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A different way to study Taxonomy… Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Chapter 19 The Diversity of Prokaryotes and Viruses Lecture Outlines by Gregory Ahearn, University of North Florida Copyright © 2011 Pearson Education Inc. Chapter 19 At a Glance 19.1 Which Organisms Are Members of the Domains Archaea and Bacteria? 19.2 How Do Prokaryotes Survive and Reproduce? 19.3 How Do Prokaryotes Affect Humans and Other Eukaryotes? 19.4 What Are Viruses, Viroids, and Prions? Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 19.1 Which Organisms Are Members of the Domains Archaea and Bacteria? – Earth’s first organisms were prokaryotes, singlecelled microbes that lacked organelles – Prokaryotes are still abundant, forming two of life’s three domains –Bacteria –Archaea Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 19.1 Which Organisms Are Members of the Domains Archaea and Bacteria? Bacteria and Archaea are fundamentally different – They have some similarities –They are both prokaryotic –They are both single-celled organisms Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 19.1 Which Organisms Are Members of the Domains Archaea and Bacteria? Differences between Bacteria and Archaea – Structural and biochemical features –Bacteria have peptidoglycan in their cell walls; Archaea don’t –There are also differences in the plasma membrane composition, ribosomes, and RNA polymerases between the two domains –Differences in transcription and translation also exist between the two Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Archaea Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 4.1 What Is the Cell Theory? Three principles comprise the cell theory – Every living organism is made of one or more cells – The smallest organisms are single cells, and cells are the functional units of multicellular organisms – All cells arise from preexisting cells Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 4.2 What Are the Basic Attributes of Cells? There are two basic types of cells: –Prokaryotic cells (“before the nucleus”) form the bodies of bacteria and archaea, the simplest forms of life –Eukaryotic cells (“true nucleus”) form the bodies of animals, plants, fungi, and protists Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. The Sizes of Microorganisms 1 m Staphylococcus cyanobacterium Escherichia coli Eukaryotic cells (10–100 m) Biology: Life on Earth, 9e Prokaryotic cells (0.2–10 m) Viruses (0.05–0.2 m) Fig. 19-10 Copyright © 2011 Pearson Education Inc. 4.3 What Are the Major Features of Eukaryotic Cells? Eukaryotic cells are usually larger than prokaryotic cells – The cytoplasm of eukaryotic cells includes a variety of organelles, such as the nucleus and mitochondria – The cytoskeleton gives shape and organization to the cytoplasm of eukaryotic cells Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 4.4 What Are the Major Features of Prokaryotic Cells? Prokaryotic cells are small and possess specialized surface features Prokaryotic cells have fewer specialized structures within their cytoplasm Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 4.4 What Are the Major Features of Prokaryotic Cells? Most prokaryotic cells (bacteria) are less than 5 µm long, with a simple internal structure compared to eukaryotic cells – They usually have a stiff cell wall – Prokaryotic cells can take several shapes: –Rod-shaped –Spiral-shaped –Spherical Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Prokaryotic Cells Are Simpler Than Eukaryotic Cells chromosome (nucleoid region) cell wall plasma membrane ribosomes capsule chromosome (nucleoid region) (d) Internal structure pili (c) Cocci ribosomes food granule (b) Spirilla prokaryotic flagellum plasmid (DNA) cytoplasm capsule or slime layer cell wall plasma membrane photosynthetic membranes (a) Generalized prokaryotic cell (bacillus) (e) Photosynthetic prokaryote Fig. 4-19 Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 4.4 What Are the Major Features of Prokaryotic Cells? Prokaryotic cells – Some bacteria and archaea are propelled by flagella – Infectious bacteria may have polysaccharide adhesive capsules and slime layers on their surfaces – Pili are protein projections in some bacteria that further enhance adhesion Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 4.4 What Are the Major Features of Prokaryotic Cells? Prokaryotic cells (continued) – In the central region of the cell is an area called the nucleoid, which is separate from the cytoplasm –Within the nucleoid is a single, circular chromosome of DNA –Small rings of DNA (plasmids) are located in the cytoplasm Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 4.4 What Are the Major Features of Prokaryotic Cells? Prokaryotic cells (continued) – Prokaryotic cells have no nuclear membrane or membrane-bound organelles present – Some have internal