Viruses and Bacteria

Transcription

1 Viruses and Bacteria

2 Structure of Viruses Viruses are not cells Virus -very small infectious particle, nucleic acid enclosed in a protein coat and, in some cases, a membranous envelope Virus -consists of a nucleic acid surrounded by a protein coat Viruses were detected indirectly long before they were actually seen 2011 Pearson Education, Inc.

3 Viral Genomes Viral genomes may consist of either Double- or single-stranded DNA, or Double- or single-stranded RNA Depending on its nucleic acid, a virus is called -DNA virus -RNA virus (retrovirus) 2011 Pearson Education, Inc.

4 Capsids and Envelopes Capsid -protein shell enclosing viral genome Capsids -built from protein subunits called capsomeres Capsids have various structures 2011 Pearson Education, Inc.

5 Figure 19.3 RNA Capsomere of capsid Capsomere DNA Membranous envelope RNA Capsid Head DNA Tail sheath Tail fiber Glycoprotein Glycoproteins nm nm (diameter) nm (diameter) nm 20 nm 50 nm 50 nm 50 nm (a) Tobacco mosaic virus (b) Adenoviruses (c) Influenza viruses (d) Bacteriophage T4

6 Some viruses - membranous envelopes that help them infect hosts Viral envelopes surround the capsids of influenza viruses and many other viruses found in animals Viral envelopes -derived from the host cell s membrane, contain a combination of viral and host cell molecules 2011 Pearson Education, Inc.

7 A Borrowed Life Bacteriophages can infect and set in motion a genetic takeover of bacteria, such as Escherichia coli Viruses lead a kind of borrowed life between lifeforms and chemicals The origins of molecular biology lie in early studies of viruses that infect bacteria 2011 Pearson Education, Inc.

8 Figure mm

9 Viruses replicate only in host cells Viruses -obligate intracellular parasites-they can replicate only within host cell Each virus has a host range-limited number of host cells it can infect (membrane recognition receptors must match) 2011 Pearson Education, Inc.

10 General Features Once viral genome enters cell, cell manufactures viral proteins Virus uses host s: enzymes, ribosomes, trnas, amino acids, ATP, etc. Viral nucleic acid molecules and capsomeres spontaneously self-assemble into new viruses 2011 Pearson Education, Inc.

11 Figure Entry and uncoating DNA Capsid VIRUS 3 Transcription and manufacture of capsid proteins 2 Replication HOST CELL Viral DNA mrna Viral DNA Capsid proteins 4 Self-assembly of new virus particles and their exit from the cell

12 Replicative Cycles of Phages Phages -best understood of all viruses Phages have two reproductive mechanisms: Lytic cycle Lysogenic cycle 2011 Pearson Education, Inc.

13 The Lytic Cycle Lytic cycle -phage replicative cycle -culminates in death (lysis) of host cell Lytic cycle produces new phages and lyses (breaks open) host s cell wall, releasing new viruses Phage -reproduces only by the lytic cycle is called a virulent phage Bacteria have defenses against phages, including restriction enzymes -recognize and cut up certain phage DNA 2011 Pearson Education, Inc.

14 Figure Attachment 5 Release 2 Entry of phage DNA and degradation of host DNA Phage assembly Head Tail Tail fibers 4 Assembly 3 Synthesis of viral genomes and proteins

15 The Lysogenic Cycle Lysogenic cycle -replicates the phage genome without destroying the host immediately. Viral DNA molecule -incorporated into the host cell s chromosome Integrated viral DNA is known -prophage Every time host divides-copies phage DNA, passes copies to daughter cells 2011 Pearson Education, Inc.

16 Environmental signal -triggers virus genome to exit bacterial chromosome and switch to lytic mode Phages uses both the lytic and lysogenic cycles -called temperate phages 2011 Pearson Education, Inc.

