Cell Cycle Phases: Mitosis and Meiosis Explained

Posted on Jan 28, 2025 in Biology

Cell Cycle

The cell cycle is a series of changes undergone by the cell from which it is formed until it divides.

Interphase

Interphase is a period that elapses between two successive mitoses (cell divisions). It occupies most of the cell cycle, and cell metabolic activity increases in size.

There are three periods:

  • Phase G1: Covers the period between division and DNA synthesis. Mainly, proteins are synthesized, and the cell enlarges. In cells that do not divide, this phase is ongoing and is called G0.
  • Phase S: DNA replication occurs, and histones are synthesized. As a result, each chromosome consists of two chromatids.
  • Phase G2: Genes are transcribed and translated to synthesize proteins necessary for cell division. Centrioles are duplicated.

Phase M

Phase M occurs in mitosis (division of the nucleus) and cytokinesis (division of the cytoplasm). During cell division, two daughter cells are obtained with identical genetic material.

Mitosis

Also called karyokinesis, mitosis equitably distributes the hereditary material, which has doubled in the S phase, between the two daughter cells that will form.

  • Prophase: Chromatin condenses, which makes the chromosomes become visible and appear composed of two chromatids joined at the centromere. The centrioles, which doubled in the G2 phase, begin to separate until they are at opposite poles of the cell. Polar microtubules are formed between them, forming the mitotic spindle or achromatic spindle. The nuclear membrane and nucleolus disappear, and the formed chromosomes disperse. Kinetochores are formed, from which microtubules are formed.
  • Metaphase: The chromosomes reach their maximum degree of condensation. The spindle is formed and extends between the two poles of the cell. The microtubules join the chromosomes, which lie in the midplane of the mitotic spindle, where the centromeres form a plaque placed at each equatorial. The chromatids are oriented toward one pole.
  • Anaphase: The two chromatids begin to separate until each one reaches a pole, carried by the microtubules.
  • Telophase: The nucleoli reappear, and chromosomes start to decondense. From the rough endoplasmic reticulum, a new nuclear envelope forms around each group of chromosomes, identifying two core areas, one at each pole.

Meiosis

From a diploid cell, four haploid cells are derived, genetically distinct and different from the mother cell. These cells are the gametes. There is an exchange of genetic material between chromatids of homologous chromosomes: genetic recombination.

In the S phase of interphase, DNA duplication of chromosomes occurs, which are composed of two chromatids joined at the centromere.

Meiosis I

In the first meiotic division, the homologous chromosomes pair, there is an exchange of genetic material between homologous chromatids, and homologous chromosomes are separated.

Prophase I

  • Leptotene: The chromosomes condense to become visible at the optical microscope. Each consists of two chromatids.
  • Zygotene: The homologous chromosomes pair, forming a tetrad due to a protein complex called the synaptonemal complex, to be fully aligned (synapsis).
  • Pachytene: Crossing-over occurs, or genetic material is exchanged between chromatids of homologous chromosomes.
  • Diplotene: Homologous chromosomes begin their separation, staying together through the points where crossing-over has taken place, called chiasmata.
  • Diakinesis: Each pair of chromatids are joined at the centromere and the homologous chromosomes by chiasmata (between non-sister chromatids). The nucleolus disappears, and the nuclear membrane breaks down.

Metaphase I

Tetrads linked by chiasmata are arranged in the equatorial plate.

Anaphase I

The pairs of homologous chromosomes begin to separate, being dragged by the spindle fibers to opposite poles. Each chromosome of a pair of homologs, consisting of two chromatids, which has been genetically recombined, is directed toward one pole of the cell.

Telophase I

The nuclear membrane and nucleolus reappear. Two daughter cells are obtained with half the chromosomes as the parent cell and with two chromatids each chromosome.

Meiosis II

Meiosis II occurs simultaneously in the two cells. There is a brief interphase without DNA synthesis.

  • Prophase II: The nuclear membrane disappears, chromosomes condense, and the spindle forms.
  • Metaphase II: Chromosomes are in the equatorial plate, each consisting of two chromatids.
  • Anaphase II: Centromeres separate, and each chromatid migrates to opposite poles.
  • Telophase II: The nuclear membrane forms around the chromosomes, which decondense. Cytokinesis occurs.