Cancer in multicellular organisms is primarily caused by which abnormal cellular process?

Introduction / Context:
Cancer is one of the most serious diseases affecting humans and other animals. At the cellular level, it is closely related to how cells divide and how their growth is controlled. Understanding the basic cause of cancer in terms of cell division is a key learning outcome in school biology and general science GK. This question asks which abnormal cellular process primarily leads to cancer.

Given Data / Assumptions:

• Cancer involves abnormal growth of cells forming tumours and possibly spreading to other parts of the body.
• The options mention loss of immunity, uncontrolled meiosis, uncontrolled mitosis, and rupturing of cells.
• We assume basic knowledge of the difference between mitosis and meiosis.
• We also assume that the question focuses on the fundamental cell division abnormality in cancer.

Concept / Approach:
In normal tissues, cell division by mitosis is carefully controlled so that old cells are replaced and injured tissues are repaired. In cancer, this control is lost, and cells undergo mitosis repeatedly and unchecked, forming masses called tumours. Meiosis is the special type of cell division that produces gametes and does not directly cause cancer when functioning normally. While overall immunity and cell death processes can influence cancer risk and progression, the primary cellular hallmark of cancer is uncontrolled mitotic division. Therefore, the correct answer is uncontrolled mitosis leading to excessive cell division.

Step-by-Step Solution:
Step 1: Recall that mitosis is the type of cell division used for growth, repair, and replacement of somatic cells. Step 2: In healthy tissues, mitosis is regulated by signals that tell cells when to divide and when to stop. Step 3: In cancer, mutations in genes that control the cell cycle cause cells to keep dividing even when they should stop. Step 4: This leads to uncontrolled mitosis, producing large numbers of cells that can form tumours and invade other tissues. Step 5: Compare this description with the options and identify uncontrolled mitosis leading to excessive cell division as the best match.

Verification / Alternative check:
Biology textbooks describe cancer as a disease of the cell cycle in which normal regulatory mechanisms fail, resulting in uncontrolled cell division. Diagrams often show normal cells dividing and then stopping, versus cancer cells dividing repeatedly to form a tumour. These explanations consistently refer to mitosis, not meiosis. Although impaired immunity can allow cancer cells to escape detection, it is not the primary cause of their uncontrolled growth. This confirms that uncontrolled mitotic division is the main cellular feature of cancer.

Why Other Options Are Wrong:
Loss of immunity in body cells is not the primary cause; it may affect the body's ability to fight cancer but does not itself create cancer cells. Uncontrolled meiosis during gamete formation is incorrect because meiosis is largely restricted to reproductive cells and its malfunction leads to genetic disorders in offspring rather than typical cancers. Simple rupturing and death of cells is wrong; while cell death occurs in many conditions, cancer is defined by excessive survival and division of abnormal cells, not by increased rupturing.

Common Pitfalls:
Students sometimes confuse any abnormality in cell division with cancer or think that cancer is mainly an immune system disease. Another mistake is not remembering which type of division, mitosis or meiosis, is involved in body growth. To avoid these errors, link mitosis to growth and repair, and meiosis to gamete formation. Then remember that cancer is essentially uncontrolled growth due to unregulated mitosis. This mental connection makes it straightforward to select uncontrolled mitosis as the correct answer.

Final Answer:
The correct choice is Uncontrolled mitosis leading to excessive cell division, because cancer arises when normal control over mitotic cell division is lost.