Gene Expression Cells use information in genes to build hundreds of different proteins, each with a specific function. But, not all proteins are required by the cell at any one time. By regulating gene expression, cells are able to control which proteins are made.

Role of Gene Expression Gene expression is the activation of a gene that results in the formation of a protein. A gene is said to be expressed when transcription occurs

Role of Gene Expression Some proteins may play a structural role, while other are enzymes that catalyze chemical reactions Cells do not always need all of the proteins present in the genome

Role of Gene Expression Gene expression occurs in two steps: transcription and translation Gene expression begins when RNA polymerase transcribes DNA into mRNA. This sets off a chain of events that results in the production of a specific protein.

Gene Expression in Prokaryotes Scientists first studied what controlled gene expression by looking at prokaryotes. The DNA of a prokaryote exists as a single circular chromosome of DNA Two scientists (Francios Jacob and Jaques Monod studied the bacteria Escherichia coli and it’s ability to metabolize the sugar lactose

Gene Expression in Prokaryotes E. coli is found in the intestines of humans Normally E. coli uses glucose as a food source However, when you drink cow’s milk, the sugar present is a two part sugar, or disaccharide, composed of glucose and galactose E. coli must alter it’s proteins in order to break down this new sugar.

Gene Expression in Prokaryotes The metabolism of lactose is controlled by three particular enzymes. The genes for these three enzymes are adjacent to each other on the E. coli chromosome.

Gene Expression in Prokaryotes The production of these enzymes is controlled by three “regulatory elements” found on the DNA of E. coli. These elements are:

Structural genes--- this is the actual code for each of the three enzymes Promoter----In front of the genes that code for our enzymes is a region of DNA that does not code for a protein, but instead acts as a binding site for RNA polymerase. (recall: what does RNA polymerase do?) Operator--just next to the promoter is a second region of DNA that does not code for a protein but is also a binding site. At this site a protein that blocks transcription can bind.

Gene Expression in Prokaryotes Together, all of these elements make up a unit called an operon. This clustered pattern of related genes and their regulators is a common pattern in bacterial DNA. This particular operon is called the lac operon because the structural genes involved regulate lactose metabolism

Gene Expression in Prokaryotes In their work with E. coli, Jacob and Monod found that the genes for lactose metabolic proteins were only expressed when lactose was present. How was that possible? Because the presence of lactose induced the genes to be transcribed. Here is how it works.

Gene Expression in Prokaryotes Repression: in the absence of lactose a protein called a repressor protein attaches to one of the binding sites in front of the structural genes…the operator. The repressor protein physically blocks RNA polymerase from being able to transcribe the genes. The action of a repressor protein blocking transcription is called repression.

Gene Expression in Prokaryotes Activation: when lactose is present in the E. coli cell it binds to the repressor protein. When this binding occurs, it changes the physical shape of the repressor protein and renders it unable to bind to the DNA. Now RNA polymerase can bind at the promoter region of the gene, and transcription can occur.

Gene Expression in Prokaryotes An inducer is a molecule that initiates gene expression. In this example lactose acts as an inducer The initiation of transcription by removing a repressor protein is called activation The advantage to this type of system is that the cell doesn’t waste energy producing proteins it doesn’t need.

Gene Expression in Eukaryotes Question: Kwashiorkor is a disease in children caused by a diet high in carbohydrates but lacking in complete protein. When children with kwashiorkor are suddenly put on a diet rich in protein they may become very ill with ammonia poisoning, and some even die. The high level of ammonia in their blood is due to the inadequate metabolism of protein. What does this tell you about the enzymes that metabolize protein?

Gene Expression in Eukaryotes Eukaryotes are vastly different than prokaryotes Genomes are much larger Several linear chromosomes instead of single circular one. Multiple, specialized cells, that only produce a subset of proteins instead of single cells Therefore gene expression is much more complicated in eukaryotes than prokaryotes!

Gene Expression in Eukaryotes Much of the control of gene expression in eukaryotes occurs at the level of the chromosome. Recall that during cell division the DNA coils into chromosomes, but after cell division it relaxes into chromatin. In some cells, gene regulation is accomplished when certain regions of the DNA remain tightly coiled to prevent transcription of genes.

Gene Expression in Eukaryotes Another form of gene regulation has to do with the way that eukaryotic genes are organized. Remember that genes are preceeded by a short segment of DNA called the promoter region, where RNA polymerase can bind. After the promoter, the structural genes of eukaryotes are made up of introns and exons.

Gene Expression in Eukaryotes Introns are segments of the gene that do not code for amino acids. Exons do code for amino acids. We are not sure why eukaryotic genes are organized in this fashion, but think it might have something to do with crossing over

Gene Expression in Eukaryotes When a gene is transcribed, both the introns and exons are transcribed into mRNA. This mRNA that contains both introns and exons is called pre-mRNA. True RNA is formed when the introns are removed by and the exons are spliced beck together by enzymes. The nucleotides from the introns are then broken down and recycled into new RNA molecules

Gene Expression in Eukaryotes

Gene Expression in Eukaryotes So, what does this have to do with which genes are expressed? Well, the splicing of these genes determines whether a protein is translated. Sometimes, a single gene can be spliced in alternative ways and result in totally different proteins

Gene Expression in Eukaryotes Enzymes drive this splicing process How these enzymes are triggered and controlled is still being figured out.

Gene Expression in Eukaryotes Eukaryotic genes also often have regions that are non-coding that enhance the transcription of genes. These regions are called enhancer regions. Much like the promoter region of prokaryotic genes, these regions determine whether RNA polymerase will transcribe a gene. Again, how these enhancer regions work is still being understood by scientists.