t4 bacteriophage infecting an e. coli cell 0.5 m
TRANSCRIPT
T4 bacteriophage infecting an E. coli cell
0.5 m
Science as a Process Research into tobacco mosaic disease
led to the conclusion that the pathogen was smaller than a bacterial cell
The pathogen was named virus Characteristics of viruses:
Smaller than bacteria Not cellular Composed of nucleic acid and protein Obligate intracellular parasites
Comparing the size of a virus, a bacterium, and an animal cell 0.25 m
Virus
Animalcell
Bacterium
Animal cell nucleus
Infection by tobacco mosaic virus
Figure 18.4 Viral structure
18 250 mm 70–90 nm (diameter) 80–200 nm (diameter) 80 225 nm
20 nm 50 nm 50 nm 50 nm
(a) Tobacco mosaic virus (b) Adenoviruses (c) Influenza viruses (d) Bacteriophage T4
RNA
RNACapsomereof capsid
DNACapsomere
Glycoprotein Glycoprotein
Membranousenvelope
CapsidDNA
Head
Tail fiber
Tail sheath
Capsids and Envelopes
Capsid =
Capsids and Envelopes
Capsid = protein coat that surrounds the viral genome
viral envelope =
Capsids and Envelopes
Capsid = protein coat that surrounds the viral genome
viral envelope = derived from host cell or nuclear membranes, it helps the virus invade
Viral Genome
Double stranded DNA Single Stranded DNA Double stranded RNA Single stranded RNA A virus has only one of these types
of nucleic acids
Viral Replication What are the possible patterns of viral
replication? DNA --> DNA RNA --> RNA, where viral genes code for
viral RNA and proteins (class IV and V) RNA --> DNA --> RNA; where viral gene
uses reverse transcriptase to create a “provirus” in the nucleus that does not leave host cell…viral RNA and protein is also made (class VI)
Bacterial Viruses Which scientists used
bacteriophages to prove that DNA was the hereditary material?
Hershey and Chase What are the two mechanisms of
phage infection? Lytic and Lysogenic cycles (of
DNA viruses)
Lytic Cycle Virulent phage … example T4 phage Steps:
1. Attachment2. Entry of phage DNA and degradation of
host DNA3. Synthesis of viral genome and protein4. Assembly 5. Release … (host cell dies while releasing
100-200 phages)
Lysogenic Cycle Temperate phage … examplelambda phage Steps:
1. Entry2. Integration of viral DNA into bacterial chromosome
creating a prophage3. Bacterium reproduces normally copying prophage and
transmitting it to daughter cells4. Under certain environmental conditions, a switchover
to lytic cycle is triggered Other prophage genes may alter host’s
phenotype and have medical significance Ex. bacteria causing diphtheria is harmless unless
infected by a phage…phage experiences a lysogenic cycle and prophage causes host cell to make a toxin that causes illness!
Bacterial Defense
What defense do bacteria have against phage infection?
Restriction enzymes (a.k.a. restriction endonucleases)
What do restriction enzymes do? They cut up DNA. The bacterial
DNA is modified to protect it from the restriction endonucleases.
Animal Viruses
What is the viral envelope? An outer membrane (outside of
the capsid) that helps the virus to invade the animal cell.
The invasion of the virus has the following stages ...
1. Attachment
2. Entry
3. Uncoating
4. RNA and protein synthesis
5. Assembly and release
Herpes virus Consists of double stranded DNA Envelope derived from host cell nuclear
envelope not from plasma membrane It, therefore, reproduces within the nucleus May integrate its DNA as a provirus
(becoming like mini-chromosomes in nucleus)
Tends to recur throughout lifetime of infected individual. Often triggered by environmental situations.
RNA Viruses Different classes of RNA viruses:
single stranded range from class IV to class VI
Class IV: invades as mRNA, is ready for translation
Class V: RNA serves as template for mRNA synthesis
Class VI: Retrovirus RNA DNA (using enzyme reverse transcriptase) RNA
The structure of HIV, the retrovirus that causes AIDS
Reversetranscriptase
Viral envelope
Capsid
Glycoprotein
RNA(two identicalstrands)
The reproductive cycle of HIV, a retrovirus
Vesicles transport theglycoproteins from the ER tothe cell’s plasma membrane.
7
The viral proteins include capsid proteins and reverse transcriptase (made in the cytosol) and envelope glycoproteins (made in the ER).
6
The double-stranded DNA is incorporatedas a provirus into the cell’s DNA.
4
Proviral genes are transcribed into RNA molecules, which serve as genomes for the next viral generation and as mRNAs for translation into viral proteins.
5
Reverse transcriptasecatalyzes the synthesis ofa second DNA strandcomplementary to the first.
3
Reverse transcriptasecatalyzes the synthesis of aDNA strand complementaryto the viral RNA.
2
New viruses budoff from the host cell.9
Capsids areassembled aroundviral genomes and reverse transcriptase molecules.
8
mRNA
RNA genomefor the nextviral generation
Viral RNA
RNA-DNAhybrid
DNA
ChromosomalDNA
NUCLEUSProvirus
HOST CELL
Reverse transcriptase
New HIV leaving a cell
HIV entering a cell
0.25 µm
HIV Membrane of white blood cell
The virus fuses with thecell’s plasma membrane.The capsid proteins areremoved, releasing the viral proteins and RNA.
1
Reasons for success of HIV
Has an envelope Creates a provirus which stays in
the nucleus of the host cell Is an RNA virus…high rate of
mutation
Viral Disease
Some viruses have toxic components
Some cause infected cells to release enzymes from lysosomes
Recovery involves ability to repair damaged region of the body.
Vaccines / Drugs
What are vaccines and how do they work? Introduce body to harmless or weakened
strain of the virus, so that your immune system learns to recognize the virus prior to invasion
Few drugs around to fight viruses, most interfere with DNA, RNA or protein synthesis Often mimic nucleosides that would allow for
nucleic acid synthesis Ex. AZT HIV replication
acyclovor herpes
Emerging Viruses
HIV, Ebola, SARS, West Nile Virus, Influenza, Hantavirus
From where do these viruses emerge? From mutated versions of current
viruses Jump from current host to new host Move from a previously isolated region
of the world
SARS (severe acute respiratory syndrome)
(a) Young ballet students in Hong Kong wear face masks to protect themselves from the virus causing SARS.
(b) The SARS-causing agent is a coronavirus like this one (colorized TEM), so named for the “corona” of glycoprotein spikes protruding from the envelope.
Viroids and Prions
Viroids are naked circular RNA that infect plants Prions are proteins that infect cells (cause tangles of
proteins in brain) Examples of prions seen in scrapies in sheep, mad-cow
disease, and Creutzfeldt-Jakob disease (CJD) in humans Timeline of Mad Cow Disease Outbreaks
How can a prion spread infection? Altered versions of proteins that can alter other
proteins (altered protein is thought to be a result of a mutated gene)
Or can be ingested by eating contaminated meats…
Figure 18.13 Model for how prions propagate
Prion
Normalprotein
Originalprion
Newprion
Many prions
Viral Evolution How did viruses evolve? Because viruses depend on cells for
their own reproduction, they most likely evolved after the first cells appeared.
Possible link to mobile genetic elements. (transposons, plasmids)
Much debate about viral evolution…lots to learn about viruses!