Viruses Bacterial Genetic Abdulina G A General

Viruses. Bacterial Genetic Abdulina G. A.

General Characteristics of Viruses • Contain a single type of nucleic acid, either DNA or RNA. Nucleic acid may be single- or doublestranded, linear or circular, or divided into several separate molecules. The proportion of nucleic acid in relation to protein in viruses ranges from about 1 % to about 50%. • Capsid and Envelope The protein coat surrounding the nucleic acid of a virus is called the capsid. The capsid is composed of subunits, capsomeres, which can be a single type of protein or several types.

• • • Capsid sometimes itself enclosed by an envelope of lipids, proteins, and carbohydrates Some envelopes are covered with protein complexes called spikes. Multiply inside living cells by using the synthesizing machinery of the cell. Cause the synthesis of specialized structures that can transfer the viral nucleic acid to other cells. Viral sizes are determined with the aid of electron microscopy. Viruses range from 20 to IOOO nm in length. Viruses are obligatory intracellular parasites.

General Morphology Helical viruses their capsids are hollow cylinders surrounding the nucleic acid. .

Polyhedral viruses (for example, adenovirus) are manysided. Usually the capsid is an icosahedron

Enveloped Viruses

Nonenveloped (naked) viruses

Nomenclature The genome of a virus may consist of DNA or RNA, which may be single stranded (ss) or double stranded (ds), linear or circular. The entire genome may occupy either one NA molecule (monopartite genome) or several NA segments (multipartite genome). The different types of genome induce different replication strategies. Genome structure and mode of replication are criteria applied in the classification and nomenclature of viruses. The genomic RNA strand of single-stranded RNA viruses is called sense (positive sense) it can serve as m. RNA, and antisense (negative sense) if a complementary strand synthesized by a viral RNA transcriptase serves as m. RNA.

Cultivation of Viruses in: 1. Embryonated Eggs 2. living Animal 3. Cell Cultures

Types of the Cell Cultures Primary cell lines, derived from tissue slices, tend to die out after only a few generations. Diploid cell lines, developed from human embryos can be maintained for about 100 generations and are widely used for viruses that require a human host Continuous cell lines. These are transformed (cancerous) cells that can be maintained through an indefinite number of generations, and they are sometimes called immortal cell lines One of these, the He. La cell line, was isolated from the cancer of a woman (Henrietta Lacks) who died in 1951

Complex Viruses Some viruses, particularly bacterial viruses, have complicated structures and are called complex viruses

1. Attachment or adsorption, occurs. T-even bacteriophages use fibers at the end of the tail as attachment sites. The complementary receptor sites are on the bacterial cell wall

Penetration T-even bacteriophage injects its DNA (nucleic acid) into the bacterium. To do this, the bacteriophage's tail releases an enzyme, phage lysozyme, which breaks down a portion of the bacterial cell wall. During the process of penetration, the tail sheath of the phage contracts, and the tail core is driven through the cell wall.

3. Biosynthesis Once the bacteriophage DNA has reached the cytoplasm of the host cell, the biosynthesis of viral nucleic acid and protein occurs by the host cell

4. Maturation Bacteriophage DNA and capsids are assembled into complete virions. The viral components essentially assemble into a viral particle spontaneously, eliminating the need for many nonstructural genes and gene products

5. Release Lysozyme, which is encoded by a phage gene, is synthesized within the cell. This enzyme causes the bacterial cell wall to break down, and the newly produced bacteriophages are released from the host cell. The released bacteriophages infect other susceptible and the viral multiplication cycle is repeated

Some Bacteriophages do not cause lysis and death of the host cell. These lysogenic ( temperate) phages are capable of incorporating their DNA into the host cell's DNA. In lysogeny, the phage remains inactive. The bacterial host cells are known as lysogenic cells. There are three important results of lysogeny. 1. lysogenic cells are immune to reinfection by the same phage 2. The second result of lysogeny is phage conversion; that is, the host cell may exhibit new properties. C. diphtheriae can produce toxin only when it carries a lysogenic phage, because the prophage carries the gene coding for the toxin. 3. lysogeny is that it makes spepecialized transduction

