Cell division is a core process of life in the microworld. Mitosis and meiosis are two types of reproduction in microbes such as algae, yeast and protozoans. In mitosis, a mother cell splits into two daughter cells. In meiosis, four genetically diverse cells are created.
Mitosis: Asexual Reproduction in Microbes
Mitosis is a type of asexual reproduction. A single mother cell divides into two genetically identical daughter cells. This cell division is predominant in eukaryotic microbes such as protozoans, yeast, and algae. It's essential for growth and repair.
Amoeba
Amoebae are protozoans and primarily reproduce through mitosis. Amoeba proteus undergoes mitosis by duplicating its genetic material. After DNA replication, the nucleus divides in a series of phases: prophase, metaphase, anaphase and telophase.
The phases are needed to provide a complete set of chromosomes to each daughter cell. After mitosis, cytoplasmic division or cytokinesis results in two newly-formed amoebae.
Yeast
Yeasts, such as the familiar Saccharomyces cerevisiae, also reproduce asexually through mitotic division. Under favorable conditions, this unicellular fungus undergoes a budding process.
Initially, the yeast cell develops a small protrusion or bud, which gradually grows larger. The nuclear material is then equally partitioned, with the nucleus migrating into the bud.
Eventually, the bud detaches, resulting in two individual yeast cells. This method is not only efficient but also allows rapid population growth, especially in nutrient-rich environments.
Under optimal conditions yeast can double its population in about two hours.
Algae
For algae, species such as Chlamydomonas replicate through mitosis as well, ensuring stability of their population in a favorable environment. These organisms are found in in nutrient-rich waters and can produce a lot of biomass very quickly.
Ciliates: Complex Mitosis
Ciliates, such as Paramecium, demonstrate a more complex form of mitosis. The process of nuclear division occurs along with organization of cilia and genetic material.
Ciliates have two types of nuclei: a macronucleus and one or more micronuclei. The macronucleus governs daily cellular functions. In reproduction the macronucleus divides while the micronuclei are involved in genetic recombination.
Ciliates can reproduce sexually through conjugation. In conjugation a microbe transfers genetic material to another through direct contact. One microbe donates the genetic material, and the other receives it.
Meiosis: Sexual Reproduction in Microbes
Meiosis is a specialized form of cell division resulting in four genetically diverse daughter cells, typically associated with formation of gametes. Though traditionally recognized in higher organisms, certain microbes engage in meiosis.
The process reduces the chromosome number by half. This results in four genetically diverse haploid cells from one diploid cell. Meiosis is especially prevalent in response to environmental stressors.
Haploid & Diploid Cells
A diploid cell has two complete sets of chromosomes. Most cells in humans are diploid, comprising 23 chromosome pairs, so 46 chromosomes in total.
Haploid defines a cell with one set of chromosomes. This term also describes the chromosome count in egg or sperm cells, known as gametes. In humans, gametes are haploid cells with 23 chromosomes, each representing one chromosome from a pair found in diploid cells.
Meiosis Among Ciliates
The ciliate Tetrahymena displays unique sexual reproduction involving meiosis and conjugation. During conjugation, two Tetrahymena cells join to exchange genetic material.
Following this exchange, meiosis occurs to produce new haploid cells. This mechanism significantly enhances genetic variety, which is crucial for adapting to changing environments.
Above: When nutrients are scarce, two individuals (A) pair with each other and begin sexual reproduction (conjugation). (B) The diploid micronucleus in each individual undergoes meiosis to form four haploid nuclei, and three of these are degraded. (C) The remaining haploid nucleus divides mitotically to form two pronuclei in each cell.
(D) One of the two pronuclei in each cell is exchanged with the mating partner, and fusion leads to he formation of the diploid zygotic nucleus. (E) The zygotic nucleus divides twice mitotically to form four nuclei.
(F) Two nuclei become micronuclei, and the other two differentiate to become macronuclei; the original parental macronucleus is degraded. (G) Cell division occurs and the nuclei are distributed to the daughter cells so that each progeny receives one micronucleus and one macronucleus.
Algae and Meiosis
Some algae, such as Chlamydomonas, undergo meiosis as part of the reproductive cycle. In haploid phase, these unicellular green algae can fuse to form a diploid zygospore, which then produces four haploid spores through meiosis.
Zygospores are those produced by fungus and protists. These spores are robust and resistant, formed by the fusion of two haploid nuclei to result in gametangia, specialized organs or cells in which gametes are formed.
Spores grow into new haploid organisms, continuing the life cycle. This switching between haploid and diploid stages is a fascinating aspect of algal biology.
Fungi and Meiosis
In the fungal kingdom, meiosis is a critical component of sexual reproduction. For instance, the ascomycete fungus Neurospora crassa, a bread mold, undergoes meiosis during the formation of ascospores, specialized sacs created from a cell membrane.
In this process, two haploid cells fuse to create a diploid cell. The cell undergoes meiosis, yielding four genetically diverse ascospores. This method of reproduction ensures variation among descendants, important to adaptation and evolution in changing environments.
The bread mold N. crassa generates haploid spores through meiosis. When these spores germinate, they can produce genetically diverse individuals, allowing the mold to adapt to different conditions.
Cultivation of yeasts and fungi is invaluable in fermentation processes which produce bread, beer, and dairy products such as cheese and yogurt. The fermentation of Saccharomyces cerevisiae alone accounts for a significant part of global wine and beer production.
Genetic variations from meiosis in protozoans and fungi play a key role in evolution and adaptation. The continuous interplay of genetic exchange enables evolution of new traits needed for survival.
Bacteria: A Special Case of Division
Bacteria reproduce by binary fission. In this method, a bacterial cell grows to a specific size, duplicates its circular DNA, and then divides into two cells. The cell elongates, and once it reaches a certain point, a septum forms and eventually separates the cell into two new entities.
These organisms can reproduce at astonishing rates. For example, under ideal conditions, Escherichia coli, a common bacterium, can divide every 20 minutes. From a single bacterium, a population of over 16 million cells can be reached in just 12 hours.
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