Chapter 17Chapter 17

Microbial Models:Microbial Models:

The Genetics of Viruses and The Genetics of Viruses and BacteriaBacteria

VirusesViruses Def’n: Def’n:

infectious particles consisting usually of only infectious particles consisting usually of only the viral genes enclosed in a protein shellthe viral genes enclosed in a protein shell

VIRUSES ARE NOT ALIVE!!!VIRUSES ARE NOT ALIVE!!! Viruses can be classified according to the Viruses can be classified according to the

type of nucleic acid they contain:type of nucleic acid they contain: Double-stranded DNADouble-stranded DNA Single-stranded DNASingle-stranded DNA Double-stranded RNADouble-stranded RNA Single-stranded RNASingle-stranded RNA

VirusesViruses

The protein shell The protein shell protects the protects the genome and is genome and is called the called the capsidcapsid

The capsid can The capsid can come in many come in many different shapes… different shapes… see photo at right!see photo at right!

VirusesViruses

Viral envelopesViral envelopes surround some surround some viral capsidsviral capsids

Viruses that infect Viruses that infect bacteria are known bacteria are known as as bacteriophagesbacteriophages (phages)(phages)

Viral ReproductionViral Reproduction

Viruses can only reproduce Viruses can only reproduce within a host cellwithin a host cell an isolated virus is unable to replicate an isolated virus is unable to replicate

itself or do anything elseitself or do anything else Unless it’s in a host cell, it can’t do Unless it’s in a host cell, it can’t do

anything…anything…

Viral ReproductionViral Reproduction

Each type of virus can infect and Each type of virus can infect and parasitize only a limited range of host parasitize only a limited range of host cells cells known as its known as its host rangehost 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 Rabies virus can only affect a number of mammalian species (rodents, dogs, humans)mammalian species (rodents, dogs, humans)

Lytic CycleLytic Cycle Viral reproductive cycle that ends in the Viral reproductive cycle that ends in the

death of the host celldeath of the host cell Basic Breakdown:Basic Breakdown:

1.1. Phage sticks to outside of bacterial cellPhage sticks to outside of bacterial cell

2.2. Phage injects its DNA into the cellPhage injects its DNA into the cell

3.3. Empty capsid is left outsideEmpty capsid is left outside

4.4. Cell’s metabolic machinery produces phage Cell’s metabolic machinery produces phage proteinsproteins

5.5. Phage directs the production of an enzyme that Phage directs the production of an enzyme that breaks down the bacterial cell wall, causing the breaks down the bacterial cell wall, causing the cell to burst – phage particles are releasedcell to burst – phage particles are released

Lysogenic CycleLysogenic Cycle Viral reproductive cycle that does NOT end in Viral reproductive cycle that does NOT end in

the death of the host cellthe death of the host cell Basic Breakdown:Basic Breakdown:

1.1. Phage binds to the surface of the cellPhage binds to the surface of the cell2.2. Phage injects its DNA into the cellPhage injects its DNA into the cell3.3. Phage DNA is incorporated into the host cell’s Phage DNA is incorporated into the host cell’s

chromosome chromosome now known as a prophage now known as a prophage1.1. Phage DNA is “silent”Phage DNA is “silent”

4.4. Every time the bacterial cell prepares to divide, it Every time the bacterial cell prepares to divide, it replicates the viral genome in addition to its own DNA, replicates the viral genome in addition to its own DNA, passing on the viral genome to its daughter cellspassing on the viral genome to its daughter cells

5.5. One day, the prophage commands the host cell to One day, the prophage commands the host cell to manufacture phages and then self-destructmanufacture phages and then self-destruct

RetrovirusesRetroviruses Important retrovirus: Important retrovirus: HIVHIV RNA viruses in which genetic RNA viruses in which genetic

information flows information flows backwardsbackwards Reverse transcriptase:Reverse transcriptase:

An enzyme that can transcribe DNA from An enzyme that can transcribe DNA from an RNA templatean RNA template

This changes the cell’s genome! This changes the cell’s genome!

Viroids and PrionsViroids and Prions Viroids:Viroids:

Simple infectious molecules of naked Simple infectious molecules of naked RNARNA

Prions:Prions: Simple infectious proteinsSimple infectious proteins

Bacterial GeneticsBacterial Genetics Bacterial genome is a single double-Bacterial genome is a single double-

stranded DNA molecule arranged in a stranded DNA molecule arranged in a circle (circle (chromosomechromosome))

The chromosome is found within the The chromosome is found within the nucleoid region of a bacterial cellnucleoid region of a bacterial cell

Bacterial cells divide by Bacterial cells divide by binary fissionbinary fission This is an asexual processThis is an asexual process Rapid reproductionRapid reproduction Therefore, most bacteria in a colony are Therefore, most bacteria in a colony are

genetically identicalgenetically identical

Sources of Genetic DiversitySources of Genetic Diversityin Bacteriain Bacteria

MutationMutation TransformationTransformation

Alteration of a bacterial cell’s genotype by the Alteration of a bacterial cell’s genotype by the uptake of naked, foreign DNA from the uptake of naked, foreign DNA from the surrounding environmentsurrounding environment

This is what we did in lab!This is what we did in lab! TransductionTransduction

Phages transfer genes from one host cell to Phages transfer genes from one host cell to anotheranother

Conjugation and PlasmidsConjugation and Plasmids Direct transfer of genetic material between two Direct transfer of genetic material between two

bacterial cells that are temporarily joinedbacterial cells that are temporarily joined

TransposonsTransposons

Transposon:Transposon: Pieces of DNA that can move from Pieces of DNA that can move from

location to another in a cell’s genomelocation to another in a cell’s genome Often called “jumping genes”Often called “jumping genes”

OperonsOperons Structural genes:Structural genes:

Genes that code for polypeptidesGenes that code for polypeptides Operator:Operator:

Segment of DNA that controls the access of RNA Segment of DNA that controls the access of RNA polymerase to the structural genespolymerase to the structural genes

Promoter: Promoter: REVIEW!!REVIEW!!

