Origin of Life Prokaryotes - Lecture Notes

Origin of life/ prokaryotes Why do some scientists believe that RNA, rather than DNA, was the first genetic material? RNA has both information storage and catalytic properties. Explanation: RNA has been shown to catalyze certain reactions (as part of ribosomes, rRNA participates in the catalysis of the peptide bonds that form proteins). It can also store information and perform some self-replication. Which group was not described in Woese's tree of life analysis? Prokarya Explanation: Prokarya was not one of the groups described in Woese's tree of life analysis. What characteristics were used to classify organisms before Woese's tree of life analysis? Presence of a membrane-bound nucleus Explanation: Before the tree of life analysis, organisms were classified as prokaryotes if they lacked a membrane-bound nucleus or eukaryotes if they possessed a membrane-bound nucleus. True or false? Data showing that the Bacteria were the first lineage to diverge from the common ancestor of all living organisms suggest that the Archaea and Eukarya are more closely related to each other than they are to the Bacteria. True Which of the following statements about phylogenetic trees is true? A paraphyletic group consists of a common ancestor and some of its descendants. Explanation: This statement is true; the common characteristics of a paraphyletic group may have arisen from lateral gene transfer to some, but not all, lineages. Which molecule did Carl Woese study to produce his tree of life? Ribosomal RNA Explanation: Ribosomal RNA was the molecule used to produce Woese's phylogeny because it evolves slowly and is critical to the function of the ribosome, which serves the same function in all three domains in the tree of life. What is lateral gene transfer? Physical transfer of a gene from a species in one lineage to a species in another lineage. Explanation: Lateral gene transfer is the physical transfer of a gene between distantly related lineages. Classify each statement or picture as applying to gram-positive bacteria, gram-negative bacteria, or both. Drag each item to the appropriate bin. Gram-positive bacteria: appear purple after gram staining, have a thick peptidoglgycan layer, alcohol rinse does not remove crystal violet Gram-negative bacteria: have a thin peptidoglycan layer, appear pink after gram staining, alcohol rinse easily removes crystal violet, have an outer membrane as part of their cell wall structure. Both: have a plasma membrane. Explanation: Gram staining is a technique for classifying bacteria based on differences in the structure of their cell walls. Gram-negative bacteria and gram-positive bacteria both have a plasma membrane surrounded by a cell wall that contains peptidoglycan. However, the cell wall of gram-negative bacteria is composed of a thin layer of peptidoglycan and an outer membrane, whereas the cell wall of gram-positive bacteria is composed of a thick layer of peptidoglycan. Gram-negative bacteria appear red or pink because the alcohol rinse washes away the crystal violet dye. Gram-positive bacteria appear purple because the alcohol rinse does not wash away the crystal violet dye. a. autotrophs b. heterotrophs c. chemoautotrophs d. photoautotrophs e. photoheterotrophs f. chemoheterotrophs g. phototrophs h. chemotrophs Explanation: Some bacteria obtain energy from light (phototrophs), whereas other bacteria obtain energy from chemicals (chemotrophs). Autotrophs (literally “self-feeders”) require only an inorganic substance, such as carbon dioxide, as their carbon source; heterotrophs (literally “other-feeders”) require at least one organic nutrient as their carbon source. Azotobacter is a genus of bacteria that live in soil and have the following characteristics:  They are bacilli.  They are gram-negative.  They are obligate aerobes.  They can fix nitrogen. (Unlike some other nitrogen-fixing bacteria, which associate with the roots of plants, Azotobacter species are free-living.) Select the four statements that are true for bacteria in the genus Azotobacter.  They are shaped like rods.  They have a relatively thin layer of peptidoglycan in their cell wall.  They can convert atmospheric nitrogen to ammonia.  They use O2 for cellular respiration. Explanation: Azotobacter is a genus of rod-shaped bacteria (bacilli). After Gram staining, bacteria in this genus appear pink due to the thin peptidoglycan layer in their cell wall. They require oxygen for cellular respiration and can convert atmospheric nitrogen to ammonia, which they incorporate into amino acids and other organic molecules How does the large amount of genetic variation observed in prokaryotes arise? They have extremely short generation times and large population. They can exchange DNA with other types of prokaryotes by way of horizontal gent transfer. Explanation: The short generation times and large population sizes in most prokaryotic species, coupled with their ability to exchange genes, helps to increase genetic variability beyond what we would expect in asexually reproducing organisms. Place the steps involved in the process of bacterial transformation in the correct order. Drag the labels to the appropriate locations in this flowchart. Explanation: Bacterial transformation is a process in which a recipient cell takes up DNA from a lysed donor cell. The donor cell DNA can become part of the recipient cell chromosome if homologous recombination occurs in regions where the two types of DNA are similar. The figure shows how bacteria reproduce through binary fission. 