membranes used to capture light – The cytoplasm may contain food granules and ribosomes, the latter with a similar function as that of ribosomes in eukaryotic cells Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 19.1 Which Organisms Are Members of the Domains Archaea and Bacteria? Classification of prokaryotes within each domain is difficult – Prokaryotes are very small and structurally simple – Prokaryotes do not have a huge array of anatomical differences that can be used to distinguish between groups Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 19.1 Which Organisms Are Members of the Domains Archaea and Bacteria? Classification of prokaryotes within each domain is difficult (continued) – Features used in prokaryotic classification are: –Shape –Means of locomotion –Pigments –Nutrient requirements –Colony appearance –Gram staining characteristics –Nucleotide sequences Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 19.1 Which Organisms Are Members of the Domains Archaea and Bacteria? Prokaryotes differ in shape and structure – Both bacteria and archaea are very small, ranging from 0.2 to 10 micrometers in diameter – Prokaryotes have three common shapes –Spherical –Rod-like –Corkscrew-shaped Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Three Common Prokaryote Shapes Fig. 19-1 Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 19.2 How Do Prokaryotes Survive and Reproduce? Some prokaryotes are motile – Flagella are the primary means of locomotion in prokaryotes – Flagella are found singly, in pairs, as a tuft at one end of the cell, or scattered over the entire cell surface – In bacteria, a “wheel-and-axle” arrangement anchors the flagellum within the cell wall and plasma membrane, enabling the flagellum to rotate rapidly – The archaeal flagella are thinner than those of bacteria and are made of different proteins Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. The Prokaryote Flagellum Fig. 19-2 Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 19.2 How Do Prokaryotes Survive and Reproduce? Many bacteria form films on surfaces – Some bacteria secrete sticky layers of polysaccharide or protein slime – Aggregates (communities) of slime-secreting bacteria are called biofilms; dental plaque is a biofilm – Bacteria embedded in biofilms are protected from disinfectants and antibiotics Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. The Cause of Tooth Decay Fig. 19-3 Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 19.2 How Do Prokaryotes Survive and Reproduce? Protective endospores allow some bacteria to withstand adverse conditions – Endospores are thickly wrapped particles of genetic material and a few enzymes – Endospores form inside some bacteria under inhospitable environmental conditions –When the endospores form, the bacterium breaks open, releasing the endospores –When a hospitable environment is found, metabolism resumes and the endospore develops into a fully functioning bacterium Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 19.2 How Do Prokaryotes Survive and Reproduce? Protective endospores allow some bacteria to withstand adverse conditions (continued) – Endospores are resistant even to extremely harsh conditions, including boiling for an hour –Endospores found sealed within a rock for 250 million years produced live bacteria when incubated in a test tube Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Spores Protect Some Bacteria Fig. 19-4 Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 19.2 How Do Prokaryotes Survive and Reproduce? Prokaryotes are specialized for specific habitats – Each species is specialized for certain environmental conditions and usually cannot survive outside a narrow range around those conditions – Prokaryotes occupy a wide range of habitats Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 19.2 How Do Prokaryotes Survive and Reproduce? Prokaryote habitats – High-pressure underground environments – Cold environments (the Antarctic sea ice) – High-salt environments (the Dead Sea) – Acidic or alkaline environments (vinegar, ammonia) – Moderate environments (the human body) – Hot environments (deep-sea vents, hot springs) Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Some Prokaryotes Thrive in Extreme Conditions Fig. 19-5 Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 19.2 How Do Prokaryotes Survive and Reproduce? Prokaryotes exhibit diverse metabolisms – Anaerobic metabolism –Some bacteria, such as the tetanus bacterium, are anaerobes and live without oxygen (and are poisoned by it) – Some bacteria can switch between aerobic and anaerobic respiration –For example, Escherichia coli in our large intestines Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 19.2 How Do Prokaryotes Survive and Reproduce? Prokaryotes exhibit diverse metabolisms (continued) – Prokaryotes can extract their energy from familiar organic compounds –Sugars, carbohydrates, fats, and proteins – Prokaryotes can also extract energy from compounds poisonous to humans –Petroleum, methane, benzene, toluene – Prokaryotes can even extract energy from inorganic molecules such as hydrogen, sulfur, ammonia, iron, and nitrite Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 19.2 How Do Prokaryotes Survive and Reproduce? Prokaryotes exhibit diverse metabolisms (continued) – Some bacteria get energy from sunlight –Cyanobacteria perform photosynthesis –Sulfur bacteria use H2S instead of water in photosynthesis Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Cyanobacteria Fig. 19-6 Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 19.2 How Do Prokaryotes Survive and Reproduce? Prokaryotes reproduce by binary fission, which is simpler than mitotic division – Asexual cell division produces identical copies – Binary fission can occur every 20 minutes A rapid reproductive rate allows for rapid evolution – Mutations in DNA replication are rapidly spread Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Reproduction in Prokaryotes Fig. 19-7 Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 19.2 How Do Prokaryotes Survive and Reproduce? Prokaryotes may exchange genetic material without reproducing – Conjugation allows for DNA transfer between donor and recipient – Sex pili connect donor to recipient cell, drawing the recipient to the donor and forming a cytoplasmic bridge Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 19.2 How Do Prokaryotes Survive and Reproduce? Prokaryotes may exchange genetic material without reproducing (continued) – Conjugation can occur between different species – Small circular DNA molecules (plasmids) that are separate from the bacterial chromosome, carry genes from donor to recipient – The plasmids may contain alleles conferring resistance to antibiotics or alleles also found on the main chromosome – Conjugation produces new genetic combinations that may allow the resulting bacteria to survive under a greater variety of conditions Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Author Animation: Bacterial Conjugation Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Conjugation: Prokaryotic ‘Mating’ Fig. 19-8 Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 19.3 How Do Prokaryotes Affect Humans and Other Eukaryotes? Prokaryotes play important roles in animal nutrition – Leaf-eating animals (e.g., rabbits, cattle) depend on digestive tract bacteria to break down cellulose in the cell walls of plants and thus release the nutrients in the cells –The animals lack the ability to digest cellulose – Many human foods are produced by bacterial action (e.g., cheese and yogurt) – Bacteria in our intestines produce vitamins (e.g., vitamins K and B12) Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 19.3 How Do Prokaryotes Affect Humans and Other Eukaryotes? Prokaryotes capture the nitrogen needed by plants – Plants cannot metabolize the nitrogen available as a gas in the air – Nitrogen-fixing bacteria that may be present in the soil, or in specialized nodules in some plant roots, convert atmospheric N2(gas) to NH4+(ammonium) that plants can use directly – Plants require nitrogen to grow and are thus dependent on these bacteria for survival – Since humans could not live without plants, they also cannot live without bacteria – Nitrogen fixation is found in alfalfa, soybeans, lupines, and clover Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Nitrogen-Fixing Bacteria in Root Nodules Fig. 19-9 Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 19.3 How Do Prokaryotes Affect Humans and Other Eukaryotes? Prokaryotes are nature’s recyclers – Many prokaryotes obtain energy by breaking down complex organic molecules into the basic building blocks of biomolecules – These prokaryotes obtain their energy from waste products and the dead bodies of plants and animals – Decomposition of dead organisms frees nutrients for reuse by new life – The recycling of nutrients by prokaryotes and other decomposers provides the basis for continued life on Earth Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 19.3 How Do Prokaryotes Affect Humans and Other Eukaryotes? Prokaryotes can clean up pollution – Nearly all human-made substances are biodegradable by some bacterial species – Oil-eating bacteria were used in clean up the Exxon Valdez oil-spill disaster – The practice of manipulating conditions to stimulate breakdown of pollutants by living organisms is known as bioremediation Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 19.3 How Do Prokaryotes Affect Humans and Other Eukaryotes? Some bacteria pose a threat to human health – Disease-producing bacteria are termed pathogenic – Pathogenic archaea have not been discovered to date Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 19.3 How Do Prokaryotes Affect Humans and Other Eukaryotes? Some bacteria pose