Biology (9700)
Topic 5 of 17Cambridge A Levels

The Mitotic Cell Cycle

The ordered sequence of events for cell growth and division into two identical daughter cells.

The mitotic cell cycle is the fundamental process by which eukaryotic cells replicate, ensuring the growth, repair, and asexual reproduction of multicellular organisms. It is a highly regulated and ordered sequence of events, divided into two main phases: Interphase and the Mitotic (M) phase.


### Interphase

This is the longest phase of the cell cycle, where the cell grows, carries out its metabolic functions, and prepares for division. It is subdivided into three stages:


  • G1 (Gap 1) Phase: Following cell division, the cell enters G1. During this stage, the cell grows in size, synthesises proteins and organelles (like mitochondria and ribosomes), and carries out its normal functions. A critical G1 checkpoint ensures the cell is large enough and has adequate resources before it commits to DNA replication.

  • S (Synthesis) Phase: The most crucial event of this phase is DNA replication. The cell synthesises an exact copy of its DNA. Each chromosome, initially a single strand of DNA, is duplicated to form two identical sister chromatids. These chromatids are joined together at a region called the centromere.

  • G2 (Gap 2) Phase: After DNA replication is complete, the cell enters the G2 phase. It continues to grow, replenishes its energy stores, and synthesises proteins necessary for mitosis, such as the protein tubulin, which forms microtubules for the spindle. The G2 checkpoint verifies that DNA replication is complete and that any DNA damage has been repaired before the cell enters mitosis.

    • ### M (Mitotic) Phase

    The M phase involves the division of the nucleus (mitosis) and the division of the cytoplasm (cytokinesis).


    Mitosis is a continuous process conventionally divided into four stages:


  • Prophase: The duplicated chromosomes condense and coil, becoming visible under a light microscope as distinct structures. The nuclear envelope and nucleolus disintegrate. In animal cells, the centrioles move to opposite poles of the cell and begin to form the spindle fibres, a network of microtubules.

  • Metaphase: The fully condensed chromosomes, each consisting of two sister chromatids, align along the cell's equator, known as the metaphase plate. Spindle fibres from opposite poles attach to the centromere of each chromosome. The spindle assembly checkpoint ensures that every chromosome is properly attached to the spindle before division proceeds.

  • Anaphase: This is the shortest stage. The proteins holding the sister chromatids together are broken down, and the centromeres divide. The sister chromatids are pulled apart by the shortening spindle fibres towards opposite poles of the cell. Once separated, each chromatid is considered an individual chromosome.

  • Telophase: The chromosomes arrive at the poles, uncoil, and decondense, becoming less visible. A new nuclear envelope reforms around each set of chromosomes at the two poles, creating two new, genetically identical nuclei.

    • Cytokinesis usually begins during late anaphase or telophase. In animal cells, a cleavage furrow forms as a ring of actin and myosin filaments contracts, pinching the cell membrane inwards and dividing the cytoplasm into two daughter cells. In plant cells, a cell plate forms at the equator and grows outwards, developing into a new cell wall that separates the two daughter cells.


    ### The Role of Telomeres and Stem Cells


    Telomeres are repetitive, non-coding sequences of DNA (e.g., TTAGGG in humans) located at the ends of linear chromosomes. Their primary role is to protect the coding regions of the DNA from being lost during successive rounds of DNA replication, as the replication machinery cannot fully copy the very end of the chromosome. With each mitotic division, telomeres shorten. This progressive shortening is linked to cellular ageing, or senescence, and limits the number of times a cell can divide (the Hayflick limit). The enzyme telomerase can rebuild telomeres, and its activity is high in cells that need to divide indefinitely, such as stem cells and, unfortunately, cancer cells.


    Stem cells are unspecialised cells that are critical for growth and repair. They have two defining properties: self-renewal (the ability to divide by mitosis to produce more stem cells) and differentiation (the ability to develop into specialised cell types).

