Ch. 18: The Genetics of Viruses and Bacteria

1. Ch. 18: The Genetics of Viruses and Bacteria

2. Intro • Bacteria and viruses are the simplest • biological systems. Most protein synthesis • research was done on bacteria. • Bacteria and viruses are also of interest • so that we can better understand the • diseases they cause. • Bacteria are prokaryotic organisms, and are • much smaller and simpler than eukaryotes. • Viruses are smaller and simpler still, lacking • the structure and most metabolic • machinery in cells.

3. -Viruses are made up of nucleic acids and a protein coat.

4. The Genetics of Viruses • Researchers discovered viruses by studying • a plant disease. • In 1883, Adolf Mayer studied tobacco • mosaic disease. • This disease causes stunted growth • and mottled plant leaves in tobacco • plant. b. Mayer found that the disease was infectious when he sprayed sap from diseased leaves onto healthy plants and caused healthy plants to become diseased.

5. 2. He said that a bacteria caused the disease until Dimitri Ivanovsky demonstrated that the sap was still infectious even after passing through a filter designed to remove bacteria. • In 1897, Martinus Beijerinck showed that • sap from one generation could infect a • second generation of plants – he showed • that the pathogen had heredity. • Beijerinck also determined that the • pathogen could reproduce only within • a host.

6. In 1935, Wendell Stanley crystallized the • pathogen, the tobacco mosaic virus • (TMV). • A virus is a genome enclosed in a protective • coat. • Since cells cannot be crystallized, • Stanley’s crystallization of viruses was an • indicator that viruses are not made up of • cells. • Viruses are infectious particles made up • of nucleic acids encased in a protein coat, • and sometimes a membranous envelope. • Viruses range in size from 20nm to barely • resolvable under a light microscope.

7. Viral nucleic acids can be: • -double-stranded DNA • -single-stranded DNA • -double-stranded RNA • -single-stranded RNA •  depending on the specific type of virus. • Some viruses only have a few genes, • while others have hundreds. • The capsid: the protein shell • Capsids are built from a large number • of protein subunits called capsomeres.

8. The tobacco mosaic virus has over • 1,000 copies of the same protein • to make the capsid. • An adenovirus • has 252 identical • proteins • arranged into a • polyhedral • capsid - as an • icosahedron.

9. Some viruses have • viral envelopes that • are membranes • derived from a host • cell. • They can also have • viral proteins and • glycoproteins.

10. The most complex capsids are found • on the phages that infect bacteria. -The T-even phages that infect E. coli have a 20-sided capsid head that encloses their DNA and protein tail piece that attaches the phage to the host and injects the phage DNA inside.

11. Viruses can only reproduce within a host: • overview • Viruses are obligate intracellular • parasites; they can only reproduce within • a host cell. a. They lack enzymes and ribosomes. • They can only infect a limited range of • hosts. • Some viruses identify host cells by a • “lock-and-key” fit between proteins on • the outside of virus and specific receptor • molecules on the host’s surface OR • some viruses have a wide range of hosts • (ex. Rabies virus).

12. Most viruses target specific tissues. Example: Human cold viruses infect only the cells lining the upper respiratory tract. The AIDS virus binds only to certain white blood cells. • Infection begins when the viral nucleic • acid is inserted into the host. • Once inside, the viral genome takes over • its host, reprogramming • the cell to copy viral nucleic acid and • manufacture proteins from the viral • genome.

13. The nucleic • acid molecules • and • capsomeres • then self- • assemble into • viral particles • and exit the • cell.

14. Phages reproduce using lytic and lysogenic • cycles • The lytic cycle: the phage reproductive • cycle culminates in the death of the host. • Virulentphages reproduce only by a • lytic cycle.

16. Some bacteria have defense • mechanisms against viruses: -Some bacterial mutants have receptors sites that are no longer recognized by a particular type of phage. -Some bacteria produce restriction nucleases that recognize and cut up foreign DNA.

17. Lysogenic cycle: the phage genome • replicates without destroying the host cell. • Temperatephages, like phage • lambda, use both lytic and lysogenic • cycles. • In the lysogenic cycle, the viral DNA • molecule, during the lysogenic cycle, is • incorporated by genetic recombination • into a specific site on the host cell’s • chromosome. • At this stage, the phage is called a • prophage, and one of its genes codes • for a protein that represses most other • prophage genes to “silent” the genome.

18. Each time the bacterial cell divides, • it will replicates its own DNA, including • the viral DNA. Each time the bacteria • divides, it will pass on the viral DNA • to the daughter cells. • Sometimes the viral genome exits the • bacterial chromosome and initiates a • lytic cycle. • This switch from lysogenic to lytic may be initiated by an environmental trigger.

19. The prophage in a lysogenic cycle will exit the bacteria genome and cause a lytic cycle.

20. Animal viruses are • diverse in their • modes of infection • and replication. • Viruses differ in • the type of • nucleic acid • they have. • They also differ • on the presence • or absence of • a protein capsid.

