OCR General Certificate of Secondary Education (GCSE) Biology Practice Exam

Why are most genes not active in specialized cells?

1. Specialized cells lack DNA

2. Most genes are irrelevant for chosen functions

3. The majority have been switched off if not required

4. Specialized cells only use external signals

The correct answer is: The majority have been switched off if not required

Most genes are not active in specialized cells because the majority have been switched off if they are not required for the specific functions of that cell type. In multicellular organisms, cells differentiate into various specialized forms, such as muscle cells, nerve cells, or skin cells. During this differentiation process, specific genes are activated while others are silenced. This allows the specialized cells to carry out their unique roles efficiently without the unnecessary expression of irrelevant genes, which would otherwise consume resources and could lead to confusion in cellular functioning. For instance, a muscle cell will activate genes essential for contraction and energy metabolism while silencing genes that are important for functions like photosynthesis or immune response, which are not relevant to its role. The notion of genes being "switched off" does not mean they are permanently lost; rather, the ability to activate those genes can still be present, allowing flexibility if the cell's environment or needs change in the future. This selective gene expression is crucial for maintaining the right balance and functioning of tissues and organs in a complex organism.