Control over Genes Chapter 15

15.1 Control Mechanisms 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

15.2 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 gene 1gene 2gene 3 lactose operon regulatory gene transcription, translation operator promoter 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 gene 1 operator promoter mRNA RNA polymerase lactose allolactose Figure 15.3c Page 241

CAP Exerts Positive Control CAP is an activator protein Adheres to promoter only when in complex with cAMP Level of cAMP depends on level of glucose

Positive Control – High Glucose There is little cAMP CAP cannot be activated The promoter is not good at binding RNA polymerase The lactose-metabolizing genes are not transcribed very much

Positive Control – Low Glucose cAMP accumulates CAP-cAMP complex forms Complex binds to promoter RNA polymerase can now bind The lactose-metabolizing genes are transcribed rapidly

15.3 Controls in Eukaryotic Cells Control of transcription Transcript processing controls Controls over translation Controls following translation

14.4 Types of Control Mechanisms 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

15.5 Signaling molecules Hormones 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

Polytene Chromosomes Occur in salivary glands of midge larvae Consist of multiple DNA molecules Can produce multiple copies of transcripts Figure 15.8 Page 246

Chromosome Puff Portion of the chromosome in which the DNA has loosened up to allow transcription Appears in response to ecdysone Translation of transcripts from puffed region produces protein components of saliva

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