The Control of Gene Expression Chapter 11 The Control of Gene Expression

To Clone or Not to Clone? A clone is an individual created by asexual reproduction and thus is genetically identical to a single parent Cloning an animal using a transplanted nucleus shows that an adult somatic cell contains a complete genome Cloning has potential benefits but evokes many concerns Does not increase genetic diversity May produce less healthy animals

GENE REGULATION 11.1 Proteins interacting with DNA turn prokaryotic genes on or off in response to environmental changes Gene regulation is the "turning on" and "turning off" of genes Helps organisms respond to environmental changes Gene expression is the process by which information flows from genes to protein Early understanding of gene control came from studies of the bacterium Escherichia coli

An operon is a cluster of genes with related functions, along with two control sequences Promoter: A sequence of genes where the RNA polymerase attaches and initiates transcription Operator: A sequence of genes between the operon and the promoter that acts as a switch for the binding of RNA polymerase A repressor binds to the operator, stopping transcription A regulatory gene, located outside the operon, codes for the repressor

The lac operon contains the genes that code for the enzymes that metabolize lactose Repressor is active when alone and inactive when bound to lactose The trp operon allows bacteria to stop making tryptophan when it is already present Repressor is inactive alone; must bind to the amino acid tryptophan to be active A third type of operon uses activators, proteins that turn operons on by binding to DNA

Lactose-utilization genes LE 11-1b OPERON Regulatory gene Promoter Operator Lactose-utilization genes DNA mRNA RNA polymerase cannot attach to promoter Active repressor Protein Operon turned off (lactose absent) DNA RNA polymerase bound to promoter mRNA Protein Inactive repressor Enzymes for lactose utilization Lactose Operon turned on (lactose inactivates repressor)

LE 11-1c Promoter Operator Genes DNA Active repressor Active repressor Tryptophan Inactive repressor Inactive repressor Lactose lac operon trp operon

11.2 Differentiation yields a variety of cell types, each expressing a different combination of genes Gene regulation is much more complex in eukaryotes than in prokaryotes In multicellular eukaryotes, cells become specialized as a zygote develops into a mature organism The particular genes that are active in each type of cell are the source of its particular function

Muscle cell Pancreas cells Blood cells

11.3 Differentiated cells may retain all of their genetic potential Though differentiated cells express only a small percentage of their genes, they retain a complete set of genes Allows for propagation of crop plants In animal cells can lead to regeneration

LE 11-3 Root of carrot plant Single cell Root cells cultured in nutrient medium Cell division in culture Plantlet Adult plant

11.4 DNA packing in eukaryotic chromosomes helps regulate gene expression DNA can fit into a chromosome because of packing DNA winds around clusters of histone proteins, forming a string of bead-like nucleosomes The beaded fiber coils, supercoils, and further folds into chromosomes DNA packing prevents gene expression most likely by preventing transcription proteins from contacting the DNA

Animation: DNA Packing LE 11-4 DNA double helix (2-nm diameter) Histones Linker “Beads on a string” TEM Nucleosome (10-nm diameter) Tight helical fiber (30-nm diameter) Supercoil (300-nm diameter) TEM 700 nm Metaphase chromosome Animation: DNA Packing

11.5 In female mammals, one X chromosome is inactive in each cell An extreme example of DNA packing is X chromosome inactivation in interphase cells of female mammals In each cell line, the X chromosome from either parent may be inactivated Leads to a random mosaic of expression of the two X chromosomes Example: coat color in tortoiseshell cat

LE 11-5 Early embryo Two cell populations in adult Cell division and random X chromosome inactivation Active X Orange fur X chromosomes Inactive X Inactive X Allele for orange fur Active X Black fur Allele for black fur

11.6 Complex assemblies of proteins control eukaryotic transcription A variety of regulatory proteins interact with DNA and with each other to turn eukaryotic genes on or off In contrast to bacteria Each eukaryotic gene has its own promoter and control sequences Activators are more important than repressors

Animation: Initiation of Transcription Eukaryotic RNA polymerase needs the assistance of transcription factors The binding of activators to enhancers initiates transcription Silencers inhibit the start of transcription Coordinated gene expression in eukaryotes seems to depend on the association of specific enhancers with groups of genes Animation: Initiation of Transcription

LE 11-6 Enhancers Promoter Gene DNA Activator proteins Transcription factors Other proteins RNA polymerase Bending of DNA Transcription