a threat to human health (continued) – Some anaerobic bacteria produce dangerous poisons – Clostridium tetani causes tetanus – It enters the body through a puncture wound – If it is lodged deeply enough in the body to avoid oxygen, it grows, producing a paralyzing poison – Bacterial diseases cause bubonic plague, Lyme disease, and tuberculosis Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 19.3 How Do Prokaryotes Affect Humans and Other Eukaryotes? Humans have battled bacterial diseases throughout history – Bubonic plague (Black Death) –It spread by rat fleas –It killed 100 million people in the 1300s (roughly a third of the population in some areas) Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 19.3 How Do Prokaryotes Affect Humans and Other Eukaryotes? Humans have battled bacterial diseases throughout history (continued) – Lyme disease (emerged in 1975) –It is caused by spiral-shaped Borrelia burgdorferi –It is carried by deer ticks, which bite humans –Flu-like symptoms can lead to arthritis and heart and nervous system problems Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 19.3 How Do Prokaryotes Affect Humans and Other Eukaryotes? Humans have battled bacterial diseases throughout history (continued) – Other historical bacterial diseases disappear and then reoccur – Tuberculosis (once thought to be vanquished from the United States) is making a comeback – Gonorrhea and syphilis (sexually transmitted diseases) have reached epidemic proportions around the globe – Cholera (water-transmitted in contaminated drinking water) is under control in developed countries but remains a major killer in poorer parts of the world Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 19.3 How Do Prokaryotes Affect Humans and Other Eukaryotes? Common bacterial species can be harmful – Streptococcus bacteria – Cause tooth decay, pneumonia, and necrotizing fasciitis (“flesh-eating” bacterial infection) – Escherichia coli – This bacterium is a common inhabitant of the human digestive system – The O157:H7 E. coli strain is pathogenic – It can be transmitted through undercooked hamburger from infected cattle – It causes intestinal bleeding and can be fatal Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 19.4 What Are Viruses, Viroids, and Prions? A virus consists of a molecule of DNA or RNA surrounded by a protein coat – Characteristics of a virus –They have no cell membranes, no cytoplasm, no ribosomes –They can reproduce only inside a host cell –They are very small (0.05–0.2 micrometers) –They come in a variety of shapes – The simplicity of viruses seems to place them outside the realm of living things Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. The Sizes of Microorganisms 1 m Staphylococcus cyanobacterium Escherichia coli Eukaryotic cells (10–100 m) Biology: Life on Earth, 9e Prokaryotic cells (0.2–10 m) Viruses (0.05–0.2 m) Fig. 19-10 Copyright © 2011 Pearson Education Inc. Viruses Come in a Variety of Shapes Fig. 19-11 Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 19.4 What Are Viruses, Viroids, and Prions? A virus consists of a molecule of DNA or RNA surrounded by a protein coat (continued) – Two major components constitute a virus –The hereditary material is single- or doublestranded DNA or RNA, which may be linear or circular –The protein coat may be surrounded by an envelope formed from the plasma membrane of the host cell Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Viral Structure and Replication glycoproteins envelope (lipid bilayer) protein coat spikes core proteins genetic material (viral RNA) coated with protein Biology: Life on Earth, 9e reverse transcriptase Fig. 19-12 Copyright © 2011 Pearson Education Inc. 19.4 What Are Viruses, Viroids, and Prions? Viruses are parasites – They cannot grow or reproduce on their own and thus parasitize living cells – They have a specialized protein coat that enables entry into a host cell – Viral genetic material “hijacks” the host cell to produce new viral components – The viral components assemble rapidly into new viruses and burst from the host cell Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 19.4 What Are Viruses, Viroids, and Prions? Each viral type is specialized to attack specific host cells – Bacteria are infected by bacteriophage viruses Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Some Viruses Infect Bacteria Fig. 19-13 Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 19.4 What Are Viruses, Viroids, and Prions? Bacteriophages can treat bacterial diseases – The rise in bacterial antibiotic resistance makes standard drugs less effective – Bacteriophages specifically target host bacteria – Bacteriophages are harmless to human body cells Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 19.4 What Are Viruses, Viroids, and Prions? In multicellular organisms, viruses specialize in attacking particular cell types – Cold viruses