Molecular Biology Chapter 19 Outline

Front 1

Obligate Intracellular Parasites

Back 1

Infectious particles that function only when existing inside a living cell.

Front 2

Nucleic acid (DNA or RNA)

Back 2

surrounded by a protein coat.
20 – 200 nm – smaller than most organelles

Front 3

Viral Genome (virus dependent)

Back 3

a) Double-stranded DNA or RNA.
b) Single-stranded DNA or RNA.
c) 4 – 1,000 genes.

Front 4

Capsid

Back 4

Protein shell that encloses a viral
genome; may be rod-shaped, polyhedral, or more
complex in shape.

Front 5

Capsomere

Back 5

Protein subunit of a capsid.

Front 6

Envelopes

Back 6

Membrane derived from membranes of the host cell that cloaks the capsid, which in turn encloses a viral genome.

Front 7

Helical capsids

Back 7

i. Arranged in a helical shape.
ii. May contain thousands of molecules, but all of a single type protein.

Front 8

Icosahedral capsids

Back 8

i. 252 identical protein molecules arranged in a polyhedral capsid with 20 triangular facets.
ii. Infect the respiratory tract of animals

Front 9

Viral envelopes

Back 9

i. Derived from host cell membranes.
ii. Contains host cell phospholipids and proteins.
iii. Contains viral proteins and glycoproteins.

Front 10

Bacteriophages

Back 10

i. Most complex capsids.
ii. Elongated icosahedral heads with DNA.
iii. Protein tailpiece with fibers for attachment to a bacterium.

Front 11

Replicative Cycles

Back 11

1. Viruses lack metabolic enzymes and organelles like ribosomes for making proteins.
2. Viruses can only infect a limited number of hosts.
3. In multicellular hosts, viruses can only infect a limited number of cells.
4. Viruses seek out cells with receptor sites to which they can attach: Measles seek out skin cells Hepatitis seeks out liver cells - Mumps seek out salivary glands - Herpes seek out nerve cells

Front 12

Viral Penetration

Back 12

a) Virus binds to host cell and the viral genome enters the host (virus/host cell specific).
Bacteriophages inject their DNA into a bacterium-Certain viruses are taken up by endocytosis -Enveloped viruses fuse with the host cell’s plasma membrane.
b) Protein encoded by the virus takes over the host, reprogramming the host to copy viral nucleic
acid in order to produce viral proteins. -Host provides enzymes and nucleotides for viral nucleic
acids (DNA & RNA). -Host provides enzymes, ribosomes, tRNA, amino acids, and ATP for protein synthesis.

Front 13

Lytic cycle

Back 13

Replicative cycle resulting in the release of new phages by lysis (and death) of the host cell.

Front 14

Virulent phage

Back 14

Phage the replicates only by the lytic cycle.

Front 15

Bacterial Defenses

Back 15

i. Natural selection alters receptors that are un-recognizable by phages.
ii. Restriction enzymes limit the activity of the phage by cutting up its DNA upon entry.
iii. Phages evolve to coexist with the host bacterium (lysogeny).

Front 16

Lysogenic cycle

Back 16

Replicative cycle in which the viral genome becomes incorporated into the bacterial host chromosome, replicating with the chromosome without killing the host.

Front 17

Temperate phage

Back 17

Phage that is capable of replicating by either the lytic or lysogenic cycle.

Front 18

Viral Construction

Back 18

a) After viral nucleic acids and capsomeres are produced, they spontaneously assemble into new viruses.
b) Hundreds to thousands of viruses emerge from the cell, usually destroying the cell in the process.

Front 19

Prophage

Back 19

phage genome that has been inserted into a specific site on a bacterial chromosome.
a) Prior to binary fission, the bacterium replicates phage DNA, passing it onto its daughter cells.
b) A switch occurs during the lysogenic cycle converting to the lytic cycle, releasing the phages
c) This allows for large number of phages to be produced prior to killing the host.

Front 20

Viral envelope

Back 20

Membrane derived from the membranes of the host cell, cloaking the
capsid which in turn encloses a viral genome.
a) Glycoproteins on the viral envelope bind to receptors on the host cell gaining entry.
b) Using the host cell’s machinery, viral genome RNA, capsid proteins, and glycoproteins are replicated and assembled.
c) The new virus buds from the host cell’s membrane.

