Gene Regulation Handout
Nearly all of the cells in an organism contain the same DNA, but these cells perform a wide variety of different functions in the organism. This variation allows for the specialized functions of different cell types, tissues and organs.
To achieve this variation, different cells express, or produce, different proteins. Gene expressionis the production of a given protein based on the DNA nucleotide sequence of its gene. Gene regulationis how gene expression is controlled, or regulated. Gene regulation allows for differences in gene expression. These differences lead to two results: cell differentiation and homeostasis. Celldifferentiation the process by whichcells adopt a specialized, and permanent, function in an organism resulting from a specific pattern of gene expression. Homeostasis is the process by which organisms, and cells, respond to a changing environment in order to maintain a stable state.
Control of Gene Expression
Cells have many ways to control how proteins are expressed, or not expressed, in the cell. The most common way that cells regulate gene expression is by controlling the transcription of a given gene. For example, cells can activate genes to produce large amounts of RNA transcript and make lots of the associated protein. Or, cells can repress genes to produce no RNA transcript and no associated protein. This transcriptional control of gene expression is controlled by combinations of DNA sequences and regulatory proteins.
The DNA sequence of a gene includes two parts: the coding sequence that has the specific information to build a protein and the regulatory sequence that helps control whether the gene is expressed or not. The regulatory sequences is a binding sites for a regulatory protein. Regulatory proteins have specific shapes that allow them to form reversible chemical bonds with specific DNA sequences to regulate gene expression. Each regulatory protein has a particular DNA sequence it is able to bind to. Regulatory proteins that increase gene expression are known as activators. Regulatory proteins that prevent gene expression are known as silencers. The expression of a single gene can be regulated bymany different regulatory proteins because there may be many different specific regulatory sequences in the regulatory region of a gene. In cells, the results is that genes include nucleotide sequences that provide the instructions to build a particular protein as well as nucleotide sequences to control if those proteins are expressed by the cell.
The ability to control, or regulate, gene expression this way allows cells, and whole organisms, to maintain stable internal (inside) conditions when the external (outside) conditions are changing. This is known as homeostasis. When cells detect a signal that the environment is changing, this signal can bind to a regulatory protein. This regulatory protein is now able to bind to its associated regulatory DNA sequence to either activate (start) or repress (stop) gene expression. By controlling the production of proteins in this way, cells are able to produce certain proteins only when they are needed. Cells are able to save resources until they need to adjust to changing conditions.
The combination of regulatory DNA sequences and regulatory proteins in cells is how organisms are able to form distinct cell types even though each cell contains identical DNA. In general, different types of cells express different genes because they have different regulatory proteins. For example, a muscle cell contains regulatory proteins that bind to regulatory DNA sequences to activate the expressionof muscle-specific proteins and repress the expression of proteins thatare not needed in a muscle cell. It is the overall combination of regulatory proteinsthat allows for the variation in the structure and function of cells of an individual.
Questions:
- What are the two parts of the DNA sequence for a gene? Explain each and why both parts are required.
- Define homeostasis and provide an example.
- Explain who it is important for organisms to regulate gene expression.