Animation: RNA Processing 11.7 Eukaryotic RNA may be spliced in more than one way After transcription, splicing removes noncoding introns Alternative splicing may generate two or more types of mRNA from the same transcript Exons or RNA splicing mRNA RNA transcript DNA Animation: RNA Processing

11.8 Translation and later stages of gene expression are also subject to regulation After eukaryotic mRNA is processed and transported to the cytoplasm, there are additional opportunities for regulation Breakdown of mRNA: The lifetime of an mRNA molecule helps determine how much protein is made Initiation of translation: A great many proteins control the start of polypeptide synthesis

Protein activation: After translation, polypeptides may be cut into smaller, active products Protein breakdown: Rapid selective breakdown of proteins allows the cell to respond to environmental changes SH Initial polypeptide (inactive) Folded polypeptide Active form of insulin Cleavage Folding of polypeptide and formation of S—S linkages HS S-S

11.9 Review: Multiple mechanisms regulate gene expression in eukaryotes Cellular differentiation results from selective turning on or off of genes at multiple control points In nucleus DNA unpacking and other changes Transcription Addition of cap and tail Splicing

Each differentiated cell still retains its full genetic potential In cytoplasm Breakdown of mRNA Translation Cleavage/modification/activation Breakdown of protein Each differentiated cell still retains its full genetic potential

LE 11-9 NUCLEUS CYTOPLASM Chromosome DNA unpacking Other changes to DNA Gene Transcription Gene Exon RNA transcript Intron Addition of cap and tail Splicing Tail mRNA in nucleus Cap Flow through nuclear envelope mRNA in cytoplasm CYTOPLASM Breakdown of mRNA Broken- down mRNA Translation Polypeptide Cleavage / modification / activation Active protein Breakdown of protein Broken- down protein

11.10 Nuclear transplantation can be used to clone animals ANIMAL CLONING 11.10 Nuclear transplantation can be used to clone animals Nuclear transplantation Nucleus of a somatic cell is transplanted into a surrogate egg stripped of nucleus Cell divides to the blastocyst stage Reproductive cloning Blastocycst is implanted into uterus Live animal is born

Embryonic stem cells are harvested from blastocyst Therapeutic cloning Embryonic stem cells are harvested from blastocyst These cells give rise to all the specialized cells of the body Donor cell Nucleus from donor cell Remove nucleus from egg cell Add somatic cell from adult donor Grow in culture to produce an early embryo (blastocyst) Implant blastocyst in surrogate mother Remove embryonic stem cells from blastocyst and grow in culture Clone of donor is born (reproductive cloning) Induce stem cells to form specialized cells (therapeutic cloning)

CONNECTION 11.11 Reproductive cloning has valuable applications, but human reproductive cloning raises ethical issues Reproductive cloning of nonhuman mammals is useful in research, agriculture, and medicine There are many obstacles, both practical and ethical, to human cloning Research continues in the absence of consensus

CONNECTION 11.12 Therapeutic cloning can produce stem cells with great medical potential In culture, embryonic stem cells Can give rise to all cell types in the body Must be obtained from human embryos Adult stem cells Can give rise to many, but perhaps not all, cell types Are present in adult tissues and, thus, are less controversial than embryonic cells

Adult stem cells in bone marrow Cultured embryonic stem cells LE 11-12 Blood cells Adult stem cells in bone marrow Nerve cells Cultured embryonic stem cells Heart muscle cells Different culture conditions Different types of differentiated cells

THE GENETIC CONTROL OF EMBRYONIC DEVELOPMENT 11.13 Cascades of gene expression and cell-to-cell signaling direct the development of an animal Studies of mutant fruit flies led to early understanding of gene expression and embryonic development Before fertilization, communication between the egg and adjacent cells determines body polarity A cascade of gene expression controls development of an animal from a fertilized egg Master control homeotic genes regulate batteries of genes that shape anatomical parts

Head of a normal fruit fly Head of a developmental mutant LE 11-13a Eye Antenna Leg Head of a normal fruit fly Head of a developmental mutant

LE 11-13b Egg cell within ovarian follicle Egg cell Egg protein signaling follicle cells Follicle cells Gene expression in follicle cells Follicle cell protein signaling egg cell Localization of “head” mRNA “Head” mRNA Fertilization and mitosis Embryo Translation of “head” mRNA Gradient of regulatory protein Gene expression Gradient of certain other proteins Gene expression Body segments 0.1 mm Larva Gene expression Adult fly Head end Tail end 0.5 mm