17 Figure 19.6 Phage Phage DNA The phage injects its DNA. Bacterial chromosome Phage DNA circularizes. Daughter cell with prophage Occasionally, a prophage exits the bacterial chromosome, initiating a lytic cycle. Cell divisions produce a population of bacteria infected with the prophage. The cell lyses, releasing phages. Lytic cycle Certain factors determine whether lytic cycle is induced or lysogenic cycle is entered Lysogenic cycle Prophage The bacterium reproduces, copying the prophage and transmitting it to daughter cells. New phage DNA and proteins are synthesized and assembled into phages. Phage DNA integrates into the bacterial chromosome, becoming a prophage.

18 Figure 19.6b Daughter cell with prophage Phage DNA circularizes. Cell divisions produce a population of bacteria infected with the prophage. Occasionally, a prophage exits the bacterial chromosome, initiating a lytic cycle. Certain factors determine whether lytic cycle is induced or lysogenic cycle is entered Lysogenic cycle Prophage The bacterium reproduces, copying the prophage and transmitting it to daughter cells. Phage DNA integrates into the bacterial chromosome, becoming a prophage.

19 Table 19.1a

20 RNA as Viral Genetic Material Broadest variety of RNA genomes is found in viruses infecting animals Retroviruses use reverse transcriptase to copy their RNA genome into DNA HIV (human immunodeficiency virus) - retrovirus that causes AIDS (acquired immunodeficiency syndrome) 2011 Pearson Education, Inc.

21 Figure 19.8 Glycoprotein Viral envelope Capsid HIV Membrane of white blood cell Reverse transcriptase HIV RNA (two identical strands) Viral RNA HOST CELL Reverse transcriptase RNA-DNA hybrid DNA RNA genome for the next viral generation Chromosomal DNA NUCLEUS Provirus mrna 0.25 m HIV entering a cell New virus New HIV leaving a cell

22 Figure 19.8a Glycoprotein Viral envelope Capsid Reverse transcriptase HIV RNA (two identical strands) Viral RNA HOST CELL Reverse transcriptase RNA-DNA hybrid DNA Chromosomal DNA NUCLEUS Provirus RNA genome for the next viral generation mrna New virus

23 Viral DNA that is integrated into the host genome is called a provirus Provirus remains permanently in host cell Host s RNA polymerase transcribes proviral DNA into RNA molecules RNA molecules functions: -as mrna for synthesis of viral proteins -as genomes for new virus particles released from the cell 2011 Pearson Education, Inc.

24 Animation: HIV Reproductive Cycle HHMI 5 min 2011 Pearson Education, Inc.

25 Prokaryotes Domain Bacteria Domain Archea

26 A tentative phylogeny of some of the major taxa of prokaryotes based on molecular systematics Domain Bacteria Domain Archaea Domain Eukarya Proteobacteria Figure Universal ancestor

27 Archaea share certain traits with bacteria And other traits with eukaryotes Table 27.2

28 Prokaryotic cells have a variety of shapes The three most common of which are spheres (cocci), rods (bacilli), and spirals 1 m 2 m 5 m (a) Spherical (cocci) (b) Rod-shaped (bacilli) (c) Spiral

29 Cell-Surface Structures One of the most important features of nearly all prokaryotic cells Is their cell wall, which maintains cell shape, provides physical protection, and prevents the cell from bursting in a hypotonic environment

30 The cell wall of many prokaryotes Is covered by a capsule, a sticky layer of polysaccharide or protein 200 nm Capsule Figure 27.4

31 Some prokaryotes have fimbriae and pili Which allow them to stick to their substrate or other individuals in a colony Fimbriae Figure nm

32 Most motile bacteria propel themselves by flagella Motility Which are structurally and functionally different from eukaryotic flagella Flagellum Filament 50 nm Cell wall Hook Basal apparatus Figure 27.6 Plasma membrane

33 Some prokaryotes Do have specialized membranes that perform metabolic functions 0.2 m 1 m Respiratory membrane Thylakoid membranes Figure 27.7a, b (a) Aerobic prokaryote (b) Photosynthetic prokaryote

