REGULATION OF GENE EXPRESSION Chapter 18

Gene expression A gene that is expressed is “turned on”. It is actively making a product (protein or RNA). Gene expression is often regulated at transcription. Newly discovered roles of RNA in gene expression

Regulation of a metabolic pathway

Prokaryotic Gene Regulation Adjust activity of enzymes already present Often through negative feedback Adjust production level of certain enzymes

OPERONS Regulation in prokaryotes Operator – switch segment of DNA in promoter Operon – the promoter, the operator, and the genes they control Regulatory gene – long distance from gene that is regulated

The trp operon: regulated synthesis of repressible enzymes trp animation trp tutorial

The trp operon: regulated synthesis of repressible enzymes

The lac operon: regulated synthesis of inducible enzymes lac operon animation lac operon tutorial

The lac operon: regulated synthesis of inducible enzymes

Regulatory gene makes protein (repressor) that inhibits operator Regulatory protein has inactive and active shape Corepressor – makes repressor active Inducer – inactivates repressor

Repressible enzymes usually used when cell makes something (ex Repressible enzymes usually used when cell makes something (ex. tryptophan) Inducible enzymes usually used when cell breaks something down (ex. lactose)

Eukaryotic Gene Regulation Expression can be regulated at any stage Differential gene expression – different cells in an organism express different genes from the genome Not all genes are turned on all of the time!

Opportunities for the control of gene expression in eukaryotic cells

GENOME ORGANIZATION 1.5% of DNA in humans codes for protein 24% introns and regulatory Most is repetitive DNA (59%) Unique noncoding is 15%

CHROMATIN Composed of DNA and proteins called histones Nucloesome – DNA wrapped around a histone

Levels of chromatin packing

Eukaryotic Regulation At DNA level Chromatin modification, DNA unpacking with histone acetylation (turned on) and DNA demethylation (turned off) At RNA level Transcription, RNA processing, transport to cytoplasm At protein level Translation, protein processing, transport to cellular destination, protein degradation

Regulation of transcription Transcription factors – mediate the binding of RNA polymerase to the promoter and other regulatory proteins Enhancers – far upstream of gene; bind to transcription factors; called distal control element

Figure 19.8 A eukaryotic gene and its transcript

Not many different control elements so the combination of control elements regulates gene action Different combos of activators (transcription factors) makes different genes turn on Different genes can be turned on by same activator

Cell-type specific transcription based on available activators Albumin is a blood protein (made in liver) Crystallin is the main protein in the lens of the eye (made in eye)

DIFFERENTIAL GENE EXPRESSION = DIFFERENT CELL TYPES Cell differentiation – process by which cells become specialized in structure and function Morphogenesis – process that gives an organism its form (shape) Pattern formation – development of spatial organization in which tissues and organs are in their correct places Positional information – molecular cues that control pattern formation Homeotic genes – control pattern formation

Caused by a mutated homeotic gene

CANCER Oncogenes- cancer causing genes in retroviruses Proto-oncogenes – normal genes that code for proteins that stimulate cell growth and division Tumor suppressor genes - make proteins that help prevent uncontrolled cell growth

Converting proto-oncogene into oncogene

Converting proto-oncogene into oncogene Movement of DNA within a chromosome May place a more active promoter near a proto-oncogene (= more cell division) Amplification of a proto-oncogene Point mutations in control element or proto-oncogene (= more expression or makes abnormal protein that doesn’t get degraded or is more active)

GENES INVOLVED IN CANCER Ras gene – makes ras (G) protein that starts cascade reactions that initiate cell division Mutations in Ras gene cause ~30% cancers p53 tumor suppressor gene – “guardian of genome” Activates p21 which halts cell cycle Turns on genes to repair DNA Activates suicide proteins that cause cell death (apoptosis) Mutations in P53 gene cause ~50% cancer

Multistep Model of Cancer Development Approximately half dozen changes have to occur at the DNA level for cancer to develop. Need at least one oncogene and loss of tumor suppressor gene(s) Most oncogenes are dominant and most tumor suppressor genes recessive so must knock out both alleles Typically telomerase is activated

A multi-step model for the development of colorectal cancer

Inherited Predisposition to Cancer 15% colorectal cancers are inherited Most from mutated APC gene (tumor suppressor gene) 5-10% breast cancers are inherited Most with mutated BRCA1 and BRCA2 A woman with one mutant BRCA1 gene (tumor suppressor gene) has a 60% chance of getting breast cancer by age 50