THE GENETICS OF VIRUSES

Chapter 19

CHARACTERISTICS OF VIRUSES

Small (20nm)Composed of RNA or DNA and

proteinCapsid- protein coat that encloses

the viral genomeViral envelopes – a membrane

that encloses capsid on some viruses (derived from host)

Comparing the size of a virus, a bacterium, and a eukaryotic cell

Viral structures

Adenovirus

Bacteriophage – a virus that infects bacteria

Viruses are obligate parasites – can only reproduce within a host cell

Host Range – virus can only infect a limited number of host cells

– Ex. HIV only attacks T-cellsConsidered nonlivingViruses infect all life on the planet

Phages

VIRAL INFECTION

Virus injects its genome (DNA or RNA) into host cell

Two major reproduction pathways for phages:–1. Lytic–2. Lysogenic

A simplified viral reproductive cycle

The lysogenic and lytic reproductive cycles of phage , a temperate phage

THE LYTIC CYCLEEnds in death of host cellViruses called virulentViral DNA inserted into hostHost’s DNA hydrolyzedViral DNA makes proteins and more viral

DNANew viruses released by bursting out of

host cell

THE LYSOGENIC CYCLEDoes not kill host cellCalled temperate virusesViral DNA is inserted into host cell Viral DNA is inserted into host cell’s DNA

(called a prophage)When host cell replicates it also replicates the

viral DNA sectionProphage genes are mostly inactiveSome can make harmful toxins (ex. diphtheria

and scarlet fever)

Classes of Animal Viruses, Grouped by Type of Nucleic Acid

Classes of Animal Viruses, Grouped by Type of Nucleic Acid

Smallpox

Measles

Polio

Influenza epidemic (Killed 40 million people in 1918-19)

Herpes

RNA VIRUSES

Can be transcribed to make proteinCan make more RNA with special

enzymes (within capsid)Retrovirus – contain reverse

transcriptase enzyme which transcribes DNA from RNA. (reverse)– Ex. HIV

Lack error checking ability during RNA replication so higher rates of mutation

HIV, a retrovirus

HIV infection

VACCINES

Harmless variants, dead or derivatives of viruses that allow us to build an immunity to the real thing

Antibiotics are powerless against viruses

EMERGING VIRUSES

Viruses have high mutation ratesDissemination of virus from small

population to larger (airplanes)Spread of existing viruses from

other animals

Cancer Causing Viruses

Some viruses can cause cancer

–Ex. Hepatitis B – liver cancer

–Ex. Papilloma – cervix cancer

Hepatitis

INFLUENZA

Three types– Type A: can cause epidemics and found in many

animals including humans– Types B and C: only in humans and no epidemics

Type A contain two proteins on capsid – H = hemagglutinin (helps virus attach to host)– N = neuroaminidase (helps release new viruses

from infected cell)

H1N1 (swine flu) and H5N1 (avian flu)

H1N1 – Caused both flu pandemic in 1918 and in 2009– Probably mutated in pigs and moved to humans– 79% people infected were under 30 in 2009

H5N1– Expanding host range and 50% mutation rate– Greater threat– Human to human transmission rare (so far)

PRIONS

Prions are infectious proteins

– Diseases caused by prions: Mad cow, Creutzfeldt-Jacob, Kuru and maybe Alzheimer’s

– Misfolded forms of proteins

– Associated with eating infected meat

– Incubate very slowly

– No cure and always deadly

A hypothesis to explain how prions propagate

Bacteria and Archaea

CHAPTER 27

Bacteria on the point of a pin

THREE MAIN LINEAGES OF LIFE

Prokaryotes

UnicellularContain cell wall, plasma

membrane, ribosomes, DNA, and cytoplasm

First organisms to inhabit earthSome are autotrophs and others

are heterotrophs

STRUCTURE AND FUNTION

Three most common shapes–Cocci – round–Bacilli – rods–Helices – spiral

Usually small (1-5μm)

Figure 27.3 The most common shapes of prokaryotes

Prokaryotic cell walls– Most walls contain peptidoglycan

(sugars cross-linked with polypeptides) except archaea

– Gram positive – large amounts of peptidoglycan (stain violet)

– Gram negative – small amounts of peptidoglycan (stain red)•Often more threatening due to lipopolysaccharides on cell walls that are often toxic

– Antibiotics often inhibit synthesis of cross-links of peptidoglycan

Figure 27.5 Gram-positive and gram-negative bacteria

Figure 27.5x Gram-positive and gram-negative bacteria

Some have pilli (some for “sex”)Some have flagella (not covered by

membrane)Some capable of taxis – movement

away from or toward stimuliProkaryotic Genome

– 1/1000 as much DNA– One circular chromosome (may be

concentrated in a nucleoid region)– Plasmids – smaller, rings of DNA; may

carry resistance genes, conjugation genes

Figure 27.6 Pili

REPRODUCTION

Binary fission – cell division; requires copying the one chromosome and then cell divides (can happen in 20 minutes)

Genetic recombination – ways that bacteria can get genes from other organisms– Conjugation (bacterial sex)– Transformation (bacteria grab

foreign DNA from environment)– Transduction (viruses infect

bacteria with foreign DNA)

Figure 27.x1 Prokaryotic conjugation

Mutation is the main source of variation in prokaryotes!!

Figure 27.11x1 Cyanobacteria: Gloeothece (top left), Nostoc (top right), Calothrix (bottom left), Fischerella (bottom right)

DOMAIN ARCHAEAProkaryotes

– Methogens – use H2 to reduce CO2 to CH4; poisoned by O2, live in swamps, decompose sewage, in guts of animals (cows and termites) help digest cellulose

– Extreme halophiles – like salt, purple-red scum due to bacteriorhodopsin

– Extreme thermophiles – in hot springs, 60° – 80° C, deep sea vents at 150° C

Figure 27.14 Extreme halophiles

Figure 27.14x1 Hot springs, home of thermophiles

Figure 27.14x2 Beggiatoa, sulfur-eating bacteria

Figure 27.1 “Heat-loving” prokaryotes