Imagine you have been given a string approximately 3 feet long, which represents an unwound, deproteinated, human chromosome. In actuality, such naked DNA does not exist in the cell nucleus. Rather, it is wound around a series of positively charged histone proteins; in the electron microscope it resembles beads on a string. This structure is wound into a cylindrical "solenoid" structure which is further packaged to fit into the nucleus along with the rest of the chromosomes. There just happens to be a small dot in the dead center of the string you have been given. It represents the gene for a particular protein called metallothionein. This gene is expressed and the protein metallothionein is made when cells are exposed to heavy metals like Cd. Your job today is to figure out a possible way (there are many) in which the cell would not express the gene, or express it to a small, constituitive level, in the absence of Cd exposure, and how the gene might be activated - through transcription of the gene and translation of the resulting mRNA to form the metallothionein protein upon exposure to Cd. (Hint: Cd doe not DIRECTLY bind to DNA.) Propose mechanisms to address the following questions. Do not propose any magical mechanisms:
- How might gene expression be repressed in the absence of Cd. List some possibilities.
- How might gene expression be activated in the presence of Cd.
The links below should be reviewed by those who have little background on the Central Dogma of Biology and of the nature of a gene.
- Overview of Central Dogma of Biology for the non-biochemistry major
- A view of genes and their products: Simplicity to Complexity
- Jmol: Simple DNA Tutorial
Proteins can then be post-translationally modified, localized to certain sites within the cells, and ultimately degraded. If functional proteins are considered the end-product of gene expression, the control of gene expression could theoretically occur at any of these steps in the process.
Figure: PROCESSES THAT AFFECT THE STEADY STATE CONCENTRATION OF A PROTEIN
Mostly, however, gene expression is controlled at the level of transcription. This makes great biological sense, since it would be less energetically wasteful to induce or inhibit the ultimate expression of a functional protein at a step early in the process. How can gene expression be regulated at the transcriptional level? Many examples have been documented. The main control is typically exerted at the level of RNA polymerase binding just upstream (5') of a site for transcriptional initiation. Other factors, called transcription factors (which are usually proteins), bind to the same region and promote the binding of RNA polymerase at its binding site, called the promoter. Proteins can also bind to sites on DNA (operator in prokaryotes) and inhibit the assembly of the transcription complex and hence transcription. Regulation of gene transcription then becomes a matter of binding the appropriate transcription factors and RNA polymerase to the appropriate region at the start site for gene transcription. Regulation of gene expression by proteins can be either positive or negative. Regulation in prokaryotes is usually negative while it is positive in eukaryotes.
Figure: Positive and Negative Regulation of Gene Transcription