Cell Cycle Phases and Cellular Division: Mitosis vs. Meiosis

Posted on Jan 10, 2025 in Biology

The Cell Cycle

Interphase: This is the initial, long stage where cell division does not occur. It is characterized by cell growth and the development of normal metabolic activities.

  • G1 Phase: Follows the previous cell’s mitotic cycle and is a stage of cell development. At the end of the G1 phase, there is a point of no return (the restriction point or R point), after which the entire process cannot be stopped. Phases S, G2, and M will inevitably follow. Some cells, before reaching the R point, undergo cell differentiation, activating the expression of certain genes. This leads to cell specialization and can last from a few days to several months. This phase, known as the G0 phase, can be reversed to reach the S phase by treatment with mitotic activators.
  • S Phase: During this phase, DNA replication occurs, ensuring that each daughter cell receives an identical copy of the genome. Transcription and translation of histones continue, while a centriole outline, called a procentriole, forms.
  • G2 Phase: Begins when replication ends and ends when chromosome condensation becomes visible, marking the beginning of the M phase.
  • M Phase (Cell Division): This is a much shorter period following interphase, in which the cell divides into two new daughter cells after the disappearance of the nuclear envelope and the nucleolus.

The duration of the G1, S, G2, and M phases depends on the cell type involved and certain physiological conditions and factors, especially temperature. For a 24-hour cell cycle, the G1 phase lasts about 11 hours, the S phase about 8 hours, the G2 phase about 4 hours, and the M phase 1 hour.

Mitosis vs. Meiosis

Mitosis

  • Involves nuclear division (mitosis) and cytoplasmic division (cytokinesis).
  • Results in two daughter cells with the same number of chromosomes as the mother cell.
  • No crossing over or synapsis occurs in prophase.
  • No chiasmata form.
  • Sister chromatids are separated in anaphase.

Meiosis

  • Involves two nuclear divisions (karyokinesis) and two cytoplasmic divisions (cytokinesis).
  • Results in four daughter cells with half the number of chromosomes as the stem cell.
  • There is a random separation of homologous chromosomes and crossing over.
  • Synapsis occurs in prophase I, and chiasmata form.
  • Homologous chromosomes are separated in anaphase I.

Biological Significance of Meiosis

Genetic Level: Meiosis is a source of genetic variability. Interbreeding leads to new combinations of genes on chromosomes, and therefore, this phenomenon is responsible for genetic recombination. Both crossing over and the distribution of chromatids depend on chance and give rise to each of the four resulting cells having a combination of different genes. These genes will later be subjected to the pressures of natural selection, so that only the better-adapted cells will survive.

Cellular Level: Meiosis results in the reduction of chromosome number, as diploid cells become haploid.

Organismal Level: Haploid cells resulting from meiosis will become reproductive cells, either sexual (gametes) or asexual (spores).