Download - Viruses and Bacteria
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Viruses and Bacteria
What you need to Know
Plus
Gene Regulation
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Phage and Bacteria
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Virus
Bacteria
Animal Cell
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Structure of Viruses
Viruses are not cells Viruses are very small infectious
particles consisting of nucleic acid enclosed in a protein coat and, in some cases, a membranous envelope
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Capsids and Envelopes
A capsid is the protein shell that encloses the viral genome
A capsid can have various structures
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Some viruses have structures have membranous envelopes that help them infect hosts
These viral envelopes surround the capsids of influenza viruses and many other viruses found in animals
Viral envelopes, which are derived from the host cell’s membrane, contain a combination of viral and host cell molecules
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General Features of Viral Reproductive Cycles Viruses are obligate intracellular
parasites, which means they can reproduce only within a host cell
Each virus has a host range, a limited number of host cells that it can infect
Viruses use enzymes, ribosomes, and small host molecules to synthesize progeny viruses
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Reproductive Cycles of Phages
Phages are the best understood of all viruses
Phages have two reproductive mechanisms: the lytic cycle and the lysogenic cycle
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The Lytic Cycle
The lytic cycle is a phage reproductive cycle that culminates in the death of the host cell
The lytic cycle produces new phages and digests the host’s cell wall, releasing the progeny viruses
A phage that reproduces only by the lytic cycle is called a virulent phage
Bacteria have defenses against phages, including restriction enzymes that recognize and cut up certain phage DNA
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LE 18-6
Attachment
Entry of phage DNAand degradation of host DNA
Synthesis of viralgenomes and proteins
Assembly
ReleasePhage assembly
Head Tails Tail fibers
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The Lysogenic Cycle
The lysogenic cycle replicates the phage genome without destroying the host
The viral DNA molecule is incorporated by genetic recombination into the host cell’s chromosome
This integrated viral DNA is known as a prophage Every time the host divides, it copies the phage DNA and
passes the copies to daughter cells Phages that use both the lytic and lysogenic cycles are
called temperate phages Go to video
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LE 18-7
Phage
Phage DNA
The phage attaches to ahost cell and injects its DNA.
Phage DNAcircularizes
Bacterial chromosome
Lytic cycle
The cell lyses, releasing phages.Lytic cycleis induced
or Lysogenic cycleis entered
Certain factorsdetermine whether
Lysogenic cycle
Occasionally, a prophageexits the bacterial chromosome,initiating a lytic cycle.
The bacterium reproducesnormally, copying the prophageand transmitting it to daughter cells.
Prophage
Many cell divisionsproduce a large population of bacteria infected withthe prophage.
Daughter cellwith prophage
Phage DNA integrates into thebacterial chromosomes, becoming aprophage.
New phage DNA and proteins aresynthesized and assembled into phages.
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Viroids and Prions: The Simplest Infectious Agents
Viroids are circular RNA molecules that infect plants and disrupt their growth
Prions are slow-acting, virtually indestructible infectious proteins that cause brain diseases in mammals
Prions propagate by converting normal proteins into the prion version
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LE 18-13
Normalprotein
New prion
Prion Original prion
Many prions
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The Bacterial Genome and Its Replication The bacterial chromosome is usually a
circular DNA molecule with few associated proteins
Many bacteria also have plasmids, smaller circular DNA molecules that can replicate independently of the chromosome
Bacterial cells divide by binary fission, which is preceded by replication of the chromosome
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LE 18-14
Origin ofreplication
Replication fork
Termination of replication
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Mutation and Genetic Recombination as Sources of Genetic Variation
Since bacteria can reproduce rapidly, new mutations quickly increase genetic diversity
More genetic diversity arises by recombination of DNA from two different bacterial cells
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Mechanisms of Gene Transfer and Genetic Recombination in Bacteria Three processes bring bacterial DNA from
different individuals together: Transformation-Transformation is the alteration of a
bacterial cell’s genotype and phenotype by the uptake of naked, foreign DNA from the surrounding environment (Griffith)
Transduction -In the process known as transduction, phages carry bacterial genes from one host cell to another
Conjugation -Conjugation is the direct transfer of genetic material between bacterial cells that are temporarily joined (Pili)