attack membranes of the respiratory tract – Rabies viruses attack nerve cells – Herpes viruses live in the body and periodically erupt as infectious sores Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 19.4 What Are Viruses, Viroids, and Prions? In multicellular organisms, viruses specialize in attacking particular cell types (continued) – HIV virus attacks a specific white blood cell type, causing AIDS – Viruses have been linked to some types of cancer, such as T-cell leukemia, liver cancer, and cervical cancer Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Author Animation: Retrovirus Replication Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Author Animation: Herpes Virus Replication Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. How does the flu virus spread and infect us? Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 19.4 What Are Viruses, Viroids, and Prions? Viral infections are difficult to treat – Antibiotics against bacteria are infective against viruses – Antiviral drugs may also kill host cells – Viruses “hide” within cells and are hard to detect – Many antiviral drugs destroy or block the function of enzymes that targeted viruses require for replication Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 19.4 What Are Viruses, Viroids, and Prions? Viral infections are difficult to treat (continued) – Viruses have high mutation rates because they lack the mechanisms to correct errors that occur during replication of their genomes –Mutations can confer resistance to an antiviral drug –Resistant viruses spread and multiply, rendering a drug ineffective Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 19.4 What Are Viruses, Viroids, and Prions? Some infectious agents are even simpler than viruses – Viroids are infectious particles with only short RNA strands (no protein coat) – These particles can enter a host cell nucleus and direct new viroid synthesis – A number of crop diseases are caused by viroids, including cucumber pale fruit disease, avocado sunblotch, and potato spindle tuber disease Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 19.4 What Are Viruses, Viroids, and Prions? Some infectious agents are even simpler than viruses (continued) – Prions are even more puzzling than viroids – A fatal degenerative disease called kuru was discovered in a New Guinea tribe (the Fore) in 1950 – Kuru causes a loss of coordination, dementia, and death – Kuru in the Fore tribe was transmitted by ritual cannibalism of the dead Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 19.4 What Are Viruses, Viroids, and Prions? Some infectious agents are even simpler than viruses (continued) – It was noted that kuru resembled certain other diseases –Creutzfeldt-Jakob (disease CJD) in humans –Scrapie in sheep –Bovine spongiform encephalopathy (BSE, or “mad cow disease”) in cattle – These diseases create holes in brain tissue Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 19.4 What Are Viruses, Viroids, and Prions? Some infectious agents are even simpler than viruses (continued) – In 1982, Stanley Prusiner demonstrated that scrapie (and by extension, other similar diseases) was caused by an infectious protein particle devoid of any genetic material – Prion-caused diseases may be heritable Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Prions: Puzzling Proteins Fig. 19-14 Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 19.4 What Are Viruses, Viroids, and Prions? Proposed model for replication of prions 1. Prion protein is a normal part of nerve cells 2. Misfolded versions of the normal protein are the infectious particles 3. The misfolded proteins induce normal copies to misfold, too 4. A high concentration of prions in nerve tissue causes cell damage and degeneration Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 19.4 What Are Viruses, Viroids, and Prions? Prions can be inherited – Certain small mutations in the gene that codes for “normal” prion protein increase the likelihood that the protein will fold into its abnormal form – If one of these mutations is genetically passed on to offspring, the tendency to develop a prion disease may be inherited Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. 19.4 What Are Viruses, Viroids, and Prions? No one is certain how these infectious particles originated – One theory is that they are evolutionary remnants of life’s early history – This is suggested by the huge variety of mechanisms for self-replication among these particles, as has been proposed for the pre-DNA world – They could be degenerate descendants of parasitic cells – Ancient parasites may have become dependent on hosts’ biochemical machinery Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Are viruses alive? Are they “life”? Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Does Agent Smith understand viruses? Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc.