Cell Division: Mitosis

Cell division: mitosis

Overview

Cells divide continuously to replace old or damaged cells and to allow organisms to grow. Cell division maintains appropriate cell size, preserves the complement of genetic material and enables reproduction in unicellular organisms.

The growth and division of a cell

Growth and its control

Growth is an increase in the size and volume of a cell or organism. As a cell grows its volume increases faster than its surface area. If the surface area to volume ratio becomes too small, the cell cannot exchange sufficient nutrients and wastes across its membrane and will divide to restore an adequate ratio.

Role of mitotic cell division

• Unicellular organisms reproduce asexually by cell division (for example, binary fission or budding), producing new individuals that are genetically similar to the parent.
• Multicellular organisms use mitosis for growth, repair and replacement of dead or damaged cells.
• Cell division helps to control cell size and maintain an efficient surface area to volume ratio for exchange of materials.

The cell cycle

The cell cycle is the sequence of events through which a cell grows and divides. It has two broad phases: interphase, during which the cell grows and duplicates its DNA, and the mitotic phase, during which karyokinesis (division of the nucleus) and cytokinesis (division of the cytoplasm) occur.

Interphase and chromosomes

During interphase the cell carries out its specialised functions and increases in size. In the nucleus the DNA is replicated so that two identical sets of chromosomes are available for distribution to the daughter cells. Each replicated chromosome consists of two sister chromatids joined at the centromere.

Surface area to volume ratio

A group of small cells has a much greater total surface area relative to its volume than a single large cell of the same total volume. Greater surface area relative to volume improves the efficiency of nutrient uptake and waste removal across the cell membrane. When a cell becomes too large for efficient exchange, the nucleus usually initiates processes that lead to cell division.

Cell-cycle checkpoints (brief)

Progression through the cell cycle is monitored at several control points called checkpoints (for example at the end of G1, at G2 and during M phase). These checkpoints ensure that the cell has grown sufficiently, that DNA replication is complete and correct, and that chromosomes are properly aligned before division proceeds. Checkpoint mechanisms help maintain genetic stability.

Mitosis

Definition and main processes

Mitosis is the process by which a eukaryotic cell separates the chromosomes in its nucleus into two identical sets, producing two genetically identical daughter nuclei. Mitosis is followed by cytokinesis, the division of the cytoplasm, to form two separate daughter cells.

Two related processes are distinguished:

• Karyokinesis - division of the nucleus, distributing one copy of each chromosome to each daughter nucleus;
• Cytokinesis - division of the cytoplasm and cell membrane to produce two separate daughter cells.

Purpose and typical locations of mitosis

• Growth: Multicellular organisms grow by increasing cell number through mitosis - organisms often begin life as a single cell that divides repeatedly.
• Repair: Worn-out or damaged cells are replaced by new cells produced by mitosis (for example, skin and blood cells).
• Asexual reproduction: Many single-celled organisms reproduce by cell division.

Examples of locations where mitosis is active:

• In plants: apical meristems (tips of roots and shoots) and lateral meristems (regions under the bark) produce new cells by mitosis.
• In animals: actively dividing tissues include bone marrow (blood cell production) and the basal layers of the skin epithelium.

Mitosis in animal cells - sequence of events

Although interphase is not a stage of mitosis, it prepares the cell for division by duplicating DNA and increasing cell contents. The classical stages associated with mitotic karyokinesis are prophase, metaphase, anaphase and telophase.

Interphase (preparatory)

• DNA is replicated; chromosomes exist as extended chromatin and, after replication, each consists of two sister chromatids.
• The cell synthesises proteins and grows in size in preparation for division.

Prophase

• Chromatin condenses and chromosomes become visible, each as two sister chromatids joined at the centromere.
• In animal cells, the centrosome, made of two centrioles, duplicates and the two centrosomes move to opposite poles of the cell.
• Spindle fibres (microtubules) begin to form between the centrosomes, establishing the mitotic spindle.
• The nuclear membrane starts to break down.

Metaphase

• The nuclear membrane is fully disintegrated.
• Chromosomes line up at the metaphase plate (the equator of the cell).
• Each chromosome attaches to spindle fibres via its centromere; correct attachment ensures that sister chromatids will separate to opposite poles.

