micro lect 1 2011 colour 2 slides per page

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3/17/20

Lecture 1

BIOSCI 101: Microbiology

and Bacteria

The evolution of prokaryotes• Prokaryotic organisms (Bacteria and Archaea)

lack a nuclear membrane and do not havemembrane-bound organelles such as

• The first microorganisms probably appearedwithin a billion years of Earth’s formation andwould have been adapted to face hot,anaerobic conditions


• The cyanobacteria are thought to be the firstO2-producing photosynthetic organisms andtheir evolution was crucial to the later evolutionof plant and animal life on Earth



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BIOSCI 101: MicrobiologyMicrobial Life, Perry et al ; 2002

Bacterial cultures• Early microbiologists cultured their microbes on

potato slices and later media solidified withgelatin. Agar plates were first used in 1887.

• Many bacteria adhere to each other and solidsurfaces via pili and fimbriae forming surface-coating colonies known as biofilms

• Bacteria in biofilms secrete signalling moleculesthat recruit neighbouring cells and recycle eachother’s wastes and nutrients


• the surfaces of implanted medical devices andare more resistant to antibiotics in suchcommunities



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Common shapes of prokaryotes

BIOSCI 101: Cellular and Molecular Biology(Campbell 27.2)

Fimbriae and pili enable bacteria toattach to surfaces and each other

Fimbriae

BIOSCI 101: Cellular and Molecular Biology(Campbell 27.5)

1 m



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The genomes of prokaryotes

• The DNA of prokaryotes is most often a circular,double-stranded, helical molecule typically of

• Prokaryotic DNA is tightly wound into supercoilsin order to package it inside the cell

• Bacteria often contain other extrachromosomal


.genes carried on plasmids generally are not

essential but may enhance the survival ofbacteria that carry them

A prokaryotic chromosome

BIOSCI 101: MicrobiologyCampbell 27.8



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Structure of viruses

• Viruses are infectious particles consisting ofnucleic acid (DNA or RNA) enclosed in a proteincoat known as the viral capsid

• The capsid is constructed from a number ofindividual subunits called capsomeres and maybe rod-shaped, polyhedral (round) or morecomplex in shape (eg the T phages of bacteria)

• In some viruses a membraneous envelopestudded with viral glycoproteins surrounds the


.host cell in which the virus was made.

• Viruses that infect bacteria are calledbacteriophages. The T-even phages of E. colihave an icosahedral head and attach to thebacteria via protein tail fibres

Viral structure

Capsomereof capsid

RNA Capsomere

DNA

Membranousenvelope RNA

Capsid

Head

DNA

Tailsheath

Glycoprotein Glycoproteins

Tailfiber

18 250 nm 80 225 nm70–90 nm (di ameter ) 80–200 nm (diameter)


20 nm 50 nm 50 nm 50 nm(a) Tobacco

mosaic virus(b) Adenoviruses (c) Influenza viruses (d) Bacteriophage T4



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Viral reproductive cycles

• Viruses can reproduce only within a host cellbecause they lack metabolic enzymes,ribosomes and other cell organelles

• The host range of a virus depends oninteraction between proteins on the surface ofthe virus and the susceptible cell

• Once a virus binds to the cell surface, the viral


genome mus en er e os ce . anybacteriophages inject their DNA via their tail

structures while most animal and plant virusesuncoat the genome after the whole virus hasentered the cell.

Viral reproductive cycles (contd)

• Host cell enzymes then produce many copies ofthe viral genome and together with host cellribosomes, produce viral proteins

• The viral genomes and capsid proteins self-assemble to form new viral particles

• The progeny virus particles are released from


the infected cell and then spread to infectneighbouring cells



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A simplified

viralreproductive

cycle


Lytic replication of T4 phages• The phage attaches to receptors on the

bacterial cell via the tail fibre proteins

• Phage DNA is injected into the bacterial cell.The bacterial DNA is hydrolysed via the actionof a phage-encoded nuclease enzyme

• Phage genomes and proteins are produced byhost cell replication enzymes and translationmachinery

• The viral proteins self-assemble to form phage


, .assembles the phage DNA is packaged inside it.

