6.4 Issues of Cell Division on Human Health
Cell division is strictly regulated by control systems at specific checkpoints during the cell cycle. However, disruptions in normal cell division can cause severe health consequences. Uncontrolled mitotic cell division leads to the formation of tumours and cancer, while errors in chromosome separation during meiosis (nondisjunction) result in genetic disorders such as Down syndrome (trisomy 21).
Learning Objectives
- Describe the effects of uncontrolled cell division that lead to the formation of benign and malignant tumours (cancer).
- Identify factors that cause cancer and describe how cancer cells develop and spread.
- Explain the abnormality of meiosis known as nondisjunction.
- Explain how nondisjunction during anaphase I or anaphase II leads to Down syndrome (trisomy 21) and describe its characteristics.
Progression of Cancer Metastasis and Mechanism of Meiotic Nondisjunction
Cancer cells spread from a single site through vascular systems to secondary organs, while nondisjunction during meiosis produces gametes with an abnormal chromosome number.
Benign tumour
Not dangerous, localized, and can be removed surgically.
Malignant tumour
Cancer that invades, spreads, and destroys neighbouring healthy tissues.
Nondisjunction
Failure of chromosomes or sister chromatids to separate evenly during anaphase.
Short Explanation
Control of the cell cycle
The cell cycle is regulated by a special control system at each phase: \(\text{G}_1\), \(\text{S}\), \(\text{G}_2\), and \(\text{M}\) phase to ensure orderly cell division. Failure of the control system leads to uncontrolled cell division, forming an abnormal mass of cells called a tumour.
Benign tumour
Not dangerous, localized, and can be removed surgically.
Malignant tumour
Also known as cancer. It invades, spreads, and destroys neighbouring healthy tissues.
Factors causing cancer
Radiation such as X-rays, gamma rays and ultraviolet rays; chemical substances such as tar in tobacco; carcinogens such as formaldehyde and benzene; genetic factors; and biological agents such as certain bacteria and viruses.
Nondisjunction
During meiosis, spindle fibres may fail to function properly during anaphase I or anaphase II. This failure of chromosomes or sister chromatids to separate evenly is termed nondisjunction.
Try to Answer First
Answer in your mind, then press “Check Answer”.
What is the difference between a benign tumour and a malignant tumour?
What cellular defect causes nondisjunction during meiosis?
How does an individual acquire 47 chromosomes in Down syndrome?
Common Mistakes
- Confusing benign tumours with malignant tumours; only malignant tumours are classified as cancer.
- Stating that nondisjunction only occurs during anaphase I; it can happen during either anaphase I (homologous chromosomes fail to separate) or anaphase II (sister chromatids fail to separate).
- Stating the total chromosome number of Down syndrome as 46 or 48 instead of 47 (\(2n + 1\)).
- Assuming that all cancer causes are strictly genetic, ignoring external factors such as radiation, carcinogens (benzene, formaldehyde), tobacco tar, bacteria, and viruses.
Concept Misunderstandings
Nondisjunction directly produces an individual with Down syndrome without fertilisation.
Nondisjunction produces abnormal gametes (\(n = 22\) or \(n = 24\)). Down syndrome only develops when an abnormal gamete (\(n = 24\) with an extra chromosome 21) fuses with a normal gamete (\(n = 23\)) during fertilisation.
Cancer cells only stay at their primary site of formation.
Malignant cancer cells detach and travel through blood vessels and lymph vessels to form secondary tumours in other organs.
Summary
- Cell cycle regulation: Checkpoints occur at \(\text{G}_1\), \(\text{S}\), \(\text{G}_2\), and \(\text{M}\) phases. Failure leads to tumours.
- Tumours: Benign (non-cancerous, surgically removable) vs. Malignant (cancerous, invasive, spreads via blood and lymph vessels).
- Cancer factors: Radiation, tobacco tar, chemical carcinogens (benzene, formaldehyde), genetic factors, bacteria, and viruses.
- Nondisjunction: Spindle fibres fail to function in anaphase I or II, causing uneven chromosome distribution (\(n = 22\) or \(n = 24\)).
- Trisomy 21 (Down syndrome): Fusion of \(n = 24\) gamete with \(n = 23\) gamete results in \(2n + 1 = 47\) chromosomes; causes stunted growth, mental retardation, slanted eyes, and a slightly protruding tongue.
Short Activity
Objective: identify the causes and consequences of uncontrolled cell division and meiotic nondisjunction.
A. Objective Quiz
Which of the following causes an individual to develop Down syndrome?
What enables cancer cells from a primary tumour to form new tumours in other organs?
B. Fill in the Blanks
An individual with Down syndrome has a total of chromosomes due to three copies of chromosome 21.
The failure of homologous chromosomes or sister chromatids to separate during anaphase is called .
C. Matching / Drag and Drop
Match each condition or agent with its correct biological description based on the textbook. Drag each term to its correct description. On a phone, tap a choice first, then tap the matching box. Tap the Answer Choices area to return a selected item.
C. Match the Term to Its Description
Write the correct term next to each description.
| Description | Term / Factor |
|---|---|
| 1. A non-dangerous mass of cells that can be removed surgically | |
| 2. A mass of cancer cells that spreads and destroys surrounding normal cells | |
| 3. Cancer-causing chemical substances such as formaldehyde and benzene | |
| 4. The presence of three copies of chromosome 21 resulting in 47 chromosomes |
Answer choices: Benign tumour • Malignant tumour • Carcinogen • Trisomy 21
