Mitosis is absolutely essential for tissue repair because it provides the mechanism for generating the large numbers of cells required to replace damaged or lost tissue. When tissue is injured, the signals released from the damaged area activate stem cells in the vicinity. These signals can be growth factors, cytokines, or direct cell-to-cell contact.

Cell signalling plays a vital role in directing stem cell behaviour. Growth factors bind to receptors on the stem cell surface, triggering a cascade of intracellular events that ultimately lead to cell cycle progression and division (mitosis). These signals also influence the differentiation pathway of the stem cell, ensuring that the newly formed cells are of the correct type to repair the damage. For example, injury to a muscle will release signals that activate muscle stem cells (satellite cells), prompting them to proliferate and differentiate into new muscle fibres.

Errors in mitosis can have serious consequences. If the cell division process is not properly regulated, it can lead to:

• Aneuploidy: An incorrect number of chromosomes in the daughter cells. This can result in non-viable cells or cells with abnormal function.
• Cell death: If the DNA damage is too severe, the cell may trigger programmed cell death (apoptosis) to prevent the propagation of the damage.
• Cancer: Uncontrolled cell division due to mutations in genes that regulate the cell cycle can lead to the formation of tumours. Cancer cells often have defects in DNA repair mechanisms, further increasing the risk of mutations.

Therefore, the accuracy and regulation of mitosis are critical for successful tissue repair. Mechanisms such as checkpoints in the cell cycle ensure that DNA damage is repaired before the cell divides, and that the daughter cells receive the correct number of chromosomes.

Telomeres are repetitive DNA sequences located at the ends of chromosomes. Their primary role is to protect the chromosome from degradation and prevent the loss of genetic information during DNA replication. During each round of DNA replication, the lagging strand cannot be fully synthesized, resulting in a gradual shortening of the chromosome ends. This is known as the 'end replication problem'.

Without telomeres, this shortening would lead to critical genes being lost with each cell division, ultimately triggering cellular senescence or apoptosis. Telomeres act as a protective cap, preventing the DNA replication machinery from treating the chromosome ends as DNA damage and triggering DNA repair mechanisms that could lead to chromosomal instability.

Telomerase is a ribonucleoprotein enzyme that counteracts telomere shortening. It contains an RNA template that is complementary to the telomere repeat sequence. Telomerase extends the 3' overhang of the telomere, providing a template for the lagging strand to complete synthesis and thus maintaining telomere length. Telomerase is highly active in germ cells and stem cells, but is typically inactive in somatic cells. Its reactivation in somatic cells is often associated with cancer cell proliferation.

In summary, telomeres prevent chromosome degradation and gene loss by acting as a protective buffer, and telomerase maintains telomere length by adding telomeric repeats.

Mitosis is fundamental to the growth of multicellular organisms. Growth requires an increase in the number of cells. Mitosis provides a mechanism for this increase by producing two genetically identical daughter cells from a single parent cell. This process allows organisms to increase in size, developing from a single fertilized egg into a complex organism with specialized tissues and organs.

Specifically, mitosis enables:

• Cell proliferation: The rapid and controlled duplication of cells is essential for increasing the overall cell number.
• Tissue development: Mitosis drives the formation of new cells within developing tissues, contributing to the expansion and differentiation of these tissues. For example, during embryonic development, mitosis is responsible for the rapid proliferation of cells to form various organs.
• Organogenesis: The formation of organs relies heavily on mitosis to generate the necessary cell mass and structure. The coordinated mitotic activity of cells within specific regions leads to the development of complex organ systems.

Without mitosis, multicellular organisms would not be able to grow beyond a single cell stage.