How Cells Divide: Mitosis, Meiosis and the Cell Cycle

The Cell Cycle and Interphase

Nutrition function: Processes necessary to obtain energy for the mother cell and its cells, so they can grow optimally and perform their activities.

Relation function: Reception of stimuli, intracellular communication and elaboration of responses.

Reproduction function: Process by which cells multiply and transmit genetic information.

Cell cycle: From the moment a cell is born until it divides, it goes through a series of stages.

Interphase

During interphase, the cell is in a long-term phase. The cell presents a nucleus and comprises:

• G1: Synthesis of mRNA and proteins. The cell grows, performs its normal functions, and may enter a resting state (G0) or specialize. Cells that are about to divide pass through G1 and reach a point of no return.
• S: Duplication of DNA, mRNA and protein synthesis take place (about 8 hours). Next to each centriole, a procentriole forms.
• G2: Synthesis of mRNA and proteins. The cell has 2 diploid nuclei and is immature (about 4 hours).

Cell Division

M phase: One mother cell gives rise to 2 daughter cells with the same number of chromosomes as the mother.

Nuclear division is the process that generates cells with the same chromosome number as the mother. In unicellular and pluricellular organisms it can occur asexually; in pluricellular organisms it is used for growth, regeneration and the renewal of individual tissues.

Mitosis

Mitosis is divided into the following stages:

• Prophase: Chromatin begins to condense into visible chromosomes, each made of 2 chromatids. The nucleolus disappears. The cell presents a diploid nucleus surrounded by pericentriolar material, which forms the centriolar complex. The pericentriolar material forms microtubules, and the fibers of the aster (astral centrosome) extend to the poles, where they form the mitotic spindle. Late in prophase, the nuclear envelope breaks down and the nucleoplasm mixes with the cytoplasm. Around the centromere, kinetochore microtubules begin to form, attaching to each chromosome.
• Metaphase: The kinetochore microtubules form a stretch between each chromosome and the poles, and the chromosomes align along the central plane, forming the equatorial plate.
• Anaphase: Separation of the two chromatids of each chromosome, so that each one holds one chromatid. The microtubules pull the chromatids toward opposite poles, and the kinetochores disappear.
• Telophase: The chromosomes reach the poles and begin to decondense. A nuclear envelope forms around each set of chromosomes, the nucleolus reappears, and the polar microtubules accumulate in the interzone with protein accumulations. Cytokinesis begins.

Cytokinesis

Cytokinesis is the division of the cytoplasm.

• In animal cells: It begins at the periphery of the plasma membrane with the formation of a cleavage furrow in the equatorial plane. A contractile ring of actin and myosin filaments constricts the cell until two daughter cells separate.
• In plant cells: A vesicle accumulation zone forms at the center, from Golgi vesicles and microtubules, creating a phragmoplast. The vesicles fuse to form a cell plate that grows outward toward the periphery, where it joins the cell wall. Plasmodesmata are left as communication channels between the cells. The cell plate becomes a middle lamella, and then a new cell wall is added on each side. Unlike animal cells, plant cells do not have centrioles, so in early prophase an organizing zone without organelles (clear zone) forms, acting as a microtubule organizing center. The two centers extend toward the poles, forming the spindle.

Types of cytoplasmic division

Bipartition or binary division (constriction, fragmentation, septation) or multiple division, pluripartition (budding) or gemmation.

Types of reproduction

Asexual (produces progeny) and sexual (produces progeny).

Meiosis (2n → n)

Meiosis occurs in the interphase of the sexual cells and consists of two divisions:

Meiosis I

• Prophase I:
  • Leptotene: DNA fibers condense and give chromosomes with 2 chromatids (4 chromosomes per 2n: 2 paternal and 2 maternal).
  • Zygotene: Homologous chromosomes pair up (synapsis), gene to gene.
  • Pachytene: The pairs continue to form bivalents or tetrads. Crossing over occurs, producing genetic recombination.
  • Diplotene: The chromosomes begin to separate (desynapsis), and chiasmata are visible.
  • Diakinesis: Continued nuclear condensation. The nuclear envelope and nucleolus disappear, and the spindle is formed.
• Metaphase I: Bivalents align on the equatorial plane.
• Anaphase I: Homologous chromosomes separate and migrate toward the poles.
• Telophase I: Chromosomes decondense and a nuclear envelope forms. Cytokinesis follows, and the cell enters prophase II.

Meiosis II

Meiosis II begins after the interkinesis (short resting phase) with no DNA replication, and proceeds similarly to mitosis, separating sister chromatids and producing four haploid cells.