OPERON: OPERON: The entire stretch of DNA required for enzyme production The entire stretch of DNA required for enzyme production

of a certain polypeptide – including the structural genes, of a certain polypeptide – including the structural genes, operator, and promoteroperator, and promoter

Repressor:Repressor: If the repressor is bound to the operator, the operon is If the repressor is bound to the operator, the operon is

blockedblocked Kind of “off/on” switch for operonKind of “off/on” switch for operon

Chapter 18Chapter 18

Genome Organization and Genome Organization and Expression in EukaryotesExpression in Eukaryotes

Cellular DifferentiationCellular Differentiation Each cell of a multicellular eukaryote Each cell of a multicellular eukaryote

expresses only a small fraction of its expresses only a small fraction of its genomegenome

50,000 – 100,000 genes in every 50,000 – 100,000 genes in every human cellhuman cell

Cellular differentiation:Cellular differentiation: The divergence in structure and function The divergence in structure and function

of different types of cells as they become of different types of cells as they become specialized during an organism’s specialized during an organism’s development and remain that waydevelopment and remain that way

The Structure of ChromatinThe Structure of Chromatin

Eukaryotic chromatin consists of DNA Eukaryotic chromatin consists of DNA and LOTS of proteinand LOTS of protein

If extended, a eukaryotic DNA If extended, a eukaryotic DNA molecule would be about 6 cm long… molecule would be about 6 cm long… WAY bigger than a cell’s nucleusWAY bigger than a cell’s nucleus

Lots of elaborate packing is required Lots of elaborate packing is required to fit the DNA into the nucleusto fit the DNA into the nucleus

The Structure of ChromatinThe Structure of Chromatin Proteins (called Proteins (called

histones)histones) are are responsible for the responsible for the first level of DNA first level of DNA packing in chromatinpacking in chromatin

NucleosomeNucleosome:: Looks like beads on a Looks like beads on a

stringstring Consists of DNA Consists of DNA

wound around wound around histoneshistones

The Structure of ChromatinThe Structure of Chromatin

Nucleosomes are Nucleosomes are then coiled into a then coiled into a fiber, which is then fiber, which is then folded into looped folded into looped domainsdomains

All this to save All this to save space! space!

Repetitive SequencesRepetitive Sequences 10 – 25 % of total DNA of 10 – 25 % of total DNA of

multicellular eukaryotes is made up multicellular eukaryotes is made up of short sequences repeated 1000s of short sequences repeated 1000s or 1,000,000s of timesor 1,000,000s of times

Known as Known as satellite DNAsatellite DNA

TelomeresTelomeres

Satellite sequences located at the ends of Satellite sequences located at the ends of chromosomeschromosomes

During replication of lagging strand, RNA During replication of lagging strand, RNA primers must be “laid down” primers must be “laid down” impossible impossible at the very end of a linear chromosomeat the very end of a linear chromosome

If this was not “dealt with” the If this was not “dealt with” the chromosome would get shorter every time chromosome would get shorter every time it was replicatedit was replicated

That’s the role of telomeres! That’s the role of telomeres! Analogy: ends of shoelacesAnalogy: ends of shoelaces

Gene AmplificationGene Amplification Def’n:Def’n:

The selective replication of certain genesThe selective replication of certain genes The number of copies of a gene or gene The number of copies of a gene or gene

family may temporarily increase in some family may temporarily increase in some tissues during a particular stage of tissues during a particular stage of developmentdevelopment

Example: Example: Ovum producing lots of extra rRNA so tons of Ovum producing lots of extra rRNA so tons of

ribosomes can be maderibosomes can be made

Cancer: It’s in the Genes?!Cancer: It’s in the Genes?! Cancer is caused by uncontrolled cell growth and Cancer is caused by uncontrolled cell growth and

division (mitosis)division (mitosis) Certain genes control cell growth and divisionCertain genes control cell growth and division These genes can be altered by environmental These genes can be altered by environmental

influences/mutations (sun, smoke, etc.)influences/mutations (sun, smoke, etc.) Oncogenes:Oncogenes:

Cancer-causing genesCancer-causing genes Proto-oncogenes:Proto-oncogenes:

Normal cellular genes that normally regulate cell growth Normal cellular genes that normally regulate cell growth and divisionand division

Tumor-suppressor genes:Tumor-suppressor genes: Genes that encode proteins that normally help prevent Genes that encode proteins that normally help prevent

uncontrolled cell growthuncontrolled cell growth

The Genetics of CancerThe Genetics of Cancer A genetic predisposition to cancer might be A genetic predisposition to cancer might be

inherited as a defect in a single tumor-inherited as a defect in a single tumor-suppressor genesuppressor gene

An individual inheriting such a recessive An individual inheriting such a recessive mutant allele will be one step closer to mutant allele will be one step closer to accumulating the necessary mutations that accumulating the necessary mutations that will result in abnormal cell growth and will result in abnormal cell growth and divisiondivision

Pre-screening/identification of these mutant Pre-screening/identification of these mutant alleles??alleles??

The Progression of The Progression of Colorectal CancerColorectal Cancer