21. Viruses with an outer envelope use the • envelope to enter a host cell. • Glycoproteins on the envelope bind • to specific receptors on the host’s • membrane. • The envelope fuses with the host’s • membrane, transporting the capsid • and viral genome inside.

22. The viral • genome • duplicates • and directs • the host’s • protein • synthesis • machinery to • synthesize • capsomeres • with free • ribosomes & • glycoproteins • with bound • ribosomes.

23. After the • capsid and • viral genome • self-assemble, • they bud from • the host cell • covered with • an envelope • derived from • the host’s • cell membrane, • including viral • glycoproteins. • These viruses do not always kill the host.

24. Some viruses have envelopes that are • not derived from plasma membrane. • The herpesvirus is derived from the • nuclear envelope of the host. • The herpesvirus has double-stranded • DNA and they reproduce within the • cell nucleus using viral and cellular • enzymes to replicate and transcribe • their DNA. • Their DNA can be incorporated into • host DNA. When they do, they are • called a provirus. • The provirus remains latent within • the nucleus until triggered by stress • to leave the genome and initiate • active viral production.

25. Viruses that have RNA as genetic material: • Some viruses have single-stranded RNA • (class IV), the genome acts as mRNA • and is translated directly. • In other cases, (class V), the RNA • genome serves as a template for mRNA • and for a complementary RNA (to make • more of the RNA genome). • All viruses that require RNA -> RNA • synthesis to make mRNA use a viral • enzyme that is packaged with the • genome inside the capsid.

26. Retroviruses (class IV) have the most • complicated reproductive cycles: • These viruses carry an enzyme, • reverse transcriptase, which • transcribes DNA from an RNA • template. • The newly made DNA is then inserted • into the animal genome as a provirus. • Proviruses never leave the host • genome, unlike prophages. • The host’s RNA polymerase transcribes • the viral DNA into more RNA molecules. • The RNA strands can serve as mRNA for viral proteins, or as genomes for new virus particles released from the cell.

27. HIV (Humanimmunodeficiency • virus (HIV), the virus that causes • AIDS (acquired immunodeficiency • syndrome) is a retrovirus. -viral envelope w/ glycoproteins -a capsid -two identical RNA strands -two reverse transcriptase enzymes

28. How does HIV infect a white blood cell? • HIV fuses with host • cell membrane. • Reverse transcriptase • synthesizes a • complementary DNA • strand to the viral RNA. • Reverse transcriptase • synthesizes a second • DNA strand • complementary to the • first DNA strand. • The new double strand • DNA is incorporated as a • provirus into the host DNA.

29. Proviral genes are • transcribed into RNA. • The RNA serves as • mRNA for translation • of HIV proteins. It is • also used as genomes • for the next generation • of viruses. • Capsids are assembled • around viral genomes • and reverse • transcriptase molecules. • The viruses bud off the • host cell.

30. Causes and prevention of viral diseases in • animals: • Some viruses cause animal cell • lysosomes to release their hydrolytic • enzymes, thus destroying the cell. • Some viral proteins are toxic to cells. • Some viruses cause the cell to produce • toxins that can kill the cell. • Viral damage can be permanent (polio • causes nerve damage) or temporary • (the cold virus). • Many temporary symptoms, such as • fever, aches, and inflammation is due to • the body’s own efforts at defending itself • against infection.

31. Vaccines are harmless variants or • derivatives of pathogens that stimulate • the immune system to act against an • actual pathogen. • The first vaccine was developed in the • late 1700s by Edward Jenner to fight • smallpox. • He found that milkmaids who were • exposed to cowpox (milder and similar • to smallpox) were resistant to • smallpox. • In 1796, Jenner infected a farmboy • with cowpox. Later, the boy was • exposed to smallpox and seemed to • resist the disease.

32. Because cowpox is so similar to small- • pox, an exposure to cowpox causes • the immune system to react vigorously • against smallpox. • Vaccines can prevent disease, but they • cannot treat or cure disease. • Antibiotics only work against bacteria. • They work by inhibiting bacterial • enzymes. Viruses have few or no • enzymes. • However, AZT inhibits HIV reproduction • by interfering with reverse transcript- • ase. Acyclovir inhibits herpesvirus • DNA synthesis.

33. Emerging viruses: • 1. HIV (1980’s) • 2. New strains of the influenza (flu) virus • 3. Ebola (fever, severe bleeding) • The causes of these viruses: -Mutations -spread from one species to another (3/4 of new human viruses come from other animal species – Ex. Hantavirus comes from deer mice) -spread from a small population to the rest of the world (HIV from Africa)

34. I. Some viruses cause cancer: • First to discover this wasPeyton Rous • when in 1911, he discovered that a virus • causes cancer in chickens. • Tumor viruses: retrovirus, papovavirus, • adenovirus, and herpesvirus types. • Hepatitis B can cause liver cancer. • Epstein-Barr virus, which causes • infectious mononucleosis, has been linked • to several types of cancer in parts of • Africa, notably Burkitt’s lymphoma. • Papilloma viruses are associated with • cervical cancers. • The HTLV-1 retrovirus causes a type of • adult leukemia.

