Control over Genes Chapter 15

Squamous Cell Carcinoma 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 • Type of cell • Internal chemical conditions 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 gene 1 gene 2 gene 3 transcription, translation 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 RNA polymerase operator promoter 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