Chapter 11 The Control of Gene Expression

To Clone or Not to Clone repairstemcell.files.wordpress.com - Began in 1950’s - Dolly (1997) - proposed for endangered species - may create new problems stop work on habitat preservation does not increase genetic diversity clones animals are less healthy Gene regulation is important for the well-being of all organisms *How are genes regulated *applications *gene regulation and embryonic development

Gene Regulation Gene expression Early understanding of gene control Came from studies of the bacterium Escherichia coli

In prokaryotes, genes for related enzymes Are often controlled together in units operons Lac Operon Can be turned on and off according to the environmental circumstances: availability of lactose

Lac Operon Lack of substrate inactivates the enzyme

Lac Operon Substrate presence inactivates the repressor of the gene

Other Kinds of Operons trp operon similar to the lac operon, but functions somewhat differently Inactive repressor TryptophanLactose Active repressor Promoter Operator Genes Lac Operon trp operon

Differentiation yields a variety of cell types, each expressing a different combination of genes multicellular eukaryotes cells become specialized as a zygote develops into a mature organism Muscle Pancreas Blood

Genetic Potential Differentiated cells may retain all of their genetic potential Most differentiated cells retain a complete set of genes root cell cultured cell division plantlet adult plant in nutrient medium in culture single cell tissue

DNA Packing DNA packing in eukaryotic chromosomes helps regulate gene expression A chromosome contains DNA Wound around clusters of histone proteins, forming a string of beadlike nucleosomes

Inactive Chromosomes In female mammals, one X chromosome is inactive in each cell An extreme example of DNA packing in interphase cells X chromosome inactivation in the cells of female mammals Early embryo cells of adult cat X chromosome Allele for orange fur Allele for black fur Cell division and random Inactivation of X chromosome active inactive

Control of Eukaryotic Transcription A variety of regulatory proteins interact with DNA and with each other Known as transcription factors “Default” state seems to be “off” They turn the transcription of genes on or off Only a small percentage of the genes

Transcription Factors Assist in initiating eukaryotic transcription Enhancers Promoter gene Transcription Factors Activator protein Other Proteins RNA polymerase Bending of DNA Transcription

Coordinating Eukaryotic Gene Expression Coordinated gene expression in eukaryotes Seems to depend on the association of enhancers with groups of genes

Eukaryotic RNA May be spliced in more than one way After transcription, alternative splicing May generate two or more types of mRNA from the same transcript DNA EXONS RNA transcript mRNA RNA splicing allows more than one type of polypeptide from a single gene Introns are removed

Regulation Translation and later stages of gene expression are also subject to regulation After eukaryotic mRNA is fully processed and transported to the cytoplasm There are additional opportunities for regulation

1. Breakdown of mRNA lifetime of an mRNA molecule Short-lived mRNA: bacteria Long-lived mRNA: eukaryotes Helps determine how much protein is made

Example of Long-lived mRNA Red blood cells of reptiles, amphibians and fish Manufactures hemoglobin Last the same as the lifetime of the RBC

2. Initiation of Translation Control of the starting point of polypeptide synthesis Example synthesis of Hemoglobin Iron in the heme group has to be present

3. Protein Activation After translation is complete Polypeptides may require an alteration to become functional Inactive polypeptide folded polypeptide active form of insulin SH HS SH HS SH S S S S S Folding of polypeptide Formation of S-S linkage Cleavage S S S S S Insulin

4. Protein Breakdown Occurs in some of the proteins that trigger metabolic changes in cells Are broken down within a few minutes or hours Allows cell to adjust the kinds and amounts of proteins in response to the environment

Review Multiple mechanisms regulate gene expression in eukaryotes

Animal Cloning Nuclear transplantation can be used to clone animals and/or therapeutic use Remove nucleus from egg Add somatic cell from adult Grow in culture to produce an early embryo (blastocyst) Nucleus from donor Surrogate mother clone of donor with implanted blastocyst Remove embryonic stem cells from blastocyst and grow in culture Induce stem cells to form speciali- zed cells (therapeutic)

