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