Cell Division: Cytokinesis, Meiosis, and Reproduction Types
Cytokinesis: Division of the Cytoplasm
Cytokinesis is the division of the cytoplasm. In animal cells, it occurs by constriction, and in plant cells, by intracellular septation.
Animal Cells
In animal cells, cytokinesis starts at the end of anaphase with the invagination of the plasma membrane in the equatorial zone, forming the cleavage furrow. Inside the cell, a contractile ring of actin and myosin forms, constricting the cell until it divides the cytoplasm, resulting in two daughter cells.
Plant Cells
In plant cells, cytokinesis begins with the accumulation of vesicles from the Golgi apparatus in the equatorial zone. These vesicles fuse to form the early cell plate (phragmoplast). This plate grows from the center to the periphery. Plasma membranes of the two cells begin to form, but there are communications between the two cells called plasmodesmata. The vesicles bring pectins and hemicellulose to form the middle lamella. Cellulose is added later to form the cell wall.
Another difference in cell division between plants and animals is the absence of centrioles in plant cells. In plant cells and some protozoa, at the beginning of prophase, an area without organelles called the clear zone appears. This zone is a microtubule-organizing center that stretches, resulting in two organizing centers called polar caps. Between these two caps, the mitotic spindle forms. This type of spindle without asters is called an anastral spindle, while a spindle with asters is called an astral spindle.
Forms of Cytoplasmic Division
- Bipartition or Binary Division: The cytoplasm of the mother cell, after the nucleus has divided, is stretched, leading to two identical cells.
- Constriction, typical of unicellular organisms.
- Cleavage in metazoa (multicellular animals).
- Septation in plants, both in some thallophytes and metaphytes.
- Multiple Division: Involves the division of the cytoplasm after the nucleus has divided several times, resulting in many nuclei and cytoplasm. Sporulation is the most typical form, seen in protozoa and sporozoa.
- Budding: The cytoplasm is divided unequally, with a small part called the bud. The nucleus divides and is placed in the bud so that one cell is larger than the other. They usually remain attached for a while. An example is yeast.
Asexual and Sexual Reproduction
Asexual Reproduction
In asexual reproduction, the descendants are genetically identical to the parent. The new individual is produced by mitosis from one or more progenitor cells. There is a single parent. Examples include bulbs, plant cuttings, and budding in corals.
Sexual Reproduction
In sexual reproduction, the offspring are genetically different from the parents. These individuals originate from reproductive cells that are haploid gametes or meiospores. They, or their ancestors, have originated by meiosis. Gametes must join in pairs to give rise to a new individual, while meiospores do not. Meiospores occur in fungi and plants, and gametes occur in animals and plants.
Meiosis: Reducing Chromosome Number
Meiosis is a process that generates cells with half the number of chromosomes as the stem cell. It consists of two successive processes: meiosis I (reductional division) and meiosis II (equational division). During interphase, DNA is duplicated.
Meiosis I
Prophase I is the most complex phase and consists of five subphases:
- Leptotene: DNA fibers condense and form chromosomes, each with two chromatids.
- Zygotene: Chromosomes of each pair come together in a process called synapsis, joining gene by gene. This is made possible by the synaptonemal complex, which consists of a lateral axis (a protein filament next to one chromatid) and a central axis between the lateral axis and the chromatid of the other chromosome.
- Pachytene: Chromosome pairs stay together, forming bivalents or tetrads. Here, crossing over occurs between homologous chromosomes, exchanging fragments between homologous chromatids. This typically happens at 2 or 3 points on the chromosome, resulting in genetic recombination.
- Diplotene: Chromosomes begin to separate but not completely (desynapsis). The junction points, called chiasmata, are visible.
- Diakinesis: Separation continues, and chiasmata are very well defined. The nuclear envelope disappears, the nucleolus dissolves, and the spindle forms.
Metaphase I: Homologous chromosome pairs are arranged in the equatorial plane randomly.
Anaphase I: Pairs of homologous chromosomes separate.
Telophase I: Chromosomes decondense, and the nuclear envelope begins to form. Almost immediately, Prophase II begins.
Meiosis II
Meiosis II starts after a small interphase or interkinesis where DNA does not duplicate. This phase is similar to a mitotic division.