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1.6. Cellular cycle

The cell cycle

Cells, prokaryotic or eukaryotic, reproduce to give rise to new daughter cells which, after growing and fulfilling their functions, will reproduce again to give rise to new cells.

The cell cycle is the sequence of events that take place from the time a cell originates by division of a previous cell, until it divides again to give rise to new daughter cells.

Although almost all cells can divide, there are some exceptions. Muscle cells or neurons cannot divide. Others, such as those of the intestinal epithelium, usually take about 8 hours to divide.

For many single-celled organisms, the cell cycle constitutes their life cycle and the birth of a new organism.

For multicellular organisms, the cell cycle of their cells allows their growth and the repair of damaged tissues.

In eukaryotic cells, the cell cycle  is divided into two stages:

Interphase

The interphase stage  is the longest stage in the life of the cell. It includes the period between the end of one division (mitosis) and the next. The main processes that occur during the interface are:

  • The cell increases in size.

  • At the end of the interface there is duplication or replication of DNA in the nucleoplasm, in such a way that each chromosome is made up of two identical filaments. Thus, each daughter cell will receive the same amount of DNA as the mother cell.

  • New cell organelles are produced.

  • The centrioles are duplicated.

  • A cell can stay at the interface forever or divide again. If it stays at the interface, the cell is transformed, and by cell differentiation, the cell specializes. An example of these specialized cells that have lost the ability to divide are neurons and muscle cells in the heart.

Remember that, at the interface, the genetic material is in the form of chromatin. In the human species, cells at the beginning of the interface have 2n = 46 chromatin chains. At the end of the interface, after DNA replication, chromatin is made up of pairs of identical strands, 2n = 92 strands (equal, two by two).

Cell division or M phase (mitosis)

The division phase or M phase is the final, shorter stage, in which a cell will have a mitotic division whereby it will originate two cells with the same amount of DNA as the cell that originated them.

The cell division phase or M phase has two stages:

  • Karyokinesis or division of the nucleus (mitosis).
  • Cytokinesis or division of the cytoplasm. Very short stage that only lasts 10% of the cell cycle, approximately.

In unicellular beings, cell reproduction coincides with the formation of a new individual.

In multicellular beings, cell reproduction serves so that the organism can grow and replace the cells that have been dying. In multicellular cells, all cells reproduce by mitosis, except for reproductive cells or gametes (ovules and sperm), which reproduce by meiosis.

Remember that when the cell is dividing, the two identical chromatin (DNA) chains spiral into chromosomes. The number of chromosomes in the human species is 2n = 46, forming 23 pairs of chromosomes.