes, but only one member of each homologous pair is present at each pole. Sister chromatids remain attached to their centromeres.
- Telophase I. One of each pair of homologous chromosomes is at each pole. The nuclear membrane is formed again. Each nucleus contains the number of haploid chromosomes, but each chromosome is a duplicate chromosome (consists of a pair of chromatids). Cytokinesis occurs, resulting in two haploid daughter cells.
Meiosis II. It is the duplicative phase: the cells from meiosis I divide, which results in the duplication of DNA.
- Prophase II. The chromosomes condense. The core envelope disappears.
- Metaphase II. The chromosomes align on the midplanes of their cells.
- Anaphase II. The chromatids separate and move toward opposite poles.
- Telophase II. The chromatids that reach each pole of the cell are now chromosomes. Nuclear envelopes form again, chromosomes gradually elongate to make chromatin fibers, and cytokinesis occurs. The two successive divisions of meiosis produce four haploid nuclei, each with one chromosome of each type. Each resulting haploid cell has a different combination of genes.
Continue with: Cell theory
References
- “Cell division” in Wikipedia.
- “Chapter 14. Cellular reproduction” in Acces Medicina.
- “Mechanisms of cellular reproduction” (video) in PuntajeNacional Chile.
- “Cell reproduction” in Biology Facts.
- “Reproduction and cell division” (video) at Khan Academy (English).
- Cellular and Molecular Biology. From Robertis Edward, Hib José. (2012) 16th ed. Promed Editorial.