35. Viruses may carry oncogenes that • trigger cancerous characteristics in cells. • (Oncogenes = genes that causes cancer.) -Viruses can also turn on proto-oncogenes (genes that code for growth factors that regulate the cell cycle). • Viroids and Prions: • Viroids are small pieces of circular • RNA that infect plants. These viroids can • stunt plant growth. • Prions are infectious proteins that spread • a disease.

36. Prions cause several degenerative • brain diseases including scrapie in • sheep, “mad cow disease”, and • Creutzfeldt-Jacob disease in humans. • Scientists hypothesize that prions are • forms brain proteins that are • misfolded. • They can convert a normal protein • into the prion version, creating a chain • reaction that increases their numbers.

37. Virus evolution: • Because viruses need cells to survive, it • is thought that they evolved after cells. • It is hypothesized that viruses originated • from fragments of cellular nucleic acids • that could move from one cell to another. • Viruses probably came from plasmids • and transposons. • Plasmids are small, circular DNA • molecules found in bacteria and yeast • that are separate from chromosomes. • Transposons are DNA segments that • can move from one location to another • within a cell’s genome.

38. The Genetics of Bacteria • The short generation span of bacteria help • them to adapt to changing environments. • Bacteria are very adaptable. • Bacteria have a circular double strand of • DNA. • In E.coli, the chromosomal DNA • consists of about 4.6 million nucleotide • pairs with about 4,300 genes. • Tight coiling of the DNA results in a • dense region called the nucleoid. • In addition to the chromosome, bacteria • have plasmids, which are smaller circles • of DNA. a. Plasmids have a few genes on them.

39. Bacteria divide • by binary fission. • The chromosome • replicates from a • single origin of • replication.

40. Bacteria replicate very rapidly: • -under optimal conditions, a population • of E. coli can double in 20 minutes, and • producing a colony of 107 to 108 bacteria • in as little as 12 hours. -In the human colon, E. coli reproduces rapidly enough to replace the 2 x 1010 bacteria lost each day in feces. • Most of the bacteria in a colony are • genetically identical to the parent cell. • However, the spontaneous mutation • rate of E. coli is 1 x 10-7 mutations per • gene per cell division. • There are ~2,000 bacteria in the human colon that have a mutation in that gene per day.

41. Genetic recombination produces new strains • of bacteria. • In addition to mutations, genetic • recombination can add to the diversity of • bacteria. • Recombination in bacteria is defined as • the combining of DNA from two • individuals into a single genome. • This recombination has 3 processes: -transformation-transduction-conjugation

43. Transformation: is the alteration of a • bacterial cell’s genotype by the uptake • of naked, foreign DNA from the • surrounding environment. -For example, harmless Streptococcus pneumoniae bacteria can be transformed to pneumonia-causing cells. -Many bacterial species have surface proteins that are specialized for the uptake of naked DNA. They will only uptake DNA from a closely related bacteria.

44. Transduction: occurs when a phage • carries bacterial genes from one host • cell to another. • General transduction: a small piece • of the host cell’s degraded DNA is • packaged within a capsid, rather than • the phage genome.

46. Specialized transduction: occurs via • a temperate phage. When the prophage viral genome exits the host chromosome, it some- times takes with it a small region of adjacent bacterial DNA. This bacterial DNA will be injected along with the viral DNA when the virus infects another bacteria.

47. -Both transduction types use a phage as a vector to transfer genes between bacteria.

48. c. Conjugation: transfers genetic material between two bacterial cells that are temporarily joined. 1.One cell (“male”) donates DNA and its “mate” (“female”) receives the genes. 2.A sex pilus from the male initially joins the two cells and creates a cytoplasmic bridge between cells.

49. 3.The “maleness” is the ability to form a sex pilus and donate DNA is the result from an F factor, a section of the bacterial chromosome or as a plasmid. 4.Plasmids, including the F plasmid, are small, circular, self-replicating DNA molecules. 5.Episomes, like the F plasmid, can undergo reversible incorporation into the cell’s chromosome. 6.Plasmids generally benefit the bacteria. They usually have only a few genes.

50. 7.The F plasmid consists of about 25 genes, most required for the production of sex pilli. 8.Cells with an F plasmid are called F+ and they pass this condition to their offspring. 9.Cells lacking the F plasmid are called F-, and they function as DNA recipients. • When an F+ and F- cells meet, the F+ cell will give the F- cell a copy of the F plasmid.