11.14 Signal transduction pathways convert messages received at the cell surface to responses within the cell Signal transduction pathway Signaling cell secretes signal molecules Signal molecules bind to receptors on target cell's plasma membrane Cascade of events leads to the activation of a specific transcription factor

Transcription factor triggers transcription of a specific gene Translation of the mRNA produces a protein

Animation: Overview of Cell Signaling Signaling cell Signal molecule Receptor protein Plasma membrane Target cell Nucleus Transcription factor (activated) Relay proteins DNA mRNA Transcription Translation New Animation: Overview of Cell Signaling Animation: Signal Transduction Pathways Animation: Cell Signaling

11.15 Key developmental genes are ancient Homeotic genes contain nucleotide sequences called homeoboxes Regulate gene expression during development Similarity of homeoboxes among organisms suggests a very early evolutionary origin

Fruit fly embryo (10 hours) LE 11-15 Fly chromosome Mouse chromosomes Mouse chromosomes Fruit fly embryo (10 hours) Mouse embryo (12 days) Adult fruit fly Adult mouse

THE GENETIC BASIS OF CANCER 11.16 Cancer results from mutations in genes that control cell division An oncogene can cause cancer when present in a single copy in a cell A cell can acquire an oncogene from A virus A mutation in a proto-oncogene, a normal gene with the potential to become an oncogene

LE 11-16a Proto-oncogene DNA Mutation within the gene Multiple copies of the gene Gene moved to new DNA locus, under new controls Oncogene New promoter Hyperactive growth- stimulating protein in normal amount Normal growth- stimulating protein in excess Normal growth- stimulating protein in excess

Tumor-suppressor genes Normally code for proteins that inhibit cell division When inactivated by mutation, can lead to uncontrolled cell division and tumors

Tumor-suppressor gene Mutated tumor-suppressor gene LE 11-16b Tumor-suppressor gene Mutated tumor-suppressor gene Normal growth- inhibiting protein Defective, nonfunctioning protein Cell division under control Cell division not under control

11.17 Oncogene proteins and faulty tumor-suppressor proteins can interfere with normal signal transduction pathways Stimulatory signal-transduction pathway Stimulates cell division in response to growth factor Can be stimulated by oncogene proteins that produce hyperactive relay proteins

LE 11-17a Growth factor Receptor Target cell Hyperactive relay protein (product of ras oncogene) issues signals on its own Normal product of ras gene Relay proteins Transcription factor (activated) DNA Nucleus Transcription Translation Protein that stimulates cell division

Inhibitory signal-transduction pathway Inhibits cell division in response to growth-inhibiting factor Faulty tumor-suppressor genes may produce proteins that fail to inhibit cell division

LE 11-17b Growth-inhibiting factor Receptor Relay proteins Nonfunctional transcription factor (product of faulty p53 tumor-suppressor gene) cannot trigger transcription Transcription factor (activated) Normal product of p53 gene Transcription Translation Protein that inhibits cell division Protein absent (cell division not inhibited)

11.18 Multiple genetic changes underlie the development of cancer Cancers result from a series of genetic changes in a cell linage More than one somatic mutation is necessary Accumulation of mutations over time leads to uncontrolled cell division Example: Colon cancer develops in a stepwise fashion

LE 11-18a Colon wall Cellular changes: Increased cell division Growth of polyp Growth of malignant tumor (carcinoma) DNA changes: Oncogene activated Tumor-suppressor gene inactivated Second tumor- suppressor gene inactivated

LE 11-18b Chromosomes 1 mutation 2 mutations 3 mutations 4 mutations Normal cell Malignant cell

11.19 Mary-Claire King discusses mutations that cause breast cancer TALKING ABOUT SCIENCE 11.19 Mary-Claire King discusses mutations that cause breast cancer Researchers have gained insight into the genetic basis of breast cancer by studying families with a history of the disease A mutation in the gene BRCA1 can put a woman at high risk for breast cancer Environmental influences also play a role

11.20 Avoiding carcinogens can reduce the risk of cancer CONNECTION 11.20 Avoiding carcinogens can reduce the risk of cancer Carcinogens are agents that induce cancer-causing mutations UV radiation, X-rays Mutagenic chemical compounds, particularly tobacco smoke Reducing exposure to carcinogens and making other lifestyle choices can help reduce cancer risk