Cells and control Revision Notes

Normal Cell Division vs. Cancerous Cell Division

Normal cells divide in a controlled manner, following a strict cell cycle with checkpoints that ensure proper growth and function. Cancerous cells, however, lose these controls due to genetic changes (mutations). They divide continuously and rapidly, forming abnormal masses of cells called tumours. This unregulated growth is a hallmark of cancer.

Genetic Changes Leading to Cancer

Cancer is fundamentally a genetic disease, meaning it's caused by changes to genes that regulate cell growth and division. These mutations can be inherited or acquired during a person's lifetime due to factors like radiation, certain chemicals (carcinogens), or viruses. These genetic alterations disrupt the normal cell signalling pathways, leading to uncontrolled proliferation and the formation of tumours.

Consequences of Uncontrolled Cell Division (Tumours)

The mass of cells formed by uncontrolled division is called a tumour. Tumours can be benign (non-cancerous) or malignant (cancerous). While benign tumours do not spread, malignant tumours can invade surrounding tissues and spread to other parts of the body through the bloodstream or lymphatic system, a process known as metastasis. This spread makes cancer particularly dangerous and difficult to treat.

The Cell Cycle: Interphase and Mitotic Phase

The cell cycle consists of interphase and the mitotic (M) phase. Interphase is the longest phase, during which the cell grows, carries out its normal functions, and duplicates its DNA and organelles in preparation for division. The mitotic phase includes mitosis (nuclear division) and cytokinesis (cytoplasmic division), leading to two identical daughter cells.

Stages of Mitosis: Prophase, Metaphase, Anaphase, Telophase

Mitosis itself is divided into four main stages:

1. Prophase: Chromosomes condense and become visible, the nuclear envelope begins to break down, and the spindle fibres start to form.
2. Metaphase: Chromosomes align at the cell's equator (metaphase plate).
3. Anaphase: Sister chromatids separate and move to opposite poles of the cell.
4. Telophase: Chromosomes decondense at opposite poles, new nuclear envelopes form around the two sets of chromosomes, and the spindle fibres disappear.

Cytokinesis: Cytoplasmic Division

Cytokinesis often overlaps with telophase and involves the physical division of the cytoplasm and its contents. In animal cells, a cleavage furrow forms, pinching the cell into two. In plant cells, a cell plate forms in the middle, developing into a new cell wall that separates the daughter cells. This process completes cell division, resulting in two separate, genetically identical daughter cells.

What are Percentile Charts?

Percentile charts are graphs that show the distribution of physical measurements (like height, weight, head circumference) for a specific population group, typically divided by age and gender. They illustrate how a child's measurements compare to those of other children their age, using curves that represent different percentiles (e.g., 2nd, 9th, 25th, 50th, 75th, 91st, 98th). These charts are crucial for monitoring healthy development.

How to Use Percentile Charts for Growth Monitoring

To use a percentile chart, plot an individual's measurement (e.g., weight) against their age at regular intervals. Connect these plotted points to form a growth curve. The position of this curve relative to the percentile lines indicates the individual's growth pattern. Sustained growth along a particular percentile curve is usually a sign of healthy development. Significant deviations, such as crossing multiple percentile lines rapidly, may indicate an underlying health issue requiring medical attention. For example, a child consistently growing below the 3rd percentile or above the 97th percentile might need further assessment.

Interpreting Percentile Lines

Each line on a percentile chart represents a specific percentile. For instance, the 50th percentile line indicates the average measurement for that age and gender. If a child's measurement falls on the 75th percentile, it means 75% of children in that group have a smaller measurement, and 25% have a larger measurement. The overall trend of a child's growth, rather than a single point, is most important for assessment. A healthy child might be on any percentile line, as long as they follow their own curve consistently.