Your cells require lots of certain proteins in order to function properly. For example, your brain cells use proteins to transmit electrical signals, your muscle cells use proteins to contract, your stomach cells use proteins to secrete acid, and so much more.
Your genome contains the DNA instructions for these proteins in the form of genes. Almost all of your cells contain two complete copies of your genome and therefore two copies of each gene (one from your mother and one from your father). With only two copies of each gene, but the need to create large amounts of certain proteins, your cells need to make more copies of particular genes. This post discusses how cells make copies of genes to drive the differential production of proteins that ultimately leads to various cellular functions.
The genome as the library in a cellular city
Think of your cells as tiny cities that together make up the country that is your body. These cities have their own specialized roles that collectively help the country (body) function properly. To carry out its role, each city has its own specific mixture of workers. Some cities have more builders, others more farmers, others more tech entrepreneurs, and so on. These workers are like the proteins in your cells.
The libraries in these cities are like your genome. They contain all of the books required to train all of the types of workers that could possibly exist in a city. However, there are only two copies of each book. Because some cities require many more plumbers than dentists, they cannot simply loan out the two books and expect all of the their needs to be met. Instead the library creates copies of each book. The more copies of a particular book the library creates, the more of that type of worker the city can train. If the city needs more actors than philosophers, its library can simply make more copies of the acting books and fewer copies of the philosophy books. In fact, the library can stop creating some copies altogether and the city will have very few of that type of worker.
Transcribing genes into mRNA copies
Your cells work in a similar way. The instructions found in the two cellular copies of your genes are required in disparate amounts, in different places, and at different times. To account for this, your cells don’t create proteins by following the instructions in genes directly, instead they transcribe the genes into copies known as mRNA. Cells then use the mRNA copies to produce much more of the encoded proteins when they’re needed. Indeed, if your cells don’t need any of a certain type of protein, they can simply stop transcribing mRNA copies of the gene that encodes it.
Our various body parts take on their different roles through transcription. Brain cells transcribe specific genes to transmit electrical signals. Muscle cells transcribe specific genes to contract. Stomach cells transcribe specific genes to secrete acid. With transcription, we’re much more than blobs of cells all doing the same thing.