Section 8.6: Gene Expression and Regulation Biology

Objectives Why do prokaryotes control transcription so carefully? How do specific sequence control transcription in prokaryotes? Describe the use of the lac operon to study transcription. What are different ways that eukaryotes control gene expression? Describe how the start of transcription requires specific patterns in eukaryotes. Describe mRNA processing.

Prokaryotes Control of gene expression allows for prokaryotes to respond to stimuli Allows them to conserve energy and materials Transcription and translation both occur simultaneously in the cytoplasm Therefore, the best way to control gene expression is to control transcription (start of protein making process)

Genes Genes include more than just the protein-coding sequence Often include sequences that control its expression Two major control sequences are: Promoters Operators

Promoters DNA segment that allows DNA to be transcribed Helps RNA polymerase find where a gene starts

Operators DNA segment that can turn gene “on” or “off” Can interact with proteins that increase the rate of transcription of stop transcription from occurring.

Bacterial Genes Bacteria have less DNA than other organisms Genes organized into operons Operon: region of DNA that includes a promoter, an operator, and the genes that code for the protein Found only in prokaryotes and round worms

The lac Operon One of the first examples of gene regulation in bacteria Consists three genes, all of which code for enzymes that breaks down lactose All under the control of a single promoter and operator

The lac Operon Turned off when lactose is absent Saves the cell’s resources When lactose is present, the lac operon is switched on

Gene Regulation In Eukaryotes Eukaryotes have different sets of genes, and these genes are only utilized in certain types of cells Many different ways to control gene expression at different points in time The start of transcription is often the most utilized way to control gene expression

Starting Transcription Many elements work together to control the start of transcription Types of regulatory DNA Sequences: Close to the gene Far away and are looped close Some are found in almost all eukaryotic cells Some genes control the transcription of other genes

Regulation of Transcription Regulatory Sequences: Sequences recognized by transcription factors that bind to the DNA, so that RNA polymerase can bind Far off regulatory sequences: If far way, often the DNA will loop so that the regulatory sequence is close Promoter: TATA box, that helps start transcription Enhancers and silencers can also speed up or slow down the rate of transcription

Regulation of Transcription Genes Controlling Genes Some genes code for proteins that control the expression of other genes Example: Sonic Hedgehog

mRNA Processing mRNA is generally edited prior to leaving the nucleus Cap: added to the beginning of mRNA helps bind to ribosome keeps it from getting broken down Tail: help mRNA exit the nucleus Editing of Code: Introns: nucleotide sequences that do not code for an amino acid Exons: nucleotide sequence that codes for an amino acid

Introns Regulate Gene Expression Protect DNA from mutations Sometimes different editing can give you a different protein Seen often in viruses