General Biology. Structure of Viruses. Viral Genomes

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1 Course No: BNG2003 Credits: 3.00 General Biology 12. Viruses and Bacteria Bacteria, Viruses and Biomedical Engineering: - Medicine ---> Biofilms etc - Energy: Biofuel Cells - Environment/Industries: Bioremediation - Biotechnology: any kind of recombinant applications production of recombinant for the treatment of human diseases. Prof. Dr. Klaus Heese Microbial Model Systems Viruses called bacteriophages can infect and set in motion a genetic takeover of bacteria, such as Escherichia coli E. coli and its viruses are called model systems because of their frequent use by researchers in studies that reveal broad biological principles Beyond their value as model systems viruses and bacteria have unique genetic mechanisms that are interesting in their own right 0. µm Recall that bacteria are prokaryotes with s much smaller and more simply organized than those of eukaryotes Viruses are smaller and simpler still A virus has a genome but can reproduce only within a host Scientists were able to detect viruses indirectly long before they were actually able to see them Virus Bacterium Animal Animal nucleus 0.2 µm Structure of Viruses Viruses are very small infectious particles consisting of nucleic acid enclosed in a protein coat and, in some cases, a membranous envelope. Viral Genomes Viral genomes may consist of double- or single-stranded double- or single-stranded s and Envelopes A capsid is the protein shell that encloses the viral genome and can have various structures Capsomere of capsid Capsomere mm nm (diameter) 20 nm 0 nm (a) Tobacco mosaic virus (b) Adenoviruses Some viruses have envelopes which are membranous coverings derived from the membrane of the host Membranous envelope nm (diameter) (c) Influenza viruses 0 nm Bacteriophages, also called phages have the most complex capsids found among viruses Head Tail sheath Tail fiber nm 0 nm (d) Bacteriophage T4 1

2 General Features of Viral Reproductive Cycles Viruses are obligated intraular parasites - they can reproduce only within a host Each virus has a host range - a limited number of host s that it can infect Viruses use enzymes, ribosomes, and small molecules of host s to synthesize progeny viruses Entry into and uncoating of Replication Viral Viral VIRUS m Transcription Reproductive Cycles of Phages Phages are the best understood of all viruses; they go through two alternative reproductive mechanisms: the lytic cycle and the lysogenic cycle The Lytic Cycle is a phage reproductive cycle that culminates in the death of the host produces new phages and digests the host s wall, releasing the progeny viruses Self-assembly of new virus particles and their exit from The lytic cycle of phage T4, a virulent phage The Lysogenic Cycle 1 Attachment. The T4 phage uses its tail fibers to bind to specific 2 Entry of phage receptor sites on the outer and degradation of host. surface of an E. coli. The sheath of the tail contracts, injecting the phage into the and leaving an empty Release. The phage directs production of an enzyme that damages the bacterial capsid outside. The s wall, allowing fluid to enter. The is hydrolyzed. swells and finally bursts, releasing 100 to 200 phage particles. Phage assembly The lysogenic cycle replicates the phage genome without destroying the host. Temperate phages Head Tails Tail fibers 4 Assembly. Three separate sets of self-assemble to form phage heads, tails, and tail fibers. The phage genome is packaged inside the capsid as the head forms. 3 Synthesis of viral genomes and. The phage directs production of phage and copies of the phage genome by host enzymes, using components within the. are capable of using both the lytic and lysogenic cycles of reproduction The lytic and lysogenic cycles of phage λ (lambda), a temperate phage Phage Phage The phage attaches to a host and injects its. Bacterial chromosome Phage circularizes Occasionally, a prophage exits the bacterial chromosome, initiating a lytic cycle. Many divisions produce a large population of bacteria infected with the prophage. Reproductive Cycles of Animal Viruses The nature of the genome is the basis for the common classification of animal viruses The lyses, releasing phages. Lytic cycle Lysogenic cycle Certain factors determine whether Lytic cycle Lysogenic cycle or Prophage is induced is entered The bacterium reproduces normally, copying the prophage and transmitting it to daughter s. New phage and are synthesized and assembled into phages. Phage integrates into the bacterial chromosome, becoming a prophage. Classes of animal viruses 2

