the genetics of viruses chapter 19. characteristics of viruses small (20nm) composed of rna or dna...
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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