What are Genes?

As we learned earlier, development begins with one cell which undergoes many changes and divisions to eventually give an organism with a large number of different cells. How is this large, diverse population of cells obtained? How can these different cells work together to form a functional organism? The answer lies in the DNA of the cells, specifically the genes. But, what are genes? This can be a very difficult term to describe. In the simplest terms, genes can be thought of as specific instructions which are contained in the DNA. The easiest way to attempt to describe genes is to think about them in the context of cells. I often like to use analogies of things we are all familiar with to describe what genes are and why they are important to the cell.

The analogy I like to use is to think of the cell as a factory. Try to imagine a factory which has only one special machine that could make any product imaginable. Even if the products are as diverse as a truck, a door, electrical cables, whatever, they can all be made by this one special machine. The only catch is that in order for this factory to make any of these products, this special machine has to be given the proper instructions. These instructions, or blueprints, contain all the information necessary for the machine to make a desired product. For example, think of a truck. For the machine to make a truck, it must be able to produce tires, a steel frame, an engine, a gas tank, and many other parts. In order to produce these parts, the machine must be given the proper instructions for each. Once all the parts are produced, the factory can put them together to make a truck. The final product of the factory can vary, depending on what instructions are given to the machine. If the machine were given instructions to make a handle, hinges, a frame, and some glass, the factory might then produce a door. One of the most important concepts to understand is that all factories contain the same instructions or information. While this means all factories have the ability to make the same products, they tend to only produce a certain set of products. As a factory becomes efficient at reading a specific set of instructions and producing that product, it will continue to do the same job. This means that different factories will develop according to the product(s) they produce. In this way, the factory becomes specialized for producing certain products.

Let's extend this concept to the cell. If the cell is simply thought of as a small factory, then the genes are the instructions which are necessary for the cell (factory) to make its product (a protein). Put this idea into development. A single cell divides to produce many diverse types of cells (think of all the different types of tissues in the human body - skin, liver, muscle, nerve, etc.). All of these cells contain the exact same genes. If they all contain the same genes, then what makes them different? Why is the red blood cell different from a nerve cell different from a skin cell? The answer is that each cell has become specialized for certain functions (producing certain products) and only a specific subset of their genes (instructions) are being used. Remember: all cells in an organism contain the exact same genes, but only a certain subset of these genes are being used in a particular cell; the rest lie dormant. This is what gives a cell its unique qualities.