Chapter 26 Viruses

Chapter 26 : Viruses Note : In this chapter the important points are in bold font and the less important are in normal font . so if you don’t have an enough time ; study only important points . Concept 26.1: A virus consists of a nucleic acid surrounded by a protein coat • The story of how viruses were discovered begins near the end of 19 th century . ❖ The Discovery of Viruses: o the beginning was with Tobacco mosaic diseases >> ▪ Tobacco mosaic diseases stunts the growth of tobacco plants and gives their leaves a mosaic coloration . • • At the beginning scientist thought the disease transmit from plant to another by microbe and then they said that was wrong ! they suggest it caused by an invisible bacteria , they tested this suggestion by the next inquiry … Figure 26.2 : What causes Tobacco mosaic disease Scientist knew that if the sap of infected plant transferred to another healthy plant that will transfer the disease , by this experiment they want to Know if bacteria is responsible for this transferring of disease ! Result : although they isolate bacteria from the sap in step #2 the disease was transferred . that mean bacteria wasn’t responsible ! but after this inquiry scientist said bacteria is too small , so it may still in the filtered sap (filter passed it with the sap) . 1|Page • all of these arguments had finished when Martinus Beijerink found by his experiences that the infectious agent in the filtered sap could reproduce. so agent is not bacteria ! ▪ In the late 1800s, Martinus Beijerink hypothesized that a particle smaller than bacteria caused the disease . and this particle can reproduce itself within the host it infected . ▪ In 1935, Wendell Stanley confirmed this hypothesis by crystallizing the infectious particle, now known as tobacco mosaic virus (TMV) . ❖ Structure of Viruses • • Viruses are not cells Viruses are very small infectious particles consisting of nucleic acid enclosed in a protein coat and, in some cases, a membranous envelope • Smallest viruses are only 20nm in diameter (smaller than a ribosome ) even largest viruses are a hundreds of nm ! a) viral Genome • Viral genomes may consist of either – Double- or single-stranded DNA, or – Double- or single-stranded RNA  Depending on its type of nucleic acid, a virus is called a DNA virus or an RNA virus • Genome is usually organized as a single linear or circular molecule of nucleic acid , sometimes it consists of multiple molecules of nucleic acid . • Smallest viruses have only 4 genes in their genome while the largest have several hundred to a thousand . b) Capsids and Envelopes • • capsid : is the protein shell that encloses the viral genome • it is built from protein subunits called capsomeres capsids can have various structures look to figure 26.3 to Know them .. 2|Page Figure 26.3 Viral structure a) Tobaco mosaic virus : it has a rigid rod-shaped capsid made from a thousand molecules of one type of protein arranged in a helix (rod-shaped usually called helical viruses). b) Adenovirus : this virus infect the respiratory system of animals , it is shaped in polyhedral capsid with a glycoprotein spike at each vertex . Its Caspid consists of 252 identical protein with 20 triangular facets (because of that it is called (icosahedrons ) and all similarly shaped viruses has the same name "icosahedrons" . c) Influenza virus : it has an outer envelope studded with glycoprotein spikes , the genome consists of 8 different RNA molecules . • Some viruses have membranous envelopes that help them infect hosts - These viral envelopes surround the capsids of influenza viruses and many other viruses found in animals - Viral envelopes, which are derived from the host cell’s membrane, contain a combination of viral molecules (proteins and Glycoprotein) and host cell molecules (phospholipids and membrane proteins) . d) Bacteriophages "phages" : are viruses that infect bacteria , They have the most complex capsids found among viruses T-even phages Have elongated icosahedral heads (contain their DNA) and a protein tail piece with fibers to attach with bacterium and inject the phage DNA inside it . 3|Page Concept 26.2: Viruses reproduce only in host cells • • Viruses are obligate intracellular parasites, which means they can reproduce only within a host cell • They lack metabolic enzymes and equipments for making proteins . Each virus has a host range, a limited variety of host cells that it can infect. (thus because of the evolution of recognition systems by the virus ) .  