T4 bacteriophage infecting an E. coli cell
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T4 bacteriophage infecting an E. coli cell
0.5 m
Science as a Process
Research into TMV led to the conclusion that the pathogen was smaller than a bacterial cell
The pathogen was named virus
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
A virus has either DNA or RNA for its genome, but not both and has no mechanism for protein synthesis (ribosomes) or metabolism (mitochondria)
It also has a protein coat called a capsid. Some have a coat made of glycoproteins.
A virus can reproduce only within a host and not on its own.
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 the host cell
Figure 18.02x2 Phages
Figure 18.9 Viral infection of plants
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
Table 18.1 Classes of Animal Viruses, Grouped by Type of Nucleic Acid
Figure 18.6 The reproductive cycle of an enveloped virus
Viral Replication
What are the possible patterns of viral replication?
DNA --> DNA RNA --> RNA, where viral
genes code for RNA replicase RNA --> DNA --> RNA; where
viral gene uses reverse transcriptase
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
Figure 18.4 The lytic cycle of phage T4
Figure 18.5 The lysogenic and lytic reproductive cycles of phage , a temperate phage
Bacterial Defense What defense do bacteria
have against phage infection?
Restriction enzymes 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 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
Herpesvirus 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
Tends to recur throughout lifetime of infected individual.
Figure 18.x6 Herpes
The structure of HIV, the retrovirus that causes AIDS
Reversetranscriptase
Viral envelope
Capsid
Glycoprotein
RNA(two identicalstrands)
Figure 18.7x1 HIV infection
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
Viral Evolution
What is the current hypothesisconcerning how viruses
evolved?
Viral Disease Some viruses have toxic components and cause infected cells to release enzymes from lysosomes
Recovery involves ability to repair damaged region of the body. Ex: polio may permanently damage nerve cells.
Figure 18.x3 Polio
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
Smallpox
Emerging Viruses HIV Ebola Influenza 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
Deer Mouse – Vector of Hantavirus
SARS (severe acute respiratory syndrome), a
recently emerging viral disease
(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 Examples of prions seen in
Scrapies in sheep, mad-cow disease, and Creutzfeldt-Jakob disease in humans
How can a prion spread infection? Altered versions of proteins that
can alter other proteins
Model for how prions propagate
Prion
Normalprotein
Originalprion
Newprion
Many prions
Replication of the Bacterial Chromosome
E. coli
E. coli Dividing
Transformation of Bacteria - Griffith
Bacterium releasing DNA with plasmids
Plasmids
Transduction
Bacterial Mating
“Male” with the F factor
Conjugation and recombination in E. coli
Viruses and Cancer Hepatitus B virus can cause
liver cancer Some viral genes can trigger
cancerous genetic conditions Oncogenes = viral genes that
trigger cancerous characteristics
proto-oncogenes = genes already found in normal cells, usually regulate growth factors
Insertion sequences, the simplest transposons
Insertion of a transposon and creation of direct repeats
Anatomy of a Composite Transposon