1. 2. 3. 4. 5. 6. Binary fission is a form of asexual reproduction because a single individual is the sole parent and passes copies of all its genes to its offspring. During binary fission, DNA replication begins at the origin of replication and continues all the way around the circular chromosome. When DNA replication is complete and the bacterium has reached twice its original size, the plasma membrane grows inward, dividing the parent cell into two daughter cells. In theory, the two daughter cells are genetically identical to each other and the parent cell. However, mistakes made during DNA replication sometimes result in mutations , changes to the nucleotide sequence of DNA. The rapid reproduction and large population sizes of many bacterial populations mean that even a small mutation rate can lead to considerable genetic variation . Explanation: Bacteria reproduce through binary fission.    During binary fission, the DNA in the bacterial chromosome is replicated, and one copy is passed to each daughter cell. Although plasmids (small, circular DNA molecules separate from the main bacterial chromosome) are not shown in the figure, any plasmids in a bacterial cell are also replicated and passed to each daughter cell during binary fission. Mutations that arise during DNA replication are one source of genetic variation in bacterial populations.  The rapid reproduction and large sizes of many bacterial populations can yield considerable genetic variation despite a low mutation rate. Because bacteria reproduce asexually, each organism typically obtains its genetic material from only one parent. However, bacteria do have mechanisms for bringing together DNA from different individuals. One of these mechanisms is bacterial conjugation, a process that results in the direct transfer of genetic material from one bacterium to another. The figure shows two examples of bacterial conjugation. In these examples, bacteria with a segment of DNA called an F factor are able to initiate conjugation.  An F factor is a piece of DNA that contains about 25 genes.  Most of the genes in the F factor are related to the production of a structure called a mating bridge. The mating bridge links two bacterial cells during conjugation, allowing for DNA transfer.  The F factor may be found on a separate piece of DNA called an F plasmid, shown in part (a) of the figure. Or it may be found within the main bacterial chromosome, shown in part (b) of the figure. F+ cell: carries the F factor in a plasmid, may donate a plasmid to another cell. Hfr cell: carries the F factor in its circular chromosome, may donate genes from its chromosome to another cell. F- cell: may receive genetic material, but can not donate genetic material, may become recombinant after conjugation with an Hfr cell, may become F+ after conjugation with an F+ cell. Explanation: Although the figure above focuses on the F factor, bacterial conjugation can occur in other ways as well. In Part C, you will learn that a different type of conjugation -- involving a plasmid known as the R plasmid -- is important in the spread of antibiotic resistance from one species of bacteria to another.  Conjugation allows one bacterial cell to transfer genetic material to another bacterial cell. In this way, conjugation contributes to genetic variation in bacterial populations.  Conjugation can occur between bacterial cells of the same species or different species. The ability to easily transfer genes from one species to another has huge implications for bacterial evolution. What is the difference between an F+ donor and an Hfr donor? The Hfr donor has an F factor integrated into its main chromosome, but the F+ donor does not. Explanation: Both the F+ donor and the Hfr donor cells have F factors. The difference between the two types of donors is that the F+ donor has its F factor in an F plasmid, whereas the Hfr donor has its F factor in its main chromosome. What usually happens to the recipient cell following conjugation with an Hfr cell? The recipient cell remains an F- cell Explanation: When an Hfr cell conjugates with an F- cell, the recipient cell does not usually receive the entire F factor from the donor cell. So, the recipient usually remains an F- cell. Which of the following typically produces a recombinant F+ recipient cell? conjugation