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Transposition of Genetic Elements
The DNA of a cell can also undergo recombination due to movement of transposable elements within the cell’s genome
Transposable elements, often called “jumping genes,” contribute to genetic shuffling in bacteria
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Transposons
Transposable elements called transposons are longer and more complex than insertion sequences
In addition to DNA required for transposition, transposons have extra genes that “go along for the ride,” such as genes for antibiotic resistance
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LE 18-19b
53
35
Transposing
Insertion sequence
Insertion sequence
Antibioticresistance gene
Transposase geneInverted repeat
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Repressible and Inducible Operons: Two Types of Negative Gene Regulation
A repressible operon is one that is usually on; binding of a repressor to the operator shuts off transcription
The trp operon is a repressible operon An inducible operon is one that is usually off; a molecule
called an inducer inactivates the repressor and turns on transcription
The classic example of an inducible operon is the lac operon, which contains genes coding for enzymes in hydrolysis and metabolism of lactose
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LE 18-22a
DNA lacl
Regulatorygene
mRNA
5
3
RNApolymerase
ProteinActiverepressor
NoRNAmade
lacZ
Promoter
Operator
Lactose absent, repressor active, operon off
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LE 18-22b
DNA lacl
mRNA5
3
lac operon
Lactose present, repressor inactive, operon on
lacZ lacY lacA
RNApolymerase
mRNA 5
Protein
Allolactose(inducer)
Inactiverepressor
-Galactosidase Permease Transacetylase
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Inducible enzymes usually function in catabolic pathways
Repressible enzymes usually function in anabolic pathways
Regulation of the trp and lac operons involves negative control of genes because operons are switched off by the active form of the repressor
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Positive Gene Regulation
Some operons are also subject to positive control through a stimulatory activator protein, such as catabolite activator protein (CAP)
When glucose (a preferred food source of E. coli ) is scarce, the lac operon is activated by the binding of CAP
When glucose levels increase, CAP detaches from the lac operon, turning it off
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LE 18-23a
DNA
cAMP
lacl
CAP-binding site
Promoter
ActiveCAP
InactiveCAP
RNApolymerasecan bindand transcribe
Operator
lacZ
Inactive lacrepressor
Lactose present, glucose scarce (cAMP level high): abundant lacmRNA synthesized
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LE 18-23b
DNA lacl
CAP-binding site
Promoter
RNApolymerasecan’t bind
Operator
lacZ
Inactive lacrepressor
InactiveCAP
Lactose present, glucose present (cAMP level low): little lacmRNA synthesized
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LE 19-2a
DNA double helix
Histonetails
His-tones
Linker DNA(“string”)
Nucleosome(“bead”)
10 nm
2 nm
Histone H1
Nucleosomes (10-nm fiber)
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LE 19-2b
30 nm
Nucleosome30-nm fiber
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LE 19-2c
300 nm
Loops
Scaffold
Protein scaffold
Looped domains (300-nm fiber)
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Concept 19.2: Gene expression can be regulated at any stage, but the key step is transcription All organisms must regulate which genes
are expressed at any given time A multicellular organism’s cells undergo
cell differentiation, specialization in form and function
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Differential Gene Expression
Differences between cell types result from differential gene expression, the expression of different genes by cells within the same genome
In each type of differentiated cell, a unique subset of genes is expressed
Many key stages of gene expression can be regulated in eukaryotic cells
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Regulation of Chromatin Structure
Genes within highly packed heterochromatin are usually not expressed
Chemical modifications to histones and DNA of chromatin influence both chromatin structure and gene expression
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Histone Modification
In histone acetylation, acetyl groups are attached to positively charged lysines in histone tails
This process seems to loosen chromatin structure, thereby promoting the initiation of transcription
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LE 19-4
Histonetails
Amino acidsavailablefor chemicalmodification
DNAdouble helix
Histone tails protrude outward from a nucleosome
Acetylation of histone tails promotes loose chromatinstructure that permits transcription
Unacetylated histones Acetylated histones
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DNA Methylation
DNA methylation, the addition of methyl groups to certain bases in DNA, is associated with reduced transcription in some species
In some species, DNA methylation causes long-term inactivation of genes in cellular differentiation
In genomic imprinting, methylation turns off either the maternal or paternal alleles of certain genes at the start of development