You may hear DNA referred to as a “blueprint” for cell structure and physiology. This refers to the fact that DNA contains the information necessary for cells to build protein and RNA products.
Most structural components of the cell are made up, at least in part, of proteins, and virtually all the functions that a cell carries out are completed with the help of proteins. Proteins help catalyze catabolic and anabolic chemical reactions, provide methods of transport across the cell membrane, and help with cell signaling, among other things. Whatever the cellular process may be, it is almost sure to involve proteins.
Just as the cell’s genome describes its full complement of DNA, a cell’s proteome is the cell’s full complement of proteins. Protein synthesis, which you learned about in a prior lesson, begins with genes. A gene is a functional segment of DNA that provides the genetic information necessary to build a protein. Each particular gene provides the code necessary to construct a particular protein.
For a cell to function properly, necessary proteins must be synthesized at the proper time and place. All cells control or regulate the synthesis of proteins from information encoded in their DNA. The process of turning on a gene to produce RNA and protein is called gene expression. Whether in a simple unicellular organism or a complex multicellular organism, each cell controls when and how its genes are expressed. For this to occur, there must be internal chemical mechanisms that control when a gene is expressed to make RNA and protein, how much of the protein is made, and when it is time to stop making that protein because it is no longer needed.
The regulation of gene expression conserves energy and space. It would require a significant amount of energy for an organism to express every gene at all times, so it is more energy efficient to turn on the genes only when they are required. In addition, only expressing a subset of genes in each cell saves space because DNA must be unwound from its tightly coiled structure to transcribe and translate the DNA. Cells would have to be enormous if every protein were expressed in every cell all the time.
Being able to turn this activity on and off ultimately dictates the structure and function of a cell by determining which proteins are made, when they are made, and how many are made. However, the control of gene expression is extremely complex. Malfunctions in this process are detrimental to the cell and can lead to the development of many diseases, including cancer.
IN CONTEXT
Have you ever been to the reference section of a library? If not, this is an area that contains library materials (i.e., books, articles, etc.) that are not allowed to leave the library. Reference material is kept out of reach, but you can request an item by asking a librarian for it, filling out a sign-out sheet (to keep track of who has what material at all times), and then using the material while remaining in the library. In a large library, a reference area may contain rare books that require special permission to use. In a school library, this area may contain textbooks or class materials that are placed there by professors for students to use. In either case, these materials are not allowed to leave the library.
Let’s say that you have a research paper to write for a class and your research requires you to use materials in the reference library. If you can’t take them out of the library, how can you complete the paper?
While reference materials are not allowed to leave the library, there is no rule against making copies of them. You can scan or take photos of specific materials which are allowed to leave the library and use those in any way and anywhere you wish to create your final product—your research paper.
Gene expression works along a similar process. DNA is like a reference book—it is not allowed to leave the nucleus. Proteins, gene expression’s final product, are synthesized by ribosomes located in the cytoplasm of the cell. Therefore, in order to use the genetic code found in DNA to make proteins, a copy of that code must be made that can leave the nucleus. This copy is made of RNA.
The Genetic Code—DNA holds all of the genetic information necessary to build a cell’s proteins. The nucleotide sequence of a gene is ultimately translated into an amino acid sequence of the gene’s corresponding protein.
terms to know
Proteome
The cell’s full complement of proteins.
Gene
A functional segment of DNA that provides the genetic information necessary to build a product.
Gene Expression
Processes that control the turning on or turning off of a gene.
1a. Evolution of Gene Regulation
Prokaryotic cells can only regulate gene expression by controlling the amount of transcription. As eukaryotic cells evolved, the complexity of the control of gene expression increased. For example, with the evolution of eukaryotic cells came compartmentalization of important cellular components and cellular processes. A nuclear region that contains the DNA was formed. Transcription and translation were physically separated into two different cellular compartments. It therefore became possible to control gene expression by regulating transcription in the nucleus, and also by controlling the RNA levels and protein translation present outside the nucleus.
Most gene regulation is done to conserve cell resources. However, other regulatory processes may be defensive. Cellular processes such as gene silencing developed to protect the cell from viral or parasitic infections. If the cell could quickly shut off gene expression for a short period of time, it would be able to survive an infection when other organisms could not. Therefore, the organism evolved a new process that helped it survive, and it was able to pass this new development to offspring.
2. Introns and Exons
At this point, you may be wondering, "If there's DNA that contains my genetic code, what else does it contain?" Quite a lot, actually. Indeed, about 98% of your DNA doesn't code for protein directly; rather, it performs subtler functions. For example, within a gene, you will have stretches of sequence that get translated into protein, called exons (coding sequences). Between them, you will have introns (non-coding sequences); instead of getting translated into protein, introns are nucleotide sequences that recruit regulatory proteins. Regulatory proteins modulate the timing and amount of a gene's expression into protein. They can even alter the protein into different versions that perform similar but subtly different functions.
Exons are coding sequences that get translated into protein, whereas introns are non-coding sequences that recruit regulatory proteins.
terms to know
Exons
Sequences present in protein-coding mRNA after completion of pre-mRNA splicing.
Introns
Non-protein-coding intervening sequences that are spliced from mRNA during processing.
Regulatory Proteins
Proteins that can stop or speed up transcription.
3. Cell Differentiation
When a cell differentiates (becomes more specialized), it may undertake major changes in its size, shape, metabolic activity, and overall function.
think about it
Because all cells in the body, beginning with the fertilized egg, contain the same DNA, how do the different cell types come to be so different?
The answer is analogous to a movie script. The different actors in a movie all read from the same script; however, they are each only reading their own part of the script. Similarly, all cells contain the same full complement of DNA, but each type of cell only “reads” the portions of DNA that are relevant to its own function. In biology, this is referred to as the unique genetic expression of each cell.
For a cell to differentiate into its specialized form and function, it need only manipulate those genes (and thus those proteins) that will be expressed and not those that will remain silent. The primary mechanism works by having proteins that bind to genes in the DNA that inhibit or activate gene expression, thereby turning gene expression “on” or “off.”
Furthermore, based on the genes that are actively used by a cell, the amount of each organelle it contains will change. For example, a cell that functions to secrete large amounts of proteins will require greater numbers of ribosomes and a larger rough endoplasmic reticulum and Golgi apparatus. A cell that requires a high amount of active membrane transport will require much more ATP and mitochondria to produce it. Each cell’s organelle content will differ based on its specific gene expression and differentiated function.
term to know
Differentiation
A process by which a cell becomes specialized.
summary
In this lesson, you learned about how protein production is regulated by gene expression. First, you learned to identify the components and processes involved in the cell performing gene expression and about the evolution of gene regulation. Then, you explored how the stretches of sequences called exons and introns facilitate protein synthesis by exons being translated into protein and introns helping regulate gene expression by recruiting regulatory proteins. Finally, you learned about how control of gene expression allows for cell differentiation.
