Chapter 17 Microbial Models: The Genetics of Viruses and Bacteria

Viruses Def’n: Def’n: infectious particles consisting usually of only the viral genes enclosed in a protein shell infectious particles consisting usually of only the viral genes enclosed in a protein shell VIRUSES ARE NOT ALIVE!!! VIRUSES ARE NOT ALIVE!!! Viruses can be classified according to the type of nucleic acid they contain: Viruses can be classified according to the type of nucleic acid they contain: Double-stranded DNA Double-stranded DNA Single-stranded DNA Single-stranded DNA Double-stranded RNA Double-stranded RNA Single-stranded RNA Single-stranded RNA

Viruses The protein shell protects the genome and is called the capsid The protein shell protects the genome and is called the capsid The capsid can come in many different shapes… see photo at right! The capsid can come in many different shapes… see photo at right!

Viruses Viral envelopes surround some viral capsids Viral envelopes surround some viral capsids Viruses that infect bacteria are known as bacteriophages (phages) Viruses that infect bacteria are known as bacteriophages (phages)

Viral Reproduction Viruses can only reproduce within a host cell Viruses can only reproduce within a host cell an isolated virus is unable to replicate itself or do anything else an isolated virus is unable to replicate itself or do anything else Unless it’s in a host cell, it can’t do anything… 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 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… Identified by “lock-and-key” mechanism… Why does that sound familiar?? Why does that sound familiar?? Example… Example… Rabies virus can only affect a number of mammalian species (rodents, dogs, humans) 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 Viral reproductive cycle that ends in the death of the host cell Basic Breakdown: Basic Breakdown: 1. Phage sticks to outside of bacterial cell 2. Phage injects its DNA into the cell 3. Empty capsid is left outside 4. Cell’s metabolic machinery produces phage proteins 5. 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 Viral reproductive cycle that does NOT end in the death of the host cell Basic Breakdown: Basic Breakdown: 1. Phage binds to the surface of the cell 2. Phage injects its DNA into the cell 3. Phage DNA is incorporated into the host cell’s chromosome  now known as a prophage 1. Phage DNA is “silent” 4. 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 5. One day, the prophage commands the host cell to manufacture phages and then self-destruct

Retroviruses Important retrovirus: HIV Important retrovirus: HIV RNA viruses in which genetic information flows backwards RNA viruses in which genetic information flows backwards Reverse transcriptase: Reverse transcriptase: An enzyme that can transcribe DNA from an RNA template An enzyme that can transcribe DNA from an RNA template This changes the cell’s genome! This changes the cell’s genome!

Viroids and Prions Viroids: Viroids: Simple infectious molecules of naked RNA Simple infectious molecules of naked RNA Prions: Prions: Simple infectious proteins Simple infectious proteins

Bacterial Genetics Bacterial genome is a single double-stranded DNA molecule arranged in a circle (chromosome) 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 The chromosome is found within the nucleoid region of a bacterial cell Bacterial cells divide by binary fission Bacterial cells divide by binary fission This is an asexual process This is an asexual process Rapid reproduction Rapid reproduction Therefore, most bacteria in a colony are genetically identical Therefore, most bacteria in a colony are genetically identical

Sources of Genetic Diversity in Bacteria Mutation Mutation Transformation Transformation Alteration of a bacterial cell’s genotype by the uptake of naked, foreign DNA from the surrounding environment 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! This is what we did in lab! Transduction Transduction Phages transfer genes from one host cell to another Phages transfer genes from one host cell to another Conjugation and Plasmids Conjugation and Plasmids Direct transfer of genetic material between two bacterial cells that are temporarily joined Direct transfer of genetic material between two bacterial cells that are temporarily joined

Transposons Transposon: Transposon: Pieces of DNA that can move from location to another in a cell’s genome Pieces of DNA that can move from location to another in a cell’s genome Often called “jumping genes” Often called “jumping genes”

Operons Structural genes: Structural genes: Genes that code for polypeptides Genes that code for polypeptides Operator: Operator: Segment of DNA that controls the access of RNA polymerase to the structural genes Segment of DNA that controls the access of RNA polymerase to the structural genes Promoter: Promoter: REVIEW!! REVIEW!! OPERON: OPERON: The entire stretch of DNA required for enzyme production of a certain polypeptide – including the structural genes, operator, and promoter The entire stretch of DNA required for enzyme production of a certain polypeptide – including the structural genes, operator, and promoter Repressor: Repressor: If the repressor is bound to the operator, the operon is blocked If the repressor is bound to the operator, the operon is blocked Kind of “off/on” switch for operon 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 Each cell of a multicellular eukaryote expresses only a small fraction of its genome 50,000 – 100,000 genes in every human cell 50,000 – 100,000 genes in every human cell Cellular differentiation: 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 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 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 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 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 Proteins (called histones) are responsible for the first level of DNA packing in chromatin Nucleosome: 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 Nucleosomes are then coiled into a fiber, which is then folded into looped domains All this to save space! 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 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 Known as satellite DNA

Telomeres Satellite sequences located at the ends of chromosomes 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 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 If this was not “dealt with” the chromosome would get shorter every time it was replicated That’s the role of telomeres! That’s the role of telomeres! Analogy: ends of shoelaces Analogy: ends of shoelaces

Gene Amplification Def’n: Def’n: The selective replication of certain genes 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 The number of copies of a gene or gene family may temporarily increase in some tissues during a particular stage of development Example: Example: Ovum producing lots of extra rRNA so tons of ribosomes can be made 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) Cancer is caused by uncontrolled cell growth and division (mitosis) Certain genes control cell growth and division Certain genes control cell growth and division These genes can be altered by environmental influences/mutations (sun, smoke, etc.) These genes can be altered by environmental influences/mutations (sun, smoke, etc.) Oncogenes: Oncogenes: Cancer-causing genes Cancer-causing genes Proto-oncogenes: Proto-oncogenes: Normal cellular genes that normally regulate cell growth and division Normal cellular genes that normally regulate cell growth and division Tumor-suppressor genes: Tumor-suppressor genes: Genes that encode proteins that normally help prevent uncontrolled cell growth 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 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 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?? Pre-screening/identification of these mutant alleles??

The Progression of Colorectal Cancer