Genetics of Viruses and

BacteriaChapter 20

Parts of 12, 13, and 14

I.Domain Bacteria A. single-celled or colonial prokaryotes

1. includes 90% of bacteria (“Eubacteria”) and blue-green algae (cyanobacteria)

a. most bacteria are beneficial• decomposition, fermentation, energy conversion, etc.

b. some pathogenic2. lack a true nucleus

a. nucleoid • region where bacterial chromosome is located

b. most species possess one or more plasmidsi. small, circular pieces of DNA, independent of main

chromosomeii. replicate independently of bacterial chromosome • are not dependent upon it

iii. consist of 5-100 genesiv. not crucial for survival of cellv. advantages of plasmids:• genes for resistance to antibiotics and/or toxins• genes for production of toxins• genes for beneficial metabolism and other useful functions

3. unique kind of DNA and ribosomes (rRNA) not found in eukaryotes4. lack all membrane-bound organelles5. cell walls composed of peptidoglycan6. most have a capsule• semi-gelatinous layer outside of cell wall

o protection, stick to surfaces, water conservation, resist defenses 7. capable of only simple activities8. will stain Gram + or -

Fig. 20.6 A bacterial cell with multiple flagella

Fig. 20.8 A bacterial cell dividing

Fig. 20.9 Different shapes of bacteria

A typical prokaryotic cell

B.most reproduce asexually by binary fission• a form of cell division, but not mitosis

o 1 cell divides into 2 identical daughter cells rapid growth rates

Fig. 9.9 Binary fission

C.how do bacteria introduce any genetic variation into their species?

1. asexual binary fission will not do it2. transformation

a. free DNA taken up by a cell integrated into receiving cell's genome

b. occurs naturally and in laboratory3. conjugation

a. plasmid DNA transferred by direct cell-to-cell contactb. close to sexual reproductionc. mating type F+ or F-

4. transductiona. a virus transfers DNA to a bacterial cell• bacteriophage

b. natural, but used extensively in genetic engineering5. transposons

a. can “jump” from bact. chrom. to plasmids and then be transferred

b. can also be used to regulate gene expression

Conjugation

II. Domain Archaea A. rare and more primitive group of bacteria

(“Archaebacteria”) B. have all basic traits of prokaryotesC. how are they different from Eubacteria?

1. unusual shapes and little clustering2. unusual lipids in plasma membranes3. cell walls lack peptidoglycan• made up of pseudomurein instead

4. ambiguous Gram staining properties5. undergo unusual metabolic processes6. live in very harsh, extreme environments7. have their own unique kind of DNA and

ribosomes (rRNA)8. none are known to be pathogenic

Sulfolobus – hot springs and acidic/sulfur-rich env.

Methanosarcina – anaerobic; digestive tracts; produces methane. Note the red cell walls

Halococcus – salt lakes; very high salt concentrations

Staphylothermus – deep ocean hydrothermal vents

Methanococcoides – ice lakes; thrives at very cold temps.

Fig. 20.13 Various habitats in which Archaea are found. Top – salt lakes; Middle – hot springs; Bottom – swamps and bogs; Bottom inset – digestive tracts of animals

A deep ocean hydrothermal vent - A common home for many Archaea.

Many Archaea are also found in the digestive tracts of various animals, such as cows and other ruminants.

III.VirusesA.not part of three domain system• acellular; not truly alive

B.structure of viruses1. two basic parts

a. DNA or RNA, never bothb. protein coat (capsid)

2. some have a semi-gelatinous envelope

3. wide variety of shapes a. simple vs. complexb. capsid determines shape

Fig. 20.1 Virus structure

C.general characteristics1. extremely small size2. obligate intracellular parasites

a. must invade a host cell in order to become active and reproduce

b. many infect bacteria – bacteriophages, or just phages3. lack cytoplasm, cell organelles, plasma membranes4. incapable of independent metabolism5. common structure with DNA or RNA (never both)6. in nature, all are harmful parasites that cause disease• can be used clinically or for genetics research

o great way of introducing new DNA into an existing genome

7. possess only 3 characteristics of life• reproduction, heredity, and organization

Virus size vs. that of a bacterial and eukaryotic cell

D.how viruses work1. dormant virus enters living organism (how?)2. attacks host's cells and begins replicating new viral particles

E.lytic infection1. ends in rupturing (lysis) of host cell host cell dies2. attachment

• virus attaches to host cell's plasma membrane3. penetration

a. an enzyme dissolves a part of the plasma membraneb. virus injects its DNA or RNA into host cell

4. biosynthesis• viral DNA or RNA takes over host cell

o hydrolyzes host cell’s DNA nonfunctionalo forces host cell to produce more new viruses

5. maturation• new viruses are assembled and put together within host cell

o burst size = # of new viruses produced; varies by type of virus6. release

a. new viruses burst forth from host celli. host plasma membrane is ruptured cell diesii. each new virus can infect its own host cells

b. envelope formation• new viruses take some of cell’s plasma membrane as they are

released c. burst time• time it takes for this entire cycle to happen; varies by type of

virus

Fig. 20.3 Viral lytic and lysogenic cycles in prokaryotes

Page 355 Attachment and penetration. Viruses attacking a bacterial (E. coli) cell.

F.lysogenic infection1. many viruses can remain dormant (latent) once inside host

cellsa. they do not cause disease right awayb. can remain latent for years before replicating and bursting c. helps them avoid host immune responsesd. cause long-term, generally more serious illnesses

2. once inside a host cell, virus enters lysogenic cyclea. attachment and penetration – same as lytic infectionb. integration

i. viral DNA integrates itself into host’s genome• does not immediately hydrolyze host cell’s DNA

ii. remains dormant and part of host's genome indefinitely• replicated along with rest of genome (mitosis or fission)o such a virus is now called a prophage

c. induction• something happens environmentally or internally

triggers virus to enter an active state and leave latent state

d. biosynthesis• virus now hydrolyzes host cell’s DNA completelyo replication proceeds as in lytic cycle

e. maturation and release – same as lytic infection

Fig. 20.3 Viral lytic and lysogenic cycles in prokaryotes

IV.Viroids and PrionsA. viroids

1. infectious RNA• no protein coat or capsule

2. cause several plant diseasesB. prions

1. infectious proteins2. cause several diseases of the

nervous system