VIRUSES BACTERIA AND THE IMMUNE SYSTEM The Molecular

VIRUSES, BACTERIA, AND THE IMMUNE SYSTEM

The Molecular Genetics of Viruses are parasites of cells. They consist of: Nucleic Acid (DNA or RNA but not both) Capsid/Protein Coat Some also are surrounded by an envelope A Bacteriophage—a virus that attacks bacteria cells

Viruses A typical virus penetrates a cell, takes over the cell’s machinery and assembles hundreds of new viruses. In the process, the host cell is usually destroyed. Viruses are specific for the kinds of cells they attack.

Lytic Cycle of a Virus

Lysogenic Cycle of Viruses In the lysogenic cycle, the viral DNA is temporarily incorporated into the DNA of the host cell. A virus in this dormant state is called a provirus. While in this stage, it will be reproduced every time a cell divides just like the normal DNA. The virus remains inactive until some trigger (often an external environmental stimulus) causes the virus to begin the lytic cycle.

More Details about Viruses The nucleic acid of a virus could be doublestranded DNA (ds. DNA) or single-stranded DNA (ss. DNA) Or: it could be doublestranded RNA (ds. RNA) or single-stranded RNA (ss. RNA)

Retroviruses Retroviruses are ss. RNA viruses that use an enzyme called reverse transcriptase to make a DNA complement of their RNA. The DNA complement can then be transcribed immediately to manufacture m. RNA, or it can begin the lysogenic cycle. HIV is a retrovirus

The Molecular Genetics of Bacteria Eukaryotic DNA A bacterial chromosome is often called a naked chromosome because it lacks the histones and other proteins associated with eukaryotic chromosomes. Bacteria have one large circular strand of DNA and may also have one or more smaller circles of DNA called plasmids.

Binary Fission Bacterial cells reproduce by binary fission. The chromosome replicates and the cell divides into two cells, each cell with one chromosome. There are no centrioles, spindles, or microtubules.

Plasmids Plasmids carry genes that are beneficial but normally essential to the survival of the bacterium. Plasmids replicate independently of the chromosome. Some plasmids, called episomes, can become incorporated into the bacterial chromosome.

Genetic Variation in Bacteria Genetic variation in bacteria can be introduced in a number of ways. Conjugation Transduction Transformation

Conjugation in Bacteria Conjugation is a process of DNA exchange between bacteria. A donor bacterium produces a tube, or pilus, that connects to a recipient bacterium. Through the pilus, the donor bacterium sends chromosomal or plasmid DNA to the recipient. In some cases, copies of large portions of a donor’s chromosomes are sent, allowing recombination with the recipient’s chromosome.

More about Recombination Due to Conjugation in Bacteria One plasmid, called the F plasmid, contains the genes that enable a bacterium to produce pili. When a recipient bacterium receives the F plasmid, it too can become a donor cell. Another group of plasmids, called the R plasmids, provide bacteria with resistance against antibiotics.

Transduction is the transfer of host DNA from one cell to another by a virus. When a virus is assembled during a lytic cycle, it is sometimes assembled with some bacterial DNA.

Transformation occurs when bacteria absorb DNA from their surroundings and incorporate it into their genome. Specialized proteins on the cell membranes of some bacteria facilitate this kind of DNA uptake.

Regulation of Gene Expression in Bacterial Cells Gene expression is the way in which cells with identical DNA become different by transcribing only certain genes at a time. In bacterial cells, certain segments of their DNA act as Operons. The operon acts like a switch that can turn several genes on or off at the same time.

Operons in Bacterial Cells Among prokaryotes, an operon is a unit of DNA that controls the transcription of a gene. It contains the following components: Promoter region Operator region Structural genes Regulatory genes

More about Operons: The promoter region is a sequence of DNA to which the RNA polymerase attaches to begin transcription. The operator region can block the action of the RNA polymerase if this region is occupied by a repressor protein.

More about Operons The structural genes contain DNA sequences that code for enzymes and proteins the cell needs for its normal daily activities. A regulatory gene, lying outside the operon region, produces repressor or activator proteins. Action of a repressor protein. This protein was made by a regulatory gene that is not shown here.

The lac Operon The lac operon in E. coli cells controls the breakdown of lactose (a disaccharide). The lac operon is an example of an inducible operon because the structural genes are normally inactive. They are activated when lactose is present. http: //www. sumanasinc. co m/webcontent/animations/c ontent/lacoperon. html http: //www. youtube. com/watc h? v=o. Bwtxd. I 1 zvk

The lac Operon When the operator region is occupied by the repressor, RNA polymerase is unable to transcribe several structural genes that code for enzymes that control the uptake and breakdown of lactose.

The lac Operon When lactose (or its’ isomer allolactose) is available, however, some of the lactose combines with the repressor to make it inactive. Then, the RNA polymerase can attach and make the enzymes needed to break down the lactose.

The trp Operon The trp operon in E. coli produces enzymes for the synthesis of the amino acid tryptophan. A regulatory gene produces an inactive repressor that does not bind to the promoter. As a result, the RNA polymerase proceeds to transcribe the genes needed to make tryptophan.

The trp Operon However, when tryptophan is available to E. coli from the surrounding environment, the bacterium no longer needs to make its own. In this case, rising levels of tryptophan induce some tryptophan to react with the inactive repressor and make it active.

