Biology 331- Chapter 11 Ppt

Chapter 11Lecture Outline

Viral Molecular Biology

Microbiology: An Evolving Science © 2009 W. W. Norton & Company, Inc. 2

Outline

11.1 Phage T4: The classic Molecular Model

11.4 A segmented (-) Strand RNA Virus: Influenza

11.5 A Retrovirus: Human Immunodeficiency Virus

Note: We will not cover 11.2, 11.3, 11.6!


Prokaryotic Viruses Must bind to host cell receptor Must cross a cell wall

In gram hosts 2 membranes to crossMust not damage host cell initially

Use host nucleotides, amino acids, ATPReplicate viral genome, build capsid,

assemble new viruses Exit through cell wall

Usually lyse host cells


Bacteriophage T4

Complicated structure170 genes10 different capsid protein

typesTail fibers bind host cell

Receptor = OmpC porin Outer membrane protein

Long tail injects DNA


Phage T4 Adsorption and DNA Injection

Tail fibers bind to outer membrane (ompC, LPS).

Baseplate binds outer membrane

Tail sheath contracts, internal tube penetrates outer membrane

Injector digests peptidoglycan; internal tube penetrates cell wall

Injector contacts inner membrane; DNA expelled into cytoplasm.


Phage T4 Genome Replication Upon entry, genome forms a circle Early genes transcribed

Take control of cell, destroy cell chromosomeReplicates via Complicated structure

Uses cell nucleotides to replicate genome“Rolling circle replication”

Continuous replication of many copies of genome


Rolling Circle

Concatemer


Phage Particles Self-Assemble Late genes transcribed

Capsid particles Head polymerizes around progeny DNA Tail fibers, long tail made Head, tail, tail fibers assemble Lysis protein made

Destroys cell wall Releases progeny


Summary Phage T4 Replication1. The phage particle attaches to the outer

membrane of E. coli and injects its genome.

2. Early genes are transcribed and translated into proteins, including nucleases to cleave host DNA and proteins for phage DNA replication.

3. Phage DNA undergoes rolling-circle replication, generating a multigenome concatemer.

4. Late genes are expressed to make head and tail components.

5. Phage genomes are packaged into heads.

6. Heads are assembled onto tails.7. Tail fibers are added.8. A phage-encoded lysozyme lyses the host cell,

releasing about 200 completed phage particles.


Animal Viruses Simpler entry into cell

Don’t have to cross a cell wallCan enter through endocytosis

Or fusion of viral envelope to plasma membrane

More complex cycle in the cellsMust travel between organelles

Host transcription machinery in nucleus Translation in cytoplasm Transport via ER, Golgi, endosomes Exit via cell lysis or budding


Viral mRNA Prokaryotes: multiple proteins from 1 mRNA

OperonViral genome can be RNA or DNA

Multiple proteins created

Eukaryotes: 1 mRNA 1 proteinViral DNA genome gives multiple mRNAsRNA virus can create only 1 protein

Solutions: 1 protein cuts itself into smaller proteins (polio) Multiple RNAs in a single virion (influenza) RNA used to make DNA multiple proteins (HIV)


(-) Strand RNA Virus: Influenza Pandemic of 1918

Greatest one-year loss of life in recorded history

RNA inside shell of matrix proteins Inside lipid envelope2 major envelope proteins

Neuraminidase Hemagglutinin


(-) Strand RNA Virus: Influenza 8 separate RNAs in genome

Each encodes one protein (-) strand RNA can’t be read by ribosome

Must be transcribed to (+) RNANo replicating RNA polymerase in hostAll (-) RNA viruses must bring own polymerase protein into host cell

Influenza binds to host sialic acids


(-) Strand RNA Virus: Influenza (-) strand RNA moves to nucleus (+) strand mRNA synthesized

Move to cytoplasm Viral proteins made

Envelope proteins placed in plasma membrane (+) strand used to make progeny (-) RNA Assembly at plasma membrane Budding to release progeny viruses


(-) Strand RNA Virus: Influenza Viral RNA polymerases are inaccurate

Introduce many mutations Antigenic drift

Rapid evolution New flu virus species every year New vaccine necessary

Cell infected by 2 strains can recombine Assemble new combinations of RNAs

Reassortment = antigenic shift


Human Immunodeficiency Virus Bullet-shaped capsid

Encloses 2 identical copies of RNAPlus polymerase proteins

Surrounded by envelopeEnvelope proteins embedded

Binds to receptorCD4 protein

Immune system T cells Microglia cells in brain

AIDS-related dementia


Human Immunodeficiency Virus Viral envelope fuses with plasma membrane Genome released into cytoplasm

Polymerase replicates DNA from RNA

Reverse transcriptase RNA destroyed as DNA made Forms dsDNA

Acts as transposon Moves to nucleus Inserts into host genome

(+) ssRNA

(+) ssDNA

dsDNA

Integration into host genome


Human Immunodeficiency Virus Reverse transcriptase is inaccurate

Introduces many mutations Mutant viruses form within single individual

Single vaccine will not protect against all forms 5 major forms of HIV in humans

Many minor forms in each individual as disease progresses

From integrated DNA RNA is transcribed RNA is used for progeny RNA, envelope

and capsid proteins Assembles in cytoplasm Exits via budding


Retroviruses Replicate transposon DNA

Insert repeatedly in chromosome

Destroy host cell

Eukaryotic cells have many retroelementsOld, inactivated retrovirus copiesPresence of many copies allows recombination

Movement of sections of chromosome Moves pieces of genes together to make new genes Major mechanism of evolution


Concept QuizAnimal (-) strand RNA viruses always carry into the host cytoplasm their own:

a. genome and polymerase protein

b. genome and envelope proteins

c. genome and capsid proteins


Concept QuizWhy is it difficult to make a vaccine against HIV?

a. The virus mutates quickly inside an infected individual.

b. The envelope proteins cannot be isolated in the laboratory.

c. Different strains of HIV recombine to make new virus types.

Documents

Biology 331- Chapter 11 Ppt