    * Totipotent stem cells (e.g., a zygote) can become any cell type, including extra-embryonic tissues.

    * Pluripotent stem cells (e.g., embryonic stem cells) can become any cell type in the body.

    * Multipotent stem cells (e.g., adult stem cells like haematopoietic stem cells in bone marrow) can differentiate into a limited range of cell types.

    Through mitosis, stem cells replace old or damaged cells, maintaining tissue health throughout an organism's life.

    Key Points to Remember

    • 1The cell cycle is a regulated process comprising Interphase (G1, S, G2) for growth and DNA replication, and the Mitotic (M) phase for division.
    • 2Mitosis occurs in four stages (Prophase, Metaphase, Anaphase, Telophase) to create two genetically identical nuclei from one parent nucleus.
    • 3Cytokinesis follows mitosis, dividing the cytoplasm via a cleavage furrow in animals or a cell plate in plants to form two daughter cells.
    • 4Critical checkpoints in G1, G2, and Metaphase regulate the cell cycle, ensuring fidelity and preventing errors like tumour formation.
    • 5Telomeres are protective caps on chromosome ends that shorten with each division, contributing to cellular ageing; the enzyme telomerase can counteract this.
    • 6Stem cells are undifferentiated cells that divide by mitosis for growth, repair, and tissue maintenance, possessing the ability to self-renew and differentiate.
    • 7Uncontrolled mitosis, often due to mutations in genes that regulate the cell cycle, is the underlying cause of cancer.

    Pakistan Example

    Stem Cell Therapy for Thalassemia in Pakistan

    Thalassemia is a significant genetic blood disorder in Pakistan, requiring patients to have regular blood transfusions. A potential cure is a bone marrow transplant, which relies on the mitotic cell cycle. In this procedure, healthy **haematopoietic stem cells** are transplanted into the patient. These multipotent stem cells divide rapidly by **mitosis** to repopulate the bone marrow, producing new, healthy red blood cells. Research centres in Pakistan, such as the National Institute of Blood Diseases (NIBD) in Karachi, are actively involved in these advanced treatments, demonstrating the critical role of controlled cell division and stem cell biology in addressing national health challenges.

    Quick Revision Infographic

    Biology — Quick Revision

    The Mitotic Cell Cycle

    Key Concepts

    1The cell cycle is a regulated process comprising Interphase (G1, S, G2) for growth and DNA replication, and the Mitotic (M) phase for division.
    2Mitosis occurs in four stages (Prophase, Metaphase, Anaphase, Telophase) to create two genetically identical nuclei from one parent nucleus.
    3Cytokinesis follows mitosis, dividing the cytoplasm via a cleavage furrow in animals or a cell plate in plants to form two daughter cells.
    4Critical checkpoints in G1, G2, and Metaphase regulate the cell cycle, ensuring fidelity and preventing errors like tumour formation.
    5Telomeres are protective caps on chromosome ends that shorten with each division, contributing to cellular ageing; the enzyme telomerase can counteract this.
    6Stem cells are undifferentiated cells that divide by mitosis for growth, repair, and tissue maintenance, possessing the ability to self-renew and differentiate.
    Pakistan Example

    Stem Cell Therapy for Thalassemia in Pakistan

    Thalassemia is a significant genetic blood disorder in Pakistan, requiring patients to have regular blood transfusions. A potential cure is a bone marrow transplant, which relies on the mitotic cell cycle. In this procedure, healthy **haematopoietic stem cells** are transplanted into the patient. These multipotent stem cells divide rapidly by **mitosis** to repopulate the bone marrow, producing new, healthy red blood cells. Research centres in Pakistan, such as the National Institute of Blood Diseases (NIBD) in Karachi, are actively involved in these advanced treatments, demonstrating the critical role of controlled cell division and stem cell biology in addressing national health challenges.

    SeekhoAsaan.com — Free RevisionThe Mitotic Cell Cycle Infographic

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