Anaphase

• The centromeres split and sister chromatids separate; each chromatid is now considered a daughter chromosome.
• Spindle fibres shorten and pull the daughter chromosomes towards opposite poles of the cell.

Telophase

• Chromosomes reach the poles and begin to decondense back into chromatin.
• A nuclear membrane and nucleolus re-form around each set of chromosomes, producing two nuclei.
• Cytokinesis typically begins during telophase: in animal cells a cleavage furrow forms and the cell membrane constricts to separate the two daughter cells.

Each daughter cell has the same chromosome number and genetic content as the parent cell (genetic identity is maintained in mitotic division).

Mitosis in plant cells - similarities and differences

The major events of chromosome behaviour are similar in plant and animal cells, but there are two important differences:

• Centrioles: Plant cells usually lack centrioles, yet a mitotic spindle still forms from microtubule organising centres.
• Cytokinesis: Instead of a cleavage furrow, plant cells form a cell plate at the equator. The cell plate develops into a new cell wall that separates the two daughter cells.

As in animal cells, plant chromosomes line up at the equator, sister chromatids separate in anaphase and nuclei reform in telophase.

Cancer

Definition and basic types

Cancer is uncontrolled cell division and growth resulting from failures in the normal regulation of the cell cycle. Excessive proliferation of cells can produce a mass of abnormal cells called a tumour or neoplasm.

• Benign tumours: Cells remain at the original site; they grow locally but do not invade other tissues.
• Malignant tumours: Cells invade surrounding tissues and can spread (metastasise) to other parts of the body.

Causes of cancer - carcinogens

Cancer can be caused by many factors that damage DNA or interfere with cell-cycle control. Such factors are called carcinogens. Common carcinogens and risk factors include:

• Inherited genetic susceptibility
• Radiation (for example ultraviolet radiation from the sun, X-rays, ionising radiation)
• Tobacco smoking
• Hormonal imbalances
• Certain processed foods and dietary factors
• Environmental pollutants and pesticides
• Certain viruses (for example human papillomavirus in some cancers)

Major categories of cancer by tissue type

• Carcinomas: originate from epithelial tissues such as skin, lining of organs and glands.
• Sarcomas: originate from connective tissues such as bone, cartilage and muscle.
• Leukaemia and lymphomas: cancers of blood-forming tissues, lymphatic and immune systems.

Beliefs, attitudes and misconceptions

Public beliefs about cancer are shaped by culture, media and personal experience. Common misconceptions include:

• "Cancer is always a death sentence."
• "Specific foods or exercise alone will prevent or cure cancer."
• "Modern medicine can cure every cancer."
• "A positive attitude guarantees survival."
• "Treatments do more harm than good."
• "Treating elderly patients is a waste of resources."
• "New treatments have not been tested."
• "Treatments are always too expensive."
• "People with cancer will be discriminated against."

Accurate knowledge, early detection and evidence-based treatment improve outcomes; supportive care and clear information help patients and families make informed choices.

Cancer treatment

Treatment approaches depend on cancer type, stage and patient health. Medical biotechnology and conventional treatments include:

• Surgery: physical removal of the tumour (using scalpel or laser) when feasible.
• Chemotherapy: use of chemical compounds that preferentially kill rapidly dividing cells.
• Radiotherapy: use of ionising radiation to damage and kill cancer cells; delivered externally or, in some cases, by internal sources.

In addition, there are traditional or complementary therapies. Some patients use herbal remedies or complementary therapies alongside medical treatment. Such combinations should always be discussed with clinical specialists to avoid harmful interactions and to ensure treatments do not reduce the effectiveness of standard care.

Prevention and early detection

Prevention strategies include reducing exposure to known carcinogens (for example avoiding tobacco smoke, limiting excessive sun exposure, reducing exposure to certain pollutants), vaccination where applicable (for example HPV vaccine), healthy diet and regular physical activity. Early detection by screening and prompt medical attention for suspicious signs improve the chances of successful treatment.

Summary

Mitosis is a fundamental process that maintains genetic stability while producing new cells for growth, repair and asexual reproduction. Proper control of the cell cycle and checkpoints prevents uncontrolled proliferation. When regulation fails, cells may divide uncontrollably, leading to tumours and cancer. Understanding mitosis, its regulation and the causes of cancer underpins prevention, early detection and treatment strategies.