Gene Expression and Regulation

Gene Expression is the process by which information from a gene is used in the synthesis of a functional PROTEIN. An exon is any nucleotide sequence encoded by a gene that remains present within the final mature RNA product of that gene after introns have been removed by RNA splicing. The term exon refers to both the DNA sequence within a gene and to the corresponding sequence in RNA transcripts. In RNA splicing, introns are removed and exons are covalently joined to one another as part of generating the mature messenger RNA. An intron is any nucleotide sequence within a gene that is removed by RNA splicing while the final mature RNA product of a gene is being generated.[1][2] The term intron refers to both the DNA sequence within a gene and the corresponding sequence in RNA transcripts.[3] Sequences that are joined together in the final mature RNA after RNA splicing are exons. Introns are found in the genes of most organisms and many viruses, and can be located in a wide range of genes, including those that generate proteins, ribosomal RNA (rRNA), and transfer RNA (tRNA). When proteins are generated from intron-containing genes, RNA splicing takes place as part of the RNA processing pathway that follows transcription and precedes translation.

How does an organism know whether to turn a gene “on” or “off”? Gene Regulation Only a fraction of the genes in a cell are expressed at any given time Only 3-5% at any one time! If we look closely at the order of DNA bases we notice that certain DNA sequences serve different purposes but… How does an organism know whether to turn a gene “on” or “off”? For example: Promoters: binding sites for RNA Polymerase Start/Stop signals for transcription There are specific sequences for promoters, on a eukaryotic cell it is known as a TATA box We keep it from being transcribed and translated!! To explore this question we are going to look at the lac operon

How does an organism know whether to turn a gene “on” or “off”? E. coli provides us with the perfect example of how gene expression can be regulated E. Coli includes a cluster of 3 genes that are turned on or off together A group of genes that operate together is known as an operon When you drink a glass of milk E. coli is able to utilize lactose as a food source. In order for E. coli to use this lactose as a fod, the genes to break it down must be expressed and are known as the lac operon. People who are unable to express this protein are known as lactose intolerant.

How does an organism know whether to turn a gene “on” or “off”? Lactose is a compound made up of 2 sugars: galactose and glucose In order to use Lactose as food E. coli must do 2 things: Take lactose across the cell membrane Break the bond between glucose and galactose These 2 things are done by the proteins coded for by the genes of the lac operon You are probably asking yourself….why must e coli turn on the lac genes in order to use lactose for food?? Flip back to previous slide to see… If the bacterium is grown in a medium where lactose is there only food source, it MUST produce these proteins. If grown on another food source, such as glucose, it would have no need to express these proteins.

How does an organism know whether to turn a gene “on” or “off”? When lactose is not present, the repressor binds to the operator region, preventing RNA polymerase from beginning transcription Lactose causes the repressor to be released from the operator region When lactose is present sugar molecules bind to the repressor, moving it away

Gene Regulation: Terms to know! Promoter: Region of DNA that indicates to an enzyme where to bind to make RNA Operator: Region of chromosome to which repressors bind Repressor: binding protein that inhibits the expression of one or more genes by binding to the operator Inducer: molecule that starts gene expression Then apply the train track analogy to the lac operon model Follow this with the pGLO model stressing that it is the lac operon model in circular form