Front 21

Viral Envelope

Back 21

a

Front 22

RNA as Viral Genetic Material

Back 22

Retrovirus
Reverse transcriptase
Provirus

Front 23

Retrovirus

Back 23

RNA virus that replicates by transcribing its RNA into DNA then inserting the DNA into a cellular chromosome.

Front 24

Reverse transcriptase

Back 24

Enzyme that uses RNA as template for DNA synthesis.

Front 25

Provirus

Back 25

Viral genome that is permanently inserted into a host genome (example: HIV).

Front 26

Evolution of Viruses

Back 26

1. Viruses are biologically inert unless existing inside a living organism.
2. Viruses have infected every type of living organism, from bacteria to humans.
3. Viruses probably evolved after the first cells appeared, from plasmids or transposons.

Front 27

Plasmids

Back 27

Small, circular DNA molecules found in bacteria and yeast. Exist apart from the cell’s genome. Replicated independently of the genome.Transferred between cells.

Front 28

Transposons

Back 28

DNA segments that can move from location to another with the cell’s genome.

Front 29

Mimivirus (mimicking microbe)

Back 29

Double-stranded DNA virus with an icosahedral capsid that is 400 nm in diameter (bacterium size) and a 1.2 million-base genome comprising 1,000 genes. Some genes code for translation, DNA repair, protein folding, and polysaccharide synthesis.

Front 30

Viruses can cause varying degrees of damage during infection.

Back 30

1. Damage or destroy cells causing the release of hydrolytic enzymes.
2. Cause production of toxins from infected cells or molecular components that are toxic.

Front 31

Organisms react differently according to the type of viral infection.

Back 31

1. Rhinovirus (common cold) – epithelium lining of respiratory tract affected, but repairs itself.
2. Poliovirus (polio) – neurons affected are permanently damaged.

Front 32

Vaccine

Back 32

Harmless variant or derivative of a pathogen that stimulates a host’s immune system
to mount defenses against the pathogen. Smallpox eradicated due to vaccine.

Front 33

Acyclovir

Back 33

Impedes the herpes virus by inhibiting the polymerase that synthesizes viral DNA.

Front 34

Azidothymidine (AZT)

Back 34

Impedes the AIDS virus by interfering with the synthesis of DNA by reverse transcriptase.

Front 35

Emerging Viruses

Back 35

A newly discovered virus that is increasing in incidence or with the
potential to increase in incidence. HIV – Belgian Congo (1959) Ebola – Central Africa (1976)
-West Nile – North America (1999) -H1N1 (swine flu) – Central America (2009)

Front 36

Pathways of Emerging Viruses

Back 36

Step #1: Mutation of existing viruses. RNA is prone to unusually high mutation rates.
Step #2: Dissemination of viral diseases from small, isolated human populations.
More rapid in a global society.
Step #3: Spread of existing viruses from other animals which become sources of new diseases in humans. Swine are melting pots for bird, swine, and human viruses.

Front 37

Epidemic

Back 37

General outbreak of a disease. Seasonal flu.

Front 38

Pandemic

Back 38

Global outbreak of a disease. 1918 Spanish Influenza.

Front 39

Effects of Plant Viruses

Back 39

1. More than 2,000 types of viral plant diseases.
2. Account for $15 billion in annual losses world-wide (mostly due to diminished quality & yield).
3. General symptoms: bleached or brown spots, stunted growth, damaged flowers or roots

Front 40

Horizontal transmission

Back 40

Plant is infected from an external source of the virus – usually through a damaged portion of the plant. Outside source

Front 41

Vertical transmission

Back 41

Plant inherits a viral infection from a parent (through parent’s genome).

Front 42

Viroid

Back 42

Plant pathogen consisting of a molecule of naked, circular RNA a few hundred nucleotides long.
1. Replicate in host plant cells.
2. Create errors in regulatory growth systems resulting in stunted growth and abnormal
development.

Front 43

Prion

Back 43

Infectious agent that is a misfolded cellular protein; increase in number by converting correctly folded proteins into prions.
1. Cause degenerative brain diseases in animals.
2. Act slowly – 10 year incubation period before virtually indestructible symptoms appear.