What is gene expression?
As biochemists unraveled the mysteries of how biology functions, they often coined processes with poetic prose: Gene expression.
A more pragmatic description could be: How cells read DNA to create RNA or proteins. Gene expression is the creation of an RNA molecule and, in most cases, the subsequent creation of a protein from the RNA. Rather than speaking in the singular, there are usually hundreds, thousands, to millions of RNA molecules and proteins created during gene expression.
Chapters 4 and 5 of Zero to Genetic Engineering Hero Book describe what gene expression is in depth. In short, it is when the cell machinery (RNA polymerase) is able to bind to a particular piece of DNA and transcribe a segment of DNA into RNA. As you’ve probably learned when reading about the central dogma, most RNA can be bound by the ribosome and then translate a segment of RNA into a protein. Gene expression is the creation of an important functional end-product from DNA, which can be RNA and protein.
Creating functional RNA: People often think gene expression is only to create proteins. However, a lot of RNA have important functions and are not used by the cell to create any protein. For example, suppose the intention of the genetic system is to create RNA. In that case, gene expression is when RNA polymerase binds to a promoter and translates a coding sequence into a length of RNA. The RNA can then fold, complete the Four B’s (You can learn about this in the Zero to Genetic Engineering Hero book), and ultimately complete its important function. Examples of functional RNA include tRNAs and rRNAs (Chapter 5 of Zero to Genetic Engineering Hero) and other RNAs used in the now famous CRISPR-Cas9 system, all of which are evolutionarily part of cellular immune activities. RNAs are essential molecules that have their own roles beyond protein expression.
Creating proteins: When an RNA polymerase binds a promoter and transcribes a segment of DNA to RNA, the RNA is called messenger RNA (mRNA) if the intention is to create protein. mRNA contains all the information to bind to the ribosome. The ribosome then translates a segment of the RNA into a string of amino acids called a protein. (Chapter 5 of Zero to Genetic Engineering Hero book).
Proteins have many different functions. For example, proteins are essential for creating cellular structures (like actin and microtubules), enzymes (like the thousands of enzymes that catalyze chemical reactions), ATPase ion pumps, and many more. You can learn about enzymes in Chapter 6 of Zero to Genetic Engineering Hero book.
Did you know that the Engineer-it Kit enables you to engineer bacteria with a DNA plasmid containing a color-producing gene? Once transformed in bacteria, the gene is expressed and results in coloful bacteria on a petri dish! How fun!