BIOLOGY LEC 15

GSA: Many of you will recall studying lac operon regulation in previous biology courses, in this course, we will focus on the trp operon instead. The background material is in sections 13.1a and 13.1d. Feel free to read the lac operon material as general background to operons but our main outcomes will relate only to trp. As you study the structure of the trp operon, think like a geneticist and ask yourself what the effect might be of various mutations at various locations. This Cycle only looks at bacterial systems so don't go beyond the section 13.1d in this chapter. We will be taking the trp regulation "one step beyond" the textbook during Class 15. This will be a very engaging and active class that will be best consumed live. structure of trp operon - Some regulated genes occur in clusters called operons o Some regulated genes in prokaryotic genome occur singly, meaning gene is transcribed by RNA polymerase to produce a single polypeptide o Transcription unit – a region of DNA that transcribes a single primary transcript. Segment of DNA from the initiation point of transcription to the termination point of transcription. The transcription unit for a protein-coding gene that occurs singly in a genome corresponds to the mRNA-coding sequence of that gene. o Other regulated genes occur in clusters. Each cluster constitutes one transcription unit, meaning the set of genes in the cluster is transcribed into a single mRNA molecule. Translation of the mRNA produces polypeptides corresponding to each of the genes in the transcription unit. o Organization of genes in cluster provides efficient coordinated regulation of those genes called the operon model, for the control gene expression o Model probably applies for the control of the expression of genes for lactose metabolism in E. Coli. o Operon – a cluster of prokaryotic genes and the DNA sequences involved in their regulation o Operator – a DNA regulatory sequence that controls transcription of an operon. A short segment that is a binding sequence for a regulatory protein (DNA-binding protein that binds to a regulatory sequence and affects the expression of an associated gene or genes). A gene that is separate from the operon encodes the regulatory protein. o Repressor – a regulatory protein that prevents the operon genes from being expressed. Some operons controlled by a regulatory protein termed repressor, which when bound to the DNA, reduces likelihood that genes will be transcribed. o Activator - A regulatory protein that controls the expression of one or more genes. Other operons are controlled by a regulatory protein termed activator, which, when bound to DNA, increases likelihood that genes will be transcribed. o Many operons are controlled by more than one regulatory mechanism, and several of the repressors or activators control more than one operon. o Each operon, which can contain several to many genes, is transcribed as a unit from the promoter into a single messenger RNA (mRNA), and, as a result, the mRNA contains codes for several proteins. o Transcription unit - A region of DNA that transcribes a single primary transcript. The cluster of genes transcribed into a single mRNA. o A ribosome translates the entire mRNA from one end to the other, sequentially making each protein that is encoded in the mRNA. Typically, the proteins encoded by genes in the same operon catalyze steps in the same process, such as enzymes acting in sequence in a biochemical pathway. ..............................................CONTINUED.....................................................