Mitosis

Definition of Mitosis

• Mitosis is a cellular process where a single parent cell divides to produce two identical daughter cells.
  • Each daughter cell contains the same number of chromosomes as the parent cell.
  • Key Points:
  • DNA replicates during this process.
  • Resulting in 2 daughter cells post-division.

Overview of Mitosis

• At the end of mitosis, two cells are formed through a process involving the division of the cytoplasm known as cytokinesis.
• Cellular Components:
  • Cytoplasm and cell organelles are evenly distributed between the two new cells.

Functions of Mitosis

• In Multicellular Organisms:
  • Mitosis is crucial for growth and repair of tissues.
• In Unicellular Organisms:
  • Mitosis serves as a mechanism for asexual reproduction, as seen in organisms like Amoeba.

Location of Mitosis

• In Animals:

  • Mitosis can occur in nearly all body cells, facilitating constant production of new cells throughout the body.

• In Plants:

  • Specific growth areas known as meristems are where mitosis occurs:
  • Tips of shoots and roots are associated with active meristematic tissues.
  • Continuous removal of meristems can cause plants to age without significant growth, an example being bonsai trees.

Mitosis and Cancer

Tumour Formation

• Tumours can arise due to uncontrolled rapid mitosis.
  • This often results from faulty regulation of the normal cell division process and can cause an accumulation of cells, forming a tumour.

Genetic Factors in Cancer

• p53 Gene:

  • This gene typically regulates the cell cycle.
  • In approximately 50% of human cancer cases, this gene is found to be mutated.

• BRCA1 Gene:

  • Often referred to as the 'breast cancer' gene.
  • Mutations in BRCA1 can hinder its function in repairing broken PTEN genes, leading to cancer development.
  • Effects of BRCA1 Mutation:
  • If BRCA1 is normal, it repairs the PTEN gene.
  • If BRCA1 is mutated, PTEN cannot be repaired, resulting in:
    • Increased cell growth.
    • Inhibition of cell death.
    • Enhanced cell migration.
    • Formation of new blood vessels (angiogenesis) and metastasis.

Types of Tumours

• Benign Tumours:

  • Generally not life-threatening.
  • Cells do not invade other tissues (e.g., warts, skin tags).
  • Most breast tumors in women are typically benign.

• Malignant Tumours:

  • Referred to as cancer.
  • Can be life-threatening as cancerous cells can migrate to other tissues through a process called metastasis.
  • When a malignant tumour accumulates to about 1 million cells, it secretes proteins that stimulate blood vessel development (angiogenesis), facilitating nutrition and further growth.

Causes of Malignant Tumours

• Normal genes can convert into oncogenes (genes that promote cancer) following exposure to carcinogens, such as:
  • Cigarette smoke.
  • Ultraviolet (UV) radiation.
  • Certain viruses.

Characteristics of Cancer Cells

• Cancer cells demonstrate unique behaviors, including:
  • Indefinite cell division (e.g., HeLa cells).
  • Failure to respond to density inhibition mechanisms.
  • Lack of adherence to neighboring cells.
  • Often exhibit abnormal chromosome numbers or mutations.

Cancer Treatment Strategies

1. Chemotherapy and Cancer Drugs:
   • Designed to disrupt mitosis, often by halting DNA synthesis.
   • Benign Tumours:
     • May be surgically removed and area post-removal may be radiated to eliminate any remaining cancerous cells.
   • Malignant Diagnosis:
     • Presence of cancer cells in lymph nodes may indicate malignancy.
   • Chemotherapy Effects:
     • Targets rapidly dividing cells but may also affect healthy but fast-dividing cells like:
     • Cells in the gastrointestinal (GI) tract.
     • Hair follicles.
     • Bone marrow cells.

Phases of Mitosis

Interphase

• Represents the cell's non-dividing phase.
  • During this time, chromosomes exist as single strands and are preparing for division by making copies of themselves, effectively doubling the amount of DNA.
  • Newly formed organelles are also created in preparation for cell division.

Transition from Interphase to Prophase

• During interphase:
  • The nucleus replicates its DNA and centrosomes.
  • Chromatin begins to condense as it coils.

Prophase

• Key events in prophase include:
  • The chromatin condenses into visible individual chromosomes.
  • Breakdown of the nuclear membrane.
  • Formation of spindle fibres at opposite ends of the cell.
  • Each chromosome, at this stage, is comprised of two identical sister chromatids joined at a centromere.

Prometaphase

• Continuation of chromatin coiling leads to more compact chromosome structures.
• Nuclear envelope breaks down further allowing for interaction between kinetochore microtubules and polar microtubules, facilitating chromosome movement.

Metaphase

• Characteristics of metaphase:
  • Chromosomes align along the equatorial plate of the cell.
  • Spindle fibres attach to the centromeres of each chromosome ensuring accurate separation.

Anaphase

• Key actions during anaphase:
  • Spindle fibres contract and divide each centromere.
  • Resulting daughter chromosomes are pulled towards opposite poles of the cell, leading to a temporary increase in chromosomal count.
  • Anaphase is a quick phase, generally lasting only a few minutes.

Telophase

• Events during telophase involve:
  • Completion of chromosome migration to the cell poles.
  • Nuclear membranes and nucleoli reform around the sets of chromosomes as the cell prepares to enter interphase again.
  • Chromosomes begin to relax into chromatin.
  • The spindle fibres are reabsorbed into the cytoplasm.
  • In animal cells, cell membranes begin to pinch inward to form two distinct cells.