3 Viral Envelopes Many animal viruses have a membranous envelope Viral glyco on the envelope bind to specific receptor molecules on the surface of a host Retroviruses, such as HIV, The reproductive cycle of an enveloped virus: use the enzyme reverse transcriptase to copy their genome into (see slides below) 1 Glyco on the viral envelope bind to specific receptor molecules (not shown) on the host, promoting viral entry into the. Envelope (with glyco) 2 and viral genome enter Viral genome () Template Complementary strands also function as m, which is translated into both capsid (in the cytosol) and glyco for the viral envelope (in the ER). 3 The viral genome (red) functions as a template for synthesis of complementary strands (pink) by a viral enzyme. m ER Glyco Copy of genome () 4 New copies of viral genome are made using complementary strands as templates. 6 Vesicles transport envelope glyco to the plasma membrane. 8 New virus 7 A capsid assembles around each viral genome molecule. The reproductive cycle of HIV, a retrovirus as Viral Genetic Material The broadest variety of genomes is found among the viruses that infect animals HIV Membrane of white blood Retroviruses, such as HIV, use the enzyme reverse transcriptase to copy their genome into, which can then be integrated into the host genome as a provirus 1 The virus fuses with the s plasma membrane. The capsid are removed, releasing the viral and. 2 Reverse transcriptase catalyzes the synthesis of a strand complementary to the viral. 3 Reverse transcriptase catalyzes the synthesis of a second strand complementary to the first. Reverse transcriptase Viral - hybrid 4 The double-stranded is incorporated as a provirus into the s. 0.2 µm HIV entering a Viral envelope NUCLEUS Chromosomal genome for the next viral generation Provirus Proviral genes are transcribed into molecules, which serve as genomes for the next viral generation and as ms for translation into viral. m 6 The viral include capsid and reverse transcriptase (made in the cytosol) and envelope glyco (made in the ER). Reverse transcriptase (two identical strands) New HIV leaving a 9 New viruses bud off from the host. 8 s are assembled around viral genomes and reverse transcriptase molecules. 7 Vesicles transport the glyco from the ER to the s plasma membrane. 3

4 Retrovirus Family Single Strand (+) virus Contains 2 copies Can infect non-dividing s Contains regulatory Latent or active viral states Spread by blood and other bodily fluids Long lived virus leading to reservoir for persistent infection Virus type present in many vertebrate species Lentiviruses Evolution of Viruses Viruses do not really fit our definition of living organisms Since viruses can reproduce only within s, they probably evolved after the first s appeared, perhaps packaged as fragments of ular nucleic acid Viruses, viroids, and prions are formidable pathogens in animals and plants Diseases caused by viral infections affect humans, agricultural crops, and livestock worldwide Viral Diseases in Animals Viruses may damage or kill s by causing the release of hydrolytic enzymes from lysosomes Some viruses cause infected s to produce toxins that lead to disease symptoms Vaccines are harmless derivatives of pathogenic microbes that stimulate the immune system to mount defenses against the actual pathogen can prevent certain viral illnesses A few years ago, the Severe Acute Respiratory Syndrome (SARS) appeared in China Emerging Viruses are those that appear suddenly or suddenly come to the attention of medical scientists (a) Young ballet students in Hong Kong (b) The SARS-causing agent is a coronavirus wear face masks to protect themselves like this one (colorized TEM), so named for the from the virus causing SARS. corona of glycoprotein spikes protruding from the envelope. Outbreaks of new viral diseases in humans are usually caused by existing viruses that expand their host territory Viral Diseases in Plants More than 2,000 types of viral diseases of plants are known Common symptoms of viral infection include spots on leaves and fruits, stunted growth, and damaged flowers or roots Plant viruses spread disease in two major modes horizontal transmission, entering through damaged walls vertical transmission, inheriting the virus from a parent Viroids and Prions: The Simplest Infectious Agents Viroids are circular molecules that infect plants and disrupt their growth Prions are slow-acting, virtually indestructible infectious that cause brain diseases in mammals; prions propagate by converting normal into the prion version Prion Normal protein Original prion New prion Many prions 4

5 Rapid reproduction, mutation, and genetic recombination contribute to the genetic diversity of bacteria Bacteria allow researchers to investigate molecular genetics in the simplest true organisms The Bacterial Genome and Its Replication The bacterial chromosome is usually a circular molecule with few associated In addition to the chromosome, many bacteria have plasmids, smaller circular molecules that can replicate independently of the bacterial chromosome Bacterial s divide by binary fission which is preceded by replication of the bacterial chromosome Origin of replication Termination of replication Replication fork Mechanisms of Gene Transfer and Genetic Recombination in Bacteria Three processes bring bacterial from different individuals together Transformation Transduction Conjugation Transformation is the alteration of a bacterial s genotype and phenotype by the uptake of naked, foreign from the surrounding environment Transduction In the process known as transduction Phages carry bacterial genes from one host to another Phage Conjugation and Plasmids Conjugation is the direct transfer of genetic material between bacterial s that are temporarily joined 1 Phage infects bacterial that has alleles A + and B + A + B + 2 Host (brown) is fragmented, and phage and are made. This is the donor. A + B + Donor 3 A bacterial fragment (in this case a fragment with the A + allele) may be packaged in a phage capsid. 4 Phage with the A + allele from the donor infects a recipient A B, and crossing over (recombination) between donor (brown) and recipient (green) occurs at two places (dotted lines). A + Crossing over A + A B Recipient The genotype of the resulting recombinant (A + B ) differs from the genotypes of both the donor (A + B + ) and the recipient (A B ). A + B Recombinant Sex pilus 1 µm