Transcript Chapter 15
Control over Genes Chapter 15 Skin Cancer Basal Cell Carcinoma Squamous Cell Carcinoma Malignant Melanoma Changes in DNA Trigger Cancer Ultraviolet radiation can cause breaks In-text figure Page 238 Gene Control Which genes are expressed in a cell depends upon: • Type of cell • Internal chemical conditions • External signals • Built-in control systems Mechanisms of Gene Control Controls related to transcription Transcript-processing controls Controls over translation Post-translation controls Regulatory Proteins Can exert control over gene expression through interactions with: –DNA –RNA –New polypeptide chains –Final proteins Control Mechanisms Negative control –Regulatory proteins slow down or curtail gene activity Positive control –Regulatory proteins promote or enhance gene activities Chemical Modifications Methylation of DNA can inactivate genes Acetylation of histones allows DNA unpacking and transcription Figure 15.2 Page 240 Gene Control in Prokaryotes No nucleus separates DNA from ribosomes in cytoplasm When nutrient supply is high, transcription is fast Translation occurs even before mRNA transcripts are finished The Lactose Operon operator regulatory gene transcription, translation operator gene 1 gene 2 gene 3 promoter lactose operon repressor protein Figure 15.3a Page 241 Low Lactose Repressor binds to operator Binding blocks promoter Transcription is blocked Figure 15.3b Page 241 High Lactose allolactose lactose mRNA operator promoter operator RNA polymerase gene 1 Figure 15.3c Page 241 Lac-Operon Analogy Like an elephant that likes to sit on a railroad track blocking the train. The only thing that the elephant likes better than to sit on his spot on the track is peanuts. If peanuts are available, the elephant leaves the track and eats the peanuts. While he is gone the train can proceed down the track until the elephant goes back to sit on the track after eating all the peanuts. Controls in Eukaryotic Cells Control of transcription Transcript processing controls Controls over translation Controls following translation Most Genes Are Turned Off Cells of a multicelled organism rarely use more than 5-10 percent of their genes at any given time The remaining genes are selectively expressed Homeotic Genes Occur in all eukaryotes Master genes that control development of body parts Encode homeodomains (regulatory proteins) Homeobox sequence can bind to promoters and enhancers X Chromosome Inactivation One X inactivated in each cell of female Creates a “mosaic” for X chromosomes Governed by XIST gene Figure 15.6 Page 245 Hormones Signaling molecules Stimulate or inhibit activity in target cells Mechanism of action varies –May bind to cell surface –May enter cell and bind to regulatory proteins –May bind with enhancers in DNA Vertebrate Hormones Some have widespread effects –Somatotropin (growth hormone) Others signal only certain cells at certain times –Prolactin stimulates milk production Phytochrome Signaling molecule in plants Activated by red wavelengths, inactivated by far-red wavelengths Changes in phytochrome activity influence transcription of certain genes Controlling the Cell Cycle Cycle has built-in checkpoints Proteins monitor chromosome structure, whether conditions favor division, etc. Proteins are products of checkpoint genes Kinases Growth factors Oncogenes Have potential to induce cancer Mutated forms of normal genes Can form following insertions of viral DNA into DNA or after carcinogens change the DNA Cancer Characteristics Plasma membrane and cytoplasm altered Cells grow and divide abnormally Weakened capacity for adhesion Lethal unless eradicated Apoptosis Programmed cell death Signals unleash molecular weapons of self-destruction Cancer cells do not commit suicide on cue