Connection: Reproductive Cloning Reproductive cloning has valuable applications, but human reproductive cloning raises ethical issues Reproductive cloning of nonhuman mammals is useful in research, agriculture, and medicine

Connection: Therapeutic Cloning can produce stem cells with great medical potential Like embryonic stem cells, adult stem cells can perpetuate themselves in culture and give rise to differentiated cells Adult stem cell in bone marrow Cultured embryonic stem cells Different culture conditions Different types of Differentiated cells Blood cells Nerve cells Muscle cells

Genetic Control of Embryonic Development Cascades of gene expression and cell-to-cell signaling direct the development of an animal Early understanding of the relationship between gene expression and embryonic development Came from studies of mutants of the fruit fly Drosophila melanogaster Normal Mutant antenna leg

D. melanogaster 1. egg protein signals follicle cells 2. follicle cells signal back to the egg 3. egg responds localizing specific mRNA which indicates the localization of the fly’s head 4. regulatory protein is produced 5. other proteins are also produced and form a gradient 6. segmentation occurs 7. adult fly is the final product Follicle cells mRNA Regulatory proteins segmentation ADULT EGG LARVA PUPA

Signal Transduction Pathways Convert messages received at the cell surface to responses within the cell Signaling cell Signaling molecule Receptor protein Target cell Relay proteins Transcription factor Nucleus DNA Transcription mRNA Translation New Protein

Key Developmental Genes are very ancient Homeotic genes contain nucleotide sequences, called homeoboxes That are very similar in many kinds of organisms Fruit fly embryo (10h) mouse embryo (12d) Adult fruit fly adult mouse

THE GENETIC BASIS OF CANCER Cancer results from mutations in genes that control cell division divide uncontrollably Result from mutations in genes whose protein products affect the cell cycle

Proto-Oncogenes A mutation can change a proto-oncogene (a normal gene that promotes cell division) into an oncogene, which causes cells to divide excessively Proto-oncogene DNA Mutation within gene Normal Gene at other locus oncogene Hyperactive growth- stimulating protein in normal amount Normal growth-stimulating protein in excess New promoter Onkos= tumor

Tumor-Suppressor Genes Mutations that inactivate tumor suppressor genes Have similar effects as oncogenes Tumor-suppressor gene Mutated tumor-suppressor gene Normal growth-inhibiting protein Defective, nonfunctional protein Cell division under control Cell division not under control

Interference with Normal Signal Transduction Pathways Oncogene proteins Can stimulate signal transduction pathways Growth factor receptor Target cell Hyperactive relay protein Issues signals on its own Normal product Relay proteins Transcription factor (activated) transcription DNA translation Protein that stimulates Cell division

Interference… Faulty tumor-suppressor proteins Can inhibit signal transduction pathways Growth inhibiting factor receptor Relay protein Nonfunctional transcription Factor: cannot trigger transcription Normal product of P53 gene Transcription factor (activated) transcription translation Protein that inhibits cell division Lack of such protein

Development of Cancer Multiple genetic changes underlie the development of cancer Cancers result from a series of genetic changes in a cell lineage

Colon cancer Develops in a stepwise fashion Cellular changes Increased cell division growth of a polyp Growth of a malignant tumor (carcinoma) DNA changes Oncogene activated Tumor-suppressor gene inactivated 2 nd tumor-suppressor gene inactivated 1 23

Accumulation of mutations Can lead to cancer Chromosomes 1 mutation 2 mutations 3 mutations 4 mutations Normal cell Malignant cell

Talking about Science Mary-Claire King discusses mutations that cause breast cancer Researchers have gained insight into the genetic basis of breast cancer By studying families in which a diseasepredisposing mutation is inherited

Avoiding carcinogens can reduce the risk of cancer Reducing exposure to carcinogens (which induce cancer-causing mutations) And making other lifestyle choices can help reduce cancer risk

Cancer in the U.S. The End