Chapter 13 Early Life Forms and Viruses
Though we never think of micro organisms (like bacteria, protists, and viruses) unless we hear about them making someone sick, most of these organisms are not harmful. • Many are beneficial in that they serve as food for larger organisms and keep nutrients cycling through ecosystems.
Evolution of the First Cells • Scientists believe that the atmosphere of early Earth contained a mixture of gases such as carbon dioxide, nitrogen, hydrogen, and water vapor from spewing volcanoes with little to no oxygen. • Lightning flashes shooting through this atmosphere enabled the first organic molecules (such as amino acids and sugars to form) • The next step in the evolution of the first cells had to be the concentration of these organic molecules in order to form more complex organic molecules. • The early RNA world hypothesis suggests that, in the beginning, RNA stored both genetic information and acted as an enzyme to produce proteins. In fact, some RNA molecules still act as enzymes today. The RNA enzymes could have copied themselves to be passed on to other cells. • Next, protocells evolved. Protocells are membranous sacs composed of lipids that would have enclosed interacting organic molecules and would have been able to divide on their own.
Prokaryotes • The first cells were probably anaerobic prokaryotes that arose 3-4 billion years ago. • They probably used dissolved carbon as their carbon source and mineral ions for energy. This would make them chemosynthesizers. • The two prokaryotic domains, Archae and Bacteria, diverged very early in the history of life. • Soon after, some members of each group evolved the ability to do photosynthesis without producing oxygen. Some modern photosynthesizers still do this. • Then, about 2.7 billion years ago, a linage of prokaryotes evolved that released oxygen when doing photosynthesis. This caused oxygen to begin to build up in the atmosphere. • This oxygen 1) prevented the formation of complex organic molecules from nonliving materials, 2) selected for organisms that could thrive in high-oxygen conditions, and 3)resulted in the formation of the ozone layer.
Evolution of organelles and eukaryotes • Eukaryotes, with their membrane-bound nucleus, evolved about 1.8 million years ago. • Scientists believe that the membrane around the nucleus, which is continuous with the ER, evolved from an infolding of the plasma membrane and that this infolding membrane around the nucleus was favored because it protected the cell’s genetic material. • Mitochondria and chloroplasts, wither their own prokaryotic DNA and ribosomes are believed to have arisen according to the EndosymbioticTheory. • The first eukaryotes were also the first protists, red alga that lived about 1.2 billion years ago. • They were multicellular-some cells held the alga in place, while others produced sexual spores; therefore, this red alga was one of the first sexually reproducing organisms. • The first animal appeared about 570 million years ago. • By the end of the Cambrian period, 543 million years ago, all major animal lineages, including vertebrate, were represented in the seas.
The prokaryotes • No nucleus or other membrane-bound organelles, very small, single circular molecule of DNA called a nucleoid • Porous cell wall around cell membrane, can be spherical (coccus), spiral (spirillum), or rod-shaped (bacillus) • Move using propeller-like flagella or pili. • Existed for 2 billion years before eukaryotes evolved. • Very successful: over 5,000,000,000,000,000,000,000,000,000,000 cells prokaryotic cells exist on Earth right now. • Can be found in our bodies, in hot hydrothermal vents, glacial ice, acidic springs, and even the driest desert. • Four different ways of meeting their energy requirements: 1) photoautotrophic, chemoheterotrophic, photoheterotrophic, chemoautotrophic
Prokaryotic reproduction • Many divide every 20 minutes by a process called prokaryotic or binary fission, which yields 2 equal sized, genetically identical daughter cells. • Don’t sexually reproduce but can transfer genetic material in the form of a plasmid using a sex pilus in a process called conjugation. • In addition, bacteria can also take up a plasmid from their environment in a process called bacterial transformation. • Finally, viruses that infect bacteria, called bacteriophages, can move genes from one cell to another, changing the bacterial genome.
Prokaryotic classification • Classification can be difficult since bacteria can exchange gene between species, even with those in the other domain, and because many cannot be grown in a lab. • Scientists define prokaryotic species by focusing on shared ancestry as revealed by gene sequence studies and shared phenotypic traits. • Are also categorized into strains within species based upon specific traits. • Eukaryotes are more closely related to Archaebacteria than Bacteria.
Domain: Archae (Archaebacteria) • More recently discovered but less well known since they can live in very extreme environments. • Extreme thermophiles: can live in very hot places • Extreme halophiles: can live in very salty water • Methanogens: make methane; cannot tolerate oxygen; live in our guts and cause us to “pass gas” • Abundant in deep ocean • Not particularly harmful to humans
Domain: Bacteria (Eubacteria) • More diverse and better studied • Plasma membrane, cell wall, and flagelle are built differently than archaens. • Genetically distinct from archae • Includes cyanobacteria that are the ancestors of chloroplasts • Cyanobacteria not only produce oxygen but also participate in the nitrogen available to plants so that it can enter living things-called nitrogen fixation. • Bacteria also serve as decomposers, breaking down organic matter and turning it back into its inorganic subunits. • Beneficial prokaryotes that normally live in or on our body are referred to as our normal flora.