Virus identify host cells by a "lock and Key " Fit between viral surface proteins and specific receptor molecules on the outside of cells . • So viruses may affects a specific type of organism or a particular tissues in many organisms . ❖ General Features of Viral Reproductive Cycles Figure 26.4 A simplified viral reproductive cycle 4|Page Notes for figure 26.4 : 1) The mechanism of genome entry depends on the type of virus and type of host cell. T-even phages use their elaborate tail apparatus , other viruses depend on endocytosis , or by fusion of viral envelope with plasma membrane . ▪ Once a viral genome has entered a cell, the cell begins copying the viral nucleic acid and manufacture viral proteins 2) Most DNA-viruses use DNA polymerase of host cell to synthesize new genomes along the templates of Viral DNA . In contrast to replicate their genomes, RNA viruses use virally encoded polymerase that can use RNA as a template . 3) The virus uses host enzymes, ribosomes, tRNAs, amino acids, ATP, and other molecules needed to produce the viral proteins (capsid proteins) 4) The simplest type of viral reproductive cycle ends with the exit of hundreds or thousands of viruses from the infected host cell . ❖ Reproductive Cycles of Phages • Phages are the best understood of all viruses • Phages have two reproductive mechanisms: the lytic cycle and the lysogenic cycle . a) The Lytic Cycle • • • • The lytic cycle is a phage reproductive cycle that culminates in the death of the host cell The Word "lytic" refers to the last stage of infection .. The lytic cycle produces new phages and digests the host’s cell wall, releasing the progeny viruses A phage that reproduces only by the lytic cycle is called a virulent phage Stages of lytic cycle : look to the figure in next page 1) Attachment : the T4 phage uses its tail fibers to bind to specific receptor sites on the outer surface of an E.coil cell . 2) Entry of phage DNA and degradation of host DNA : the sheath of the tail contracts , injecting the phage DNA into the cell and leaving an empty capsid outside . the cell's DNA is hydrolyzed . 3) Synthesis of viral genomes and proteins : the phage DNA directs production of phage proteins and copies of the phage genome by host enzymes , using components within the cell . 5|Page 4) Assembly : Three separate sets of proteins self-assemble to form phage heads , tails , and tail fibers .The phage genome is packaged inside the capsid as the head forms . 5) Release : the phage directs production of an enzyme that damages the bacterial cell wall, allowing fluid to enter . The cell swells and finally bursts releasing 100 to 200 phage particles . Figure 26.5 The lytic cycle of phage T4, a virulent phage Notes : - The phage DNA is protected from breakdown because it contains a modified form of cytosine that isn’t recognized by enzymes . - lytic cycle takes only 20-30 minutes at 37 C . • Bacteria have defenses against phages, 1) By restriction enzymes that recognize and cut up certain phage DNA 2) Some mutant bacteria have difficult recognizing receptors for a particular types of phages . ** but sometimes there are mutant phages that can bind to more than one type of receptors or resistant to particular types of restriction enzymes .. 6|Page b) The Lysogenic Cycle • The lysogenic cycle replicates the phage genome without destroying the host cell . – Steps of reproduction : 1. The viral DNA molecule is incorporated into the host cell’s chromosome , This integrated viral DNA is known as a prophage . 2. A one prophage gene codes for protein , this protein prevents transcription of most of the other prophage genes . 3. this phage genome will still silent within the bacterium . 4. when bacterium prepare to divide it replicates the phage DNA with its own and passes the copies (in prophage form) to its daughter cells . <> • Phages that use both the lytic and lysogenic cycles are called temperate phages or (lambda ʎ ) • for example: T4 and it is called (phage ʎ) , (but phage ʎ differs from original T4 ; it has only a short tail fiber) when it infects any E.coil cell it begins with binds to the surface of the cell and injects its linear DNA genome , then ʎ DNa molecule forms a circle, what happens next depends on the reproductive cycle … look to figure 26.6 and study it carefully ☺ Figure 26.6 The lytic and lysogenic cycles of phage λ, a temperate phage 7|Page • Lysogenic term means prophages are capable of generating active phages that lyse their host cells , that happens when : 1- environmental signal (chemical / high energy radiation )can trigger the virus genome to exit the bacterial chromosome and switch to the lytic mode . 