• A phage-encoded enzyme degrades thebacterial cell wall resulting in rupture (lysis) ofthe infected cell and release of 100-200 phageparticles



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The lytic cycle of phage T4 Attachment

2

1

Entry of phageDNA andde radat ion

5

of host DNA

Release

Phage assembly


43 Synthesis of

viral genomesand proteins

Assembl y

Head Tail Tailfibers

Phage λ – lytic vs lysogenic

replication

• Phage λ is an example of a phage that canundergo lytic replication or alternativelyreplicate in a bacterial cell without killing it

• Such phages are called temperate phages andtheir non-lytic replication is described aslysogenic

• Lysogeny is established when the λ DNAgenome integrates into a specific site in the


• The integrated λ DNA is known as a prophage.One of the prophage genes codes for arepressor protein that prevents transcription ofmost of the other prophage genes



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Phage λ – lytic vs lysogenic

replication (contd)

• Each time the host E. coli cell divides, thero ha e DNA is re licated alon with the

bacterial chromosome

• A single infected cell soon gives rise to a largepopulation of cells carrying the integratedprophage DNA

• In response to environmental signals such asUV radiation, the λ genome exits the bacterial


, -cycle of replication that kills the host cell

Phage

PhageDNA

The phage

injects its DNA.

Phage DNA

circularizes.

The lyticcycle ofha e λ

The cell lyses, releasing phages.

ac er a

chromosome

Lytic cycle

lytic cycle

is inducedor

Certain factors

determine whether

lysogenic cycle

is entered


New phage DNA and proteins

are synthesized and assembledinto phages.



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Phage DNAcircularizes.

Daughter cell

with prophage

Occasionally, a prophage

Cell divisions

produce a

population of bacteria infectedwith the prophage.

Thelysogenic

cycle ofphage λ

lytic cycle

is inducedor

Certain factors

determine whether

lysogenic cycle

is entered

Lysogenic cycle

Prophage

exits the bacterial chromosome,

initiating a lytic cycle.

The bacterium reproduces,

copying th e prophage andtransmitting it to daughter

cells.

BIOSCI 101: MicrobiologyBIOSCI 101: MicrobiologyCampbell 19.6

Phage DNA integrates into

the bacterial chromosome,becoming a proph age.

HIV – an example of a retrovirus• Viral genomes may be double-stranded or

single-stranded DNA or RNA

• DNA viruses often use the host cell DNApolymerase to replicate their genome but RNAviruses must contain a gene that encodes theirown RNA-dependent RNA polymerase

• Retroviruses reproduce their genome by firstmaking a DNA copy using a virally encodedreverse transcriptase enzyme


retrovirus that causes AIDS. The virus particlehas a ssRNA genome and also contains copiesof a viral reverse transcriptase enzyme



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The structure of HIV


Replication of HIV• Glycoproteins on the surface of HIV bind toreceptors on human T cells

• After the virus has entered the T cell, the viralreverse transcri tase enz mes are released into thecytoplasm where they catalyse synthesis of DNAcopies of the viral genome

• The HIV DNA then enters the nucleus and integratesinto the host cell chromosome ie it becomes aprovirus

• Transcription of the proviral DNA provides new


production of viral proteins

• Immature viral particles assemble in the cytoplasmand mature by “budding” from the plasmamembrane, acquiring viral glycoproteins and a viralenvelope in the process



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Thereproductivecycle of HIV


Viruses and disease• Disease symptoms often reflect the impacts of viralreplication in infected cells eg destruction ofepithelial cells by herpes viruses

• Viral diseases may spread by different routes:

- faecal-oral eg rotavirus, polio

- respiratory eg influenza, chicken pox

- contact/abrasions eg HIV

- arthropod vectors eg Dengue fever


• Antiviral drugs typically target viral proteins

• Vaccines have been used successfully to eradicatesome viral diseases eg smallpox



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Viruses and disease (contd)

• Viral diseases in plants have enormous financialconsequences because of the damage they cause toagricultural and horticultural crops

• any p an v ruses ave genomes anassemble as rod-shaped particles (eg tobaccomosaic virus or TMV)

• Plant viruses spread by two major routes:

– horizontal transmission from an external sourceeg insect vectors


–plants (eg cuttings or infected seeds)

• Once inside a plant cell, plant viruses can spreadrapidly throughout the plant via the plasmodesmata

Viral infections in plants

BIOSCI 101: MicrobiologyCampbell 18.3 (7th Edition)

Campbell 19.10



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Emerging viruses

• New viral diseases can suddenly appear as a resultof:

– mutation/recombination of existing virusesespec a y v ruses

– spread of existing viruses to a new host species

• SARS (Severe Acute Respiratory Syndrome) is arecent example of disease caused by an emergentvirus


• Will bird flu be next??

SARS – a recently emergent

viral disease

BIOSCI 101: MicrobiologyCampbell 18.11 (7th Edition)



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Where will thenext pandemic

come from?

BIOSCI 101: MicrobiologyBIOSCI 101: MicrobiologyCampbell 19.9 (8th and 9th edns)

micro lect 1 2011 colour 2 slides per page

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