Disease-causing bacteria • Called pathogens, organisms that infect other species and cause diseases • An animal that transmits a pathogen is called a vector (for example, a tick is the vector that transmits Lyme disease). • Sexually transmitted bacterial diseases include: gonorrhea, syphilis, and chlamydia. • May also be taken in when eating or drinking contaminated food. • Many very harmful pathogens (such as anthrax and the bacterium that causes lockjaw) can form spores, which is a dormant state that the cells enter until conditions are more favorable. • These spores (also called endospores) can survive boiling, irradiation, and dessication.
protists • Most are single-celled; a few are colonial or multi-cellular • Many are photoautotrophs, but some are predators, parasites, or decomposers • Are very diverse, consist of may different lineages • Many lineages are more closely related to plants, animals, or fungi than they are to each other.
protists Flagellated protozoa foraminiferans Single-celled predators that secrete a shell of calcium carbonate Live on the sea floor Some are plankton, microscopic organisms that drift or swim in the open sea These plankton often have photosynthetic protists that live inside their cells • Heterotrophic protists that live as single cells • No cell wall • Have one or more flagella • Have a pellicle (layer of elastic proteins) that help cell keep its shape • Include Trichomonas(STD), Giardia, Trypanosomes (parasitic), Euglena (freshwater) • Euglena have contractile vacuoles that allow water that diffuses into cell to be pumped back out.
protists ciliates dinoflagellates Means “whirling flagellate” Single-celled with two flagellacell rotates as it moves forward Deposits cellulose beneath cell membrane, forming think protective plates Live in freshwater and ocean May be predators, parasites, or photosynthetic Some are bioluminescent, converting ATP into light One of the protists that undergoes algal bloom in nutrient-enriched waters • Ciliated protozoans • No cell wall • Have many cilia • Most are predators that feed on bacteria, algae, or one another • Live in the gut of mammalian grazers, helping them to digest plant material
protists apicomplexans Water mold, Diatoms, brown algae Water molds are heterotrophs also called oomycetes Most water molds decompose organic debris and dead organisms in water but some are parasites that have a significant economic impact Diatoms have a two-part silica shell that fit together like a shoe box Deposits left by these millions of years ago are quarried to make filters, abrasive cleaners and insecticides Brown algae are multicellular, live in temperate or cool seas Range from microscopic filaments to giant kelps that stand 100 feet tall in coastal waters Both diatoms and brown algae are photosynthetic • Parasitic protists • Also called sporozoans • Possess a complex of microtubules at their apical (top) end that allows them to enter a host • Example: malaria
protists Red algae and green algae Amoebozoans No cell wall, shell, or pellicle so they can change their shape Engulf prey and move by extending lobes of cytoplasm called pseudopods Single-celled Plasmodial slime molds are flat, slimy masses that hang over logs on forest floor that feeds on bacteria and is composed of a single cell with 100’s of nuclei Cellular slime molds are individual amoeba-like cells When food runs out, cells come together, forming a “slug” that migrates in response to light and heat Cellular slime molds are used in research to research how signaling pathways of multicellular animals that are responsible for coordinated behavior evolved. • Most red algae are multicellular and live in tropical seas • Have a branching structure • Some have calcium carbonate cell walls and are a component of coral reefs • Tinted red to black due to pigments called phycobilins, allowing them to carry out photosynthesis in deeper waters • Green algae has single-celled, colonial, and multicellular forms • Can be found in fresh water mostly • Both red and green algae contain chloroplasts and so they share a common ancestor. • One lineage of green algae gave rise to land plants
viruses • A non-cellular infectious agent • Made of a protein coat wrapped around genetic material (RNA or DNA) • Can only replicate inside of living cells • Inserts its genetic material into the DAN of the host cell, essentially “hijacking” the host cell’s cellular machinery to reproduce itself. • Each virus can only infect certain hosts • Bacteriophages infect prokaryotes • Since plants have thick cell wall, plant viruses can generally only infect a plant after insects or pruning has created a wound where the virus can enter. • Adenoviruses are naked (no protein coat) viruses that infect animals • Human upper respiratory problems, hepatitis, polio, the common cold, and warts are usually caused by adenoviruses • Usually, however, animal viruses have a coat that is composed of membrane derived from the host cell. • Herpes, AIDS, rabies, rubella, bronchitis, mums, measles, yellow fever, and West Nile are caused by enveloped viruses.
Viruses Bacteriophage Herpes virus
Viral multiplication • Virus attaches to host cell by binding to a protein on host cell membrane • Virus/genetic material enters host cell • Viral genes direct cell to replicate the viral DNA/RNA and to build viral proteins • These components self-assemble to form new viruses • The new viruses are released from host cell and go to infect new cells and repeat the cycle • Bacteriophages use two different pathways to get out of host cells • Lytic pathway: virus causes host cell to produce enzymes that cause cell lysis; cell membrane breaks up, viruses escape and host cell is killed • Lysogenic pathway: viral DNA becomes incorporated into host cell DNA and is copied and passed on to descendant cells, awaiting a signal to enter the lytic cycle
Viral origins • Three hypotheses: • Viruses were parasites inside of other cells and became unable to survive on their own. • Viruses are genetic elements that escaped from cells. • Viruses represent a separate evolutionary branch; they arose independently from replicating molecules that preceded the origin of life. • Viruses infect organisms in all three domains of life. • Viruses usually decrease a host’s ability to survive and reproduce. • Though this is bad new for the host, it can actually be beneficial. • It keeps population numbers of cyanobacteria in check in the ocean. • Viruses benefit humans when they target bacteria that cause human disease or insects that eat our crops. • “The enemy of my enemy is my friend.” • Virus-based pesticides are safer than chemical ones because viruses are specific for their hosts and they do not persist in the environment.