Bacteriophage ‘s Applications Government agencies in the West have for several years been looking to Georgia and the former Soviet Union for help with exploiting phages for counteracting bioweapons and toxins, such as anthrax and botulism (Phage therapy and prophylaxis) Bacteriophages are also important model organisms for studying principles of evolution and ecology Since 2006, the United States Food and Drug Administration (FDA) and USDA have approved several bacteriophage products, using bacteriophages on cheese to kill the L. monocytogenes bacteria the same bacteriophage were approved for use on all food products

Bacterial Genome Organization 1. The bacterial chromosome is a circular molecule of DNA that functions as a self-replicating genetic element (replicon). 2. Transposons are small segments of DNA that can move (be "transposed") from one region of a DNA molecule to another. These pieces of DNA are 700 to 40, 000 base pairs long. All transposons contain the information for their own transposition. The simplest transposons, also called insertion sequences (IS) 3. Extrachromosomal genetic elements such as plasmids

Plasmids They are usually much smaller than the bacterial chromosome, and replicate independently of the chromosome. Most plasmids are supercoiled, circular, double-stranded DNA molecules. Conjugative plasmids code for functions that promote transfer of the plasmid from the donor bacterium to other recipient bacteria, but nonconjugative plasmids do not. F-fertility plasmid, R- determine resistance to antibiotics, enterotoxins, encoded by Ent plasmids.

Mutation: Change in the Genetic Material I. A mutation is a change in the nitrogenous base sequence of DNA; that change causes a change in the product coded for by the mutated gene. 2. The mutation rate is the probability that a gene will mutate when a cell divides. Types of Mutations • Shift and drift mutation. • Spontaneous and Induced • Chromosome, gene and point mutation Mutagens are agents in the environment that cause permanent changes in DNA.

Genetic Transfer and Recombination 1. Genetic recombination refers to the transfer of genetic material from donor to recipient by means of Transformation Conjugation and Transduction 2. Vertical gene transfer occurs during reproduction when genes are passed from an organism to its offspring. 3. Horizontal gene transfer in bacteria involves a portion of the cell's DNA being transferred from donor to recipient. 4. When some of the donor's DNA has been integrated into the recipient's DNA, the resultant cell is called a recombinant 5. Genetic recombination types: Transformation Conjugation and Transduction

Transformation in Bacteria 1. During this process, genes are transferred from one bacterium to another as "naked" DNA in solution. 2. This process occurs naturally among a few genera of bacteria. Griffith's experiment demonstrating genetic transformation. (a) Living encapsulated bacteria caused disease and death when injected into a mouse. (b) Living nonencapsulated bacteria are readily destroyed by the phagocytic defenses of the host. so the mouse remained healthy after injection. (c) After being killed by heat. encapsulated bacteria lost the ability to cause disease. (d) However. the combination of living nonencapsulated bacteria and heat-killed encapsulated bacteria did cause disease. Somehow, the live nonencapsulated bacteria were transformed by the dead encapsulated bacteria

Conjugation in Bacteria 1. This process requires contact between living cells. 2. One type of genetic donor cell is an F+; recipient cells are F-. F cells contain plasmids caused F factors; these are transferred to the F- cells during conjugation. 3. When the plasmid becomes incorporated into the chromosome, the cell is called an Hfr (high frequency of recombination) cell. During conjugation, an Hfr cell can transfer chromosomal DNA to an F- cell. Usually, the Hfr chromosome breaks before it is fully transferred.

Transduction in Bacteria DNA is passed from one bacterium to another due to a bacteriophage and is then incorporated into the recipient's DNA. Generalized transduction, any bacterial genes can be transferred specialized transduction, only certain bacterial genes.