Chapters 15 – 17 Regulation of Gene Expression Development, Stem Cells, and Cancer Viruses

Purpose of Gene Regulation Unicellular (bacteria, protists) ▫Prevents waste of cell’s resources creating unnecessary proteins Multicellular ▫Allows for cell differentiation  All cells have identical genome, yet express different portions of it  Different cells perform different functions Works with feedback inhibition

Operons Found in unicellular organisms “On-Off switch” for genes w/related functions Components ▫Operator  Region of DNA where repressor protein binds ▫Controlled genes ▫Regulatory gene (and its product – repressor protein)  Always expressed in cell

lac Operon Classic example of operon function Involves genes associated with lactose digestion

Eukaryotic Gene Regulation Regulation occurs at multiple points ▫In the nucleus:  Chromatin modification  Protein synthesis  RNA processing ▫In the cytoplasm:  RNA transcription  Protein processing

In the Nucleus … Chromatin modification ▫Histone acetylation  Acetyl group (–COCH 3 ) added to histone proteins  DNA packaged more loosely, promoting transcription ▫DNA methylation  Methyl group (–CH 3 ) added to bases – usually cytosine  Prevents protein interaction with that region of chromatin ▫Alternative RNA splicing

In the Cytoplasm…

Embryonic Development Zygote to adult involves cell division, cell differentiation and morphogenesis ▫Cell division increases number of cells ▫Cell differentiation results in different types of cells ▫Morphogenesis produces new structures (shaping)

Stem Cells Stem cells are unspecialized cells that can reproduce indefinitely (no Hayflick limit) and can differentiate into specialized cells of one or more types ▫Totipotent – exist only early on after fertilization; can become ANY cell type ▫Pluripotent– descendants of totipotent; can become any cell derived from the three germ layers ▫Multipotent– can result in any cell type belonging to a family of cells

Abnormal Regulation of Genes Faulty gene expression regulation can lead to cancer Research has lead to the discovery of cancer causing genes: oncogenes ▫Oncogenes can arise from a change to a normally- functioning gene or from certain types of viruses  Proto-oncogenes are NORMAL genes that are involved with NORMAL cell growth and division  Ex. Tumor-supressor genes code for proteins that curb uncontrolled cell growth ▫Over HALF of all human tumors are the result of damage to the p53 gene  Viruses play a role in about 15% of human cancers

Viral Structure Viruses are obligate, intracellular parasites ▫Must infiltrate a cell and utilize it’s resources and machinery to complete viral “life cycle” ▫Infectious agent made of little more than a protein coat and nucleic acid  Capsid – protein coat surrounding genetic material ▫Bacteriophages (or phages) are viruses that infect bacteria ▫Most viruses are limited to a specific host or set of hosts they can infect – host range

Typical Viral Life Cycle

Phage Replication After infecting a bacterium, phage genetic material may become incorporated into the bacterial chromosome ▫The lysogenic cycle often doesn’t harm the bacterium ▫Integrated viral DNA known as a prophage  Viral DNA is replicated and passed to daughter cells ▫Changes to environmental conditions may trigger switch to lytic cycle Lytic cycle involves replication of viral particles and assembly of new phages ▫Typically results in rupture (and death) of cell

Animal Viruses A greater diversity of viruses exist that infect animals ▫Type of genetic material ▫Outer covering Retroviruses utilize RNA as genetic material ▫Carry reverse transcriptase to produce DNA from RNA ▫Viral genome incorporated into host’s DNA – provirus  Permanent resident of host cell  Unlike phages, cell typically isn’t ruptured during viral production

Vaccines Vaccines are derivatives of pathogens that stimulate the immune system ▫Current vaccines CANNOT cause the illness they vaccinate against  Efficacy often decreases with time ▫Anti-virals can treat viral infections after they’re contracted … not a cure  Antibiotics have NO EFFECT on viral infections  Antibiotics combat bacterial infections