between an F+ cell and an F- cell Explanation: Conjugation between an F+ cell and an F- cell typically produces a recombinant F+ recipient cell. The F+ cell transfers a copy of its F plasmid to the F- cell, turning it into an F+ cell. Which of the following could cause DNA from the main chromosome of a donor cell to be incorporated into the main chromosome of the recipient cell? conjugation between an Hfr cell and an F- cell Explanation: Conjugation between an Hfr cell and an F- cell may result in DNA being transferred from the main chromosome of the donor cell to the main chromosome of the recipient cell. This occurs when the Hfr cell transfers a piece of DNA to the recipient cell and then crossing over occurs between the transferred piece of DNA and the recipient cell's chromosome. Suppose conjugation occurs between an Hfr cell and an F- cell. Although not typical, what would have to occur for the recipient cell to become an Hfr cell? The entire F factor would have to be incorporated into the recipient cell’s chromosome. Explanation: A cell must have the entire F factor in its main chromosome to be an Hfr cell. In order for the recipient cell to become an Hfr cell, it would have to receive the entire F factor from the donor cell and incorporate this F factor into its main chromosome through crossing over. Can you distinguish among the three processes of genetic transfer in bacteria? Conjugation only: requires cell-to-cell contact, involves the transfer of DNA from an Hfr cell to an F- cell Transduction only: involves bacteriophage transferring pieces of bacterial DNA from one cell to another. Transformation only: involves DNA form the environment being taken up into a bacterial cell. Explanation: Bacteria can acquire new genes from the environment through transformation, by direct contact with another bacteria cell through conjugation, or by being infected with a bacteriophage through transduction. Which of the following is true of generalized bacteriophage transduction? The bacteriophage packages fragments of bacterial DNA into new phage particles. Explanation: During transduction, phages package fragments of bacterial DNA into new phage heads. This bacterial DNA can be transferred to another bacteria cell during the next phage infection. Which statement is true about obligate anaerobes? They are poisoned by O2 Explanation: Some obligate anaerobes live exclusively by fermentation; others extract chemical energy by anaerobic respiration, in which substances other than O2 accept electrons at the "downhill" end of electron transport chains. Oxygen is toxic to them. Photoautotrophs use __________. Light as an energy source and CO2 as a carbon source Plantlike photosynthesis that releases O2O2 occurs in __________. Cyanobacteria Eukaryotes According to the endosymbiotic theory of the origin of eukaryotic cells, how did mitochondria originate? from engulfed, originally free-living proteobacteria Biologists think that endosymbiosis gave rise to mitochondria before plastids partly because __________. all eukaryotes have mitochondria (or their remnants), whereas many eukaryotes do not have plastids Which of the following was derived from an ancestral cyanobacterium? chloroplast Plastids that are surrounded by more than two membranes are evidence of __________. secondary endosymbiosis Which of these observations gives the most support to the endosymbiotic theory for the origin of eukaryotic cells? the similarity in size between the cytosolic ribosomes of prokaryotes and the ribosomes within mitochondria and chloroplasts Which of the following is true of secondary endosymbiosis? An organism containing an endosymbiont is engulfed by another organism and becomes an endosymbiont. Explanation: This is a description of secondary endosymbiosis. In many cases secondary endosymbionts are surrounded by four membranes. Which of the following is a reason why kingdom Protista has been abandoned? Some protists are more closely related to plants, animals, or fungi than they are to other protists Explanation: Recognizing that the kingdom Protista as originally defined was polyphyletic and that some protists are more closely related to other eukaryotes than to each other, scientists have abandoned the kingdom Protista and reorganized the entire domain Eukarya. The evolution of complex multicellularity in eukaryotes __________. occurred independently in several eukaryotic lineages Which of the following is the most likely sequence of events, from earliest to most recent, in the evolution of sponges (organisms with many cells that have different functions)? 1. single-celled eukaryote 2. single-celled prokaryote 3. multicellular eukaryote 4. colonial eukaryote 2, 1, 4, 3 The evolution of multicellularity in animals required adaptations that promoted cellular adhesion and intercellular communication The evolution of multicellularity in animals has primarily occurred by changing how existing genes are used