Cell cycle, its different phases and duration

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  • Life starts as a single cell called zygote or fertilized egg in all multicellular organisms. Living organisms grow by addition of new cells that arise by the division of pre-existing cells.
  • The chromosomes, or DNA, of a parent cell must be doubled exactly if the daughter cells formed by its division are to have identical information about their structure and functioning.
  • This duplication of chromosomes, or DNA, does occur well before cell division begins. Each new cell must inherit not only a nucleus containing the entire genetic information it will need, but also the cytoplasmic organelles required to express the genetic information, such as ribosomes to synthesize proteins and mitochondria to supply the necessary energy.
  • The mode of cell division is fundamentally similar in all organisms, which emphasizes the unity of life.

Cell cycle:

  • The sequence of events by which a cell duplicates its genome and synthesizes other cell contents and eventually divides into two daughter cells is termed as cell cycle.
  • The term cell cycle was introduced by Howard and Pelc in 1953.
  • In simple words, the cell cycle refers to the sequence of events involving growth and division of a cell from the time of its formation (by division of the parent cell) to its own division into daughter cells.

What is generation time?

  • It is the duration of the cell cycle, i.e. period between two successive cell divisions.
  • It depends on the type of cell and external factors such as temperature and food and oxygen supplies. E.g. bacterial cells ( coli) divide every 20 minutes, epithelial cells of the intestinal lining every 8-10 hours, a yeast cell every 90 minutes etc.

Phases of cell cycle:

  • There are two main stages in the cell cycle of a typical eukaryotic somatic cell.
    • Interphase: a long non-dividing state also known as inter-mitosis or I-phase
    • Mitotic phase or M-phase: a shorter phase of nuclear division
    • G0 phase: for further non-dividing cells
  • These main phases are followed by a still shorter phase of cytoplasmic division known as cytokinesis or C-phase.
  • Various phases of cell cycle are regulated by proteins cyclins and cyclin dependent protein kinases (CDKs). The latter phosphorylate protein using ATP resulting in the breakdown of nuclear membrane.

Different phases of a cell cycle
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a. Interphase:               

  • Interphase involves a series of changes that a newly formed cell and its nucleus undergo to become fit for division.
  • In this phase, there is continuous synthesis of biological molecules, such as carbohydrates, lipids, proteins and nucleic acids, needed for its growth carrying out its specific functions.
  • DNA repair: Repair of damaged DNA sequences also takes place in interphase.
  • The interphase lasts for 10-20 hours, and takes 75-90 %of the entire generation time. It is further divided into three periods or sub-phases; first gap or G1 phase, synthetic or S-phase, and second gap or G2 phase. Duration of these sub-phases varies in different organisms.
    1. G1 phase (Gap 1):
      • It is a stage of initial growth of a newly formed cell. The cell is metabolically active in preparation of DNA replication. Nucleotides, ATP and amino acids are formed. Mitochondria and chloroplasts (in plant cell) multiply.
      • Endoplasmic reticulum, golgi apparatus and other cell organelles are produced.
      • RNAs (rRNA, mRNA, tRNA) and ribosomes are synthesized whereas no change occurs in the DNA content of the cell.
      • The duration of G1 phase is highly variable. Frequently dividing cells have a shorter G1 phase whereas it is longer in the cells which divide non-frequently.
      • Some unknown signal molecule (s) is/are thought to switch the cell into the S-phase.
    2. S-phase:
      • It is a stage of replication of each chromosome by the synthesis of a new DNA molecule on the template of the existing DNA.
      • Each chromosome now consists of two identical strands called sister chromatids,that are held together at the region of centromere by a complex protein called
      • At the end of S-phase, the diploid cell (2n) becomes tetraploid (4n), as the chromosome carries duplicate set of genes
      • Histone proteins are synthesized and these associate with each DNA strand, forming nucleosomes. Some non-histone proteins are also formed in S-phase.
      • In most eukaryotic cells, this phase lasts for 6 to 8 hours. With the beginning of S-phase, the cell has to undergo mitosis.
    3. G2 phase (Gap 2):
      • It is the last stage of further growth of the cell and preparation for its division.
      • During this stage, synthesis of RNAs and proteins continues. Similarly, cell organelles like centrioles, mitochondria and Golgi apparatus are doubled.
      • Proteins for spindle and asters are synthesized and active metabolism stores energy for the next division.
      • It lasts for 2 to 5 hours. Some proteins formed in G2 phase cause condensation of chromosomes to initiate mitosis.

b. Mitotic phase (M-phase):

  • Mitotic phase or M-phase follows the interphase.
  • It starts with the separation of sister chromatids (now called daughter chromosomes) by the process called karyokinesis or nuclear division. Duplicated chromosomes are equally distributed among the two daughter nuclei.
  • After karyokinesis, the equal division of cytoplasm called cytokinesis occurs, where the cell organelles and macromolecules are approximately equally divided between the daughter cells.
  • Cytokinesis is followed by the separation of daughter cells which are diploid and contain identical sets of chromosomes, and hence exactly the same hereditary information as the parent cell.
  • After the completion of mitosis, the daughter cells may enter the G1 phase of the next cell cycle or G0

c. G0 phase:

  • The cells which are not to divide further don’t proceed beyond the G1 phase and start undergoing differentiation into specific types of cells. Such cells are said to be in quiescent stage or G0 phase of the cell cycle to distinguish them from G1 cells which will soon enter S-phase.
  • Cells in G0 phase remain metabolically active but no longer proliferate unless called on to do so depending on the requirement of the organism. Some of the G0 cells, such as fibroblasts, which help in healing of wounds, grow and divide again on demand of the body. Some types of cells don’t divide after attaining full differentiation and finally die. E.g. nerve cells, skeletal muscle cells, mature circulating WBCs and RBCs.

*The most important event in the regulation of cell cycle occurs in the G1 phase. A G1 cell commits whether to restart a new cell cycle of to enter the G0 phase.

Duration of cell cycle:

  • Under optimal conditions of nutrition and temperature, the duration of the cell cycle for a particular kind of cell is constant.
  • Under less favorable conditions, it may become longer, but it is not possible to speed up the cell cycle and make cells grow and divide faster.
  • This shows that the duration of the cell cycle is the time required for carrying out some precise program that has been built into each cell. This program seems to include replication of chromosomes and doubling of all other constituents of the cell meant for growth.

Cell cycle, its different phases and duration