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Chapter 17 Microbial Models: The Genetics of Viruses and Bacteria Viruses Def’n: infectious particles consisting usually of only the viral genes enclosed in a protein shell VIRUSES ARE NOT ALIVE!!! Viruses can be classified according to the type of nucleic acid they contain: Double-stranded DNA Single-stranded DNA Double-stranded RNA Single-stranded RNA Viruses The protein shell protects the genome and is called the capsid The capsid can come in many different shapes… see photo at right! Viruses Viral envelopes surround some viral capsids Viruses that infect bacteria are known as bacteriophages (phages) Viral Reproduction Viruses can only reproduce within a host cell an isolated virus is unable to replicate itself or do anything else Unless it’s in a host cell, it can’t do anything… Viral Reproduction Each type of virus can infect and parasitize only a limited range of host cells known as its host range Identified by “lock-and-key” mechanism… Why does that sound familiar?? Example… Rabies virus can only affect a number of mammalian species (rodents, dogs, humans) Lytic Cycle Viral reproductive cycle that ends in the death of the host cell Basic Breakdown: 1. 2. 3. 4. 5. Phage sticks to outside of bacterial cell Phage injects its DNA into the cell Empty capsid is left outside Cell’s metabolic machinery produces phage proteins Phage directs the production of an enzyme that breaks down the bacterial cell wall, causing the cell to burst – phage particles are released Lysogenic Cycle Viral reproductive cycle that does NOT end in the death of the host cell Basic Breakdown: 1. 2. 3. Phage binds to the surface of the cell Phage injects its DNA into the cell Phage DNA is incorporated into the host cell’s chromosome now known as a prophage 1. 4. 5. Phage DNA is “silent” Every time the bacterial cell prepares to divide, it replicates the viral genome in addition to its own DNA, passing on the viral genome to its daughter cells One day, the prophage commands the host cell to manufacture phages and then self-destruct Retroviruses Important retrovirus: HIV RNA viruses in which genetic information flows backwards Reverse transcriptase: An enzyme that can transcribe DNA from an RNA template This changes the cell’s genome! Viroids and Prions Viroids: Simple infectious molecules of naked RNA Prions: Simple infectious proteins Bacterial Genetics Bacterial genome is a single double-stranded DNA molecule arranged in a circle (chromosome) The chromosome is found within the nucleoid region of a bacterial cell Bacterial cells divide by binary fission This is an asexual process Rapid reproduction Therefore, most bacteria in a colony are genetically identical Sources of Genetic Diversity in Bacteria Mutation Transformation Transduction Alteration of a bacterial cell’s genotype by the uptake of naked, foreign DNA from the surrounding environment This is what we did in lab! Phages transfer genes from one host cell to another Conjugation and Plasmids Direct transfer of genetic material between two bacterial cells that are temporarily joined Transposons Transposon: Pieces of DNA that can move from location to another in a cell’s genome Often called “jumping genes” Operons Structural genes: Operator: REVIEW!! OPERON: Segment of DNA that controls the access of RNA polymerase to the structural genes Promoter: Genes that code for polypeptides The entire stretch of DNA required for enzyme production of a certain polypeptide – including the structural genes, operator, and promoter Repressor: If the repressor is bound to the operator, the operon is blocked Kind of “off/on” switch for operon Chapter 18 Genome Organization and Expression in Eukaryotes Cellular Differentiation Each cell of a multicellular eukaryote expresses only a small fraction of its genome 50,000 – 100,000 genes in every human cell Cellular differentiation: The divergence in structure and function of different types of cells as they become specialized during an organism’s development and remain that way The Structure of Chromatin Eukaryotic chromatin consists of DNA and LOTS of protein If extended, a eukaryotic DNA molecule would be about 6 cm long… WAY bigger than a cell’s nucleus Lots of elaborate packing is required to fit the DNA into the nucleus The Structure of Chromatin Proteins (called histones) are responsible for the first level of DNA packing in chromatin Nucleosome: Looks like beads on a string Consists of DNA wound around histones The Structure of Chromatin Nucleosomes are then coiled into a fiber, which is then folded into looped domains All this to save space! Repetitive Sequences 10 – 25 % of total DNA of multicellular eukaryotes is made up of short sequences repeated 1000s or 1,000,000s of times Known as satellite DNA Telomeres Satellite sequences located at the ends of chromosomes During replication of lagging strand, RNA primers must be “laid down” impossible at the very end of a linear chromosome If this was not “dealt with” the chromosome would get shorter every time it was replicated That’s the role of telomeres! Analogy: ends of shoelaces Gene Amplification Def’n: The selective replication of certain genes The number of copies of a gene or gene family may temporarily increase in some tissues during a particular stage of development Example: Ovum producing lots of extra rRNA so tons of ribosomes can be made Cancer: It’s in the Genes?! Cancer is caused by uncontrolled cell growth and division (mitosis) Certain genes control cell growth and division These genes can be altered by environmental influences/mutations (sun, smoke, etc.) Oncogenes: Proto-oncogenes: Cancer-causing genes Normal cellular genes that normally regulate cell growth and division Tumor-suppressor genes: Genes that encode proteins that normally help prevent uncontrolled cell growth The Genetics of Cancer A genetic predisposition to cancer might be inherited as a defect in a single tumor-suppressor gene An individual inheriting such a recessive mutant allele will be one step closer to accumulating the necessary mutations that will result in abnormal cell growth and division Pre-screening/identification of these mutant alleles?? The Progression of Colorectal Cancer