34 Bacterial Genetics Typical prokaryotic genome ring of DNA not surrounded by a membrane and located in a nucleoid region Chromosome Figure m

35 Bacteria -Circular Chromosomes Chromosome Origin of Replication Cell Wall Cell Membrane Cytoplasm Termination of Replication

36 Genetic Recombination If bacteria were: -incapable of genetic recombination-all members of a species would be clones with differences arising only by mutations. -not sharing genetic information, there would be vastly less opportunity to adapt to different environments

37 Ways Bacteria Exchange Genetic Material (Genetic Recombination) Transformation - Bacteria take up DNA from environment and incorporate it into their genome (i.e., the Griffith experiment) Transduction - Movement of DNA between bacteria by viruses (like the phage) Conjugation - The direct transfer of DNA by bacteria usually via plasmids

38 Transformation Genetic recombination in which a DNA fragment from a dead, degraded bacterium enters a competent recipient bacterium and it is exchanged for a piece of the recipient's DNA.

39 Transformation Crossing over Insertion

40 Transduction Genetic recombination in which a DNA fragment is transferred from one bacterium to another by a bacteriophage

41 Transduction Infection Destruction of the bacteria s DNA Lysis Packaging Production of viral parts Replication of the viral genome

42 Bacterial Conjugation -Genetic recombination in which there is a transfer of DNA from a living donor bacterium to a recipient bacterium. -Often involves a sex pilus and smaller rings of DNA called plasmids.

43 F plasmid F + bacteria Conjugation Mating Bridge F - bacteria

44 Rapid reproduction and horizontal gene transfer Facilitate evolution of prokaryotes with changing environments

45 Many prokaryotes form endospores Which can remain viable in harsh conditions for centuries Endospore Figure m

46 A great diversity of nutritional and metabolic adaptations have evolved in prokaryotes. Examples of models of nutrition are found among prokaryotes Photoautotrophs Chemoautotrophs Photoheterotrophs Chemoheterotrophs

47 Major nutritional modes in prokaryotes Table 27.1

48 Metabolic Relationships to Oxygen Prokaryotic metabolism varies with respect to oxygen -Obligate aerobes Require oxygen -Facultative anaerobes Can survive with or without oxygen -Obligate anaerobes Are poisoned by oxygen

49 Nitrogen Metabolism Some prokaryotes can metabolize nitrogen in a variety of ways, ex denitrification, ammonification, ect. -ex. one process called nitrogen fixation -convert atmospheric nitrogen to ammonia

50 1 m Metabolic Cooperation -between prokaryotes Allows them to use environmental resources they could not use as individual cells Metabolic cooperation occurs in surface-coating colonies called biofilms communicate signal transduction

51 Archea- live on the edge Some archaea Live in extreme environments Extreme thermophiles Thrive in very hot environments

52 Extreme halophiles Live in high saline environments Methanogens- Live in swamps and marshes Produce methane as a waste product

53 Prokaryotes play crucial roles in the biosphere Prokaryotes are so important to the biosphere that if they were to disappear The prospects for any other life surviving would be dim

54 Chemical Recycling Prokaryotes play a major role In the continual recycling of chemical elements between the living and nonliving components of the environment in ecosystems Chemoheterotrophic prokaryotes function as decomposers Breaking down corpses, dead vegetation, and waste products

55 Prokaryotes are the principal agents in bioremediation The use of organisms to remove pollutants from the environment Figure 27.17

56 Prokaryotes are also major tools in Mining The synthesis of vitamins Production of antibiotics, hormones, and other products

57 Symbiotic Relationships Many prokaryotes Live with other organisms in symbiotic relationships such as mutualism and commensalism Figure 27.15

58 Prokaryotes have both harmful and beneficial impacts on humans Some prokaryotes are human pathogens But many others have positive interactions with humans Pathogenic prokaryotes typically cause disease By releasing exotoxins or endotoxins

59 Pathogenic Prokaryotes Prokaryotes cause about half of all human infectious diseases Lyme disease is an example