Viruses and Bacteria

What you need to Know

Plus

Gene Regulation

Phage and Bacteria

Virus

Bacteria

Animal Cell

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

Capsids and Envelopes

A capsid is the protein shell that encloses the viral genome

A capsid can have various structures

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

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

go to video

Reproductive Cycles of Phages

Phages are the best understood of all viruses

Phages have two reproductive mechanisms: the lytic cycle and the lysogenic cycle

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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Attachment

Entry of phage DNAand degradation of host DNA

Synthesis of viralgenomes and proteins

Assembly

ReleasePhage assembly

Head Tails Tail fibers

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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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.

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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Normalprotein

New prion

Prion Original prion

Many prions

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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Origin ofreplication

Replication fork

Termination of replication

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

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)

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

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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53

35

Transposing

Insertion sequence

Insertion sequence

Antibioticresistance gene

Transposase geneInverted repeat

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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DNA lacl

Regulatorygene

mRNA

5

3

RNApolymerase

ProteinActiverepressor

NoRNAmade

lacZ

Promoter

Operator

Lactose absent, repressor active, operon off

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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

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

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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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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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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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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30 nm

Nucleosome30-nm fiber

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300 nm

Loops

Scaffold

Protein scaffold

Looped domains (300-nm fiber)

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

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

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

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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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

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