2- there are in addition to the main prophage other types of prophage genes expressed and alter the hosts phenotype . ---------------------------------------------------- ❖ Reproductive Cycles of Animal Viruses • There are two key variables used to classify viruses that infect animals: – – DNA or RNA? Single-stranded or double-stranded?  We will talk about Viral Envelope and viral genetic material roles : a) Viral Envelopes • The viral Envelope is derived from the host cell plasma membrane with molecules specified by viral gene . Figure 26.7 The reproductive cycle of an enveloped RNA virus 1 Single stranded RNA 2 3 5 4 This mechanism shows the formation of new envelopes for producing a new viruses . 8|Page 7 6 8 Figure illustration : 1) Glycoproteins on the viral envelope bind to specific receptor molecules on the host cell ( prompting viral entry into the host cell). 2) The capsid and viral genome enter the cell . digestion of the capsid by cellular enzymes releases the viral genome . 3) The viral genome functions as a template for synthesis of complementary RNA strands by a viral enzyme . 4) New copies of viral genome RNA are made using complementary RNA strands as template . 5) Complementary RNA strands also function as mRNA , which is translated into both capsid proteins (in the cytosol) and glycoproteins for the viral envelope in the ER and Golgi apparatus . 6) Vesicles transport envelope glycoproteins to the plasma membrane (exocytosis) 7) A capsid assembles around each viral genome molecule 8) Each new virus buds from the cell , its envelope studded with viral glycoproteins embedded in membrane derived from the host cell . - • - This reproductive cycle doesn’t necessarily kill the host cell . Other viral membranes form from the host’s nuclear envelope and are then replaced by an envelope made from Golgi apparatus membrane such as" herpesviruses" These viruses have double –stranded DNA genome and reproduce within the host cell nucleus . b) RNA as Viral Genetic Material • • - - The broadest variety of RNA genomes is found in viruses that infect animals Remember that > In figure 26.7 the virus was from class IV , In step 3 RNA genome uses viral enzymes for RNA synthesis to produce mRNA for the next steps . - Here we will talk about another RNA animal viruses with the most complicated reproductive cycle >> Retroviruses use Reverse transcriptase (enzyme) to copy their RNA genome into DNA . Retro means backward ; it is an unusual phenomenon !! 9|Page • These viruses are enveloped viruses and contain 2 identical molecules of single stranded RNA and 2 molecules of reverse transcriptase . • One of them is >> HIV (human immunodeficiency virus) that causes AIDS (acquired immunodeficiency syndrome) . Look to figure 26.8 to know its reproductive cycle >> Figure 26.8 The reproductive cycle of HIV, the retrovirus that causes AIDS 1 2 3 4 5 6 9 7 10 10 | P a g e 8 Figure Illustration >> 1) The envelope glycoproteins enable the virus to bind to specific receptors on certain white blood cells . 2) The virus fuses with the cells plasma membrane . The capsid proteins are removed , releasing the viral proteins and RNA . 3) Reverse transcriptase catalyzes the synthesis f a DNA strand complementary to the viral RNA . 4) Reverse transcriptase catalyzes the synthesis of a second DNA strand complementary to the first . 5) The double stranded DNA is incorporated as a provirus into the cell's DNA 6) Proviral genes are transcribed into RNA molecules , which serve as genomes for the next viral generation and as mRNAs for translation into viral protein . 7) The viral proteins include capsid proteins and reverse transcriptase (made in the cytosol) and envelope glycoproteins (made in the ER). 8) Vesicles transport the glycoproteins to the cell‘s plasma membrane . 9) Capsids are assembled around viral genomes and reverse transcriptase molecules. 10) New viruses bud off from the host cell . - Note : (In step 5 ) Provirus is the integrated viral DNA , Provirus is Unlike a prophage , it remains a permanent resident of the host cell The End of Chapter 26 11 | P a g e