Tryptophan acts as a Corepressor The active repressor now binds to the operator region, which prevents the transcription of the structural genes that would make more tryptophan. Since these structural genes stop producing enzymes only in the presence of an active repressor, they are called repressible enzymes.

The Immune System The internal environment of animals provides attractive conditions for the growth of bacteria, viruses and other organisms. To protect against these, humans possess three levels of defense: Skin Internal defenses The immune response

The skin and mucous membranes provide a nonspecific first line of defense against invaders Skin—a physical and hostile barrier covered with oily and acidic secretions from sweat glands Cilia line the lungs to sweep invaders out Gastric juice of the stomach kills most microbes Symbiotic bacteria found in the digestive tract and the vagina outcompete many other organisms. Healthy skin provides a good barrier against invasion by foreign microbes.

The Second Line of Defense-Phagocytes 1. Phagocytes—white blood cells (WBCs) that engulf pathogens by phagocytosis. They include: neutrophils and monocytes. Monocytes enlarge into large phagocytic cells called macrophages. Other WBCs called natural killer cells attack abnormal body cells such as tumors or pathogen-infected body cells.

The Second Line of Defense— Complement Proteins Complement is a group of about 20 proteins that “complement” defense reactions. These proteins help attract phagocytes to foreign cells and help destroy foreign cells by promoting cell lysis (breaking open the cells).

The Second Line of Defense-Interferons are substances secreted by cells invaded by viruses that stimulate neighboring cells to produce proteins that help them defend against viruses. Therefore, the neighboring cells are less likely to be invaded by the virus.

The Second Line of Defense—The Inflammatory Response The inflammatory Response is a series of nonspecific events that occur in response to pathogens. When skin is damaged: Histamine is secreted by basophils, WBCs found in connective tissues. The histamine stimulates the dilation of blood vessels (vasodilation). This increases the blood supply to the damaged area and allows for easier movement of WBCs through blood vessel walls.

The Inflammatory Response, continued The dilation of the blood vessels also causes redness, an increase of temperature, and swelling. The increase in temperature, like a fever, may stimulate WBCs, and they make it inhospitable to pathogens. Phagocytes, attracted to the injury, arrive and engulf pathogens and damaged cells. http: //www. youtube. com/watch? v=_b NN 95 s. A 6 -8

The Inflammatory Response 1. Tissue injury: release of chemical signals such as histamine 2. Dilation and increased leakiness of local blood vessels; migration of phagocytes to the area 3. Phagocytes (macrophages and neutrophils) consume bacteria and cell debris; tissue heals

The Third Line of Defense—The Immune Response This differs from the inflammatory response because it targets specific antigens. Antigens are any molecules (usually a protein) that can be identified as foreign. It may be a toxin (such as venom), a part of the protein coat of a virus, or a molecule unique to the pathogen.

The Major Histocompatibility Complex (MHC) The MHC is the mechanism by which the immune system is able to differentiate between self and nonself cells. It is a collection of glycoproteins that exist on the membranes of all body cells. The proteins of a single individual are unique.

Lymphocytes The primary agents of the immune response are Lymphocytes. These are white blood cells that originate in the bone marrow (like all blood cells), but concentrate in lymphatic tissues such as the lymph nodes, the thymus gland, and the spleen.

B Cells B cells are lymphocytes that originate and mature in the bone marrow. B cells respond to antigens. The plasma membrane surface of B cells is characterized by specialized antigen receptors called antibodies.

Antibodies Each antibody is a variation of a basic Y-shaped protein that consists of constant regions and variable regions. The variable regions are sequences of amino acids that differ among antibodies and give them specificity to antigens. Antibodies are proteins that are specific to a particular antigen. There are 5 classes of antibodies (also called immunoglobulins): Ig. A, Ig. D, Ig. E, Ig. G, Ig. M Antibodies inactivate antigens by binding to them. Inactivation is followed by macrophage phagocytosis.

How Antibodies Help the Immune Response:

When B Cells Encounter Antigens, They Make 2 Kinds of Cells: Plasma Cells—these release specific antibodies that circulate throughout the body, binding to antigens Memory Cells—these are long-lived B cells that do not release their antibodies in response to the immediate invasion. Instead, they circulate in the body and respond quickly to eliminate any new invasion by the same antigen.

T Cells are lymphocytes that originate in the bone marrow but mature in the thymus gland. T cells also have antigen receptors. However, these receptors are not antibodies, but recognition sites for molecules displayed by non-self cells.

How Do T-Cells Distinguish Between Self and Non-self Cells? When a body cell is invaded by a virus (or any other foreign cell), it displays a combination of self and non-self markers. T-cells interpret this display as non-self. Cancer cells or tissue transplants are also recognized as non-self cells by T-cells

When T-cells Encounter Non-Self Cells: When T-cells encounter non-self cells, they divide and produce two kinds of cells: Cytotoxic T Cells (or Killer T Cells)—these recognize and destroy non-self cells by puncturing and lysing them Helper T Cells—these stimulate the proliferation of B-cells and more Killer T Cells

Clonal Selection of B-Cells and T-Cells When an antigen binds to a B-cell or when a non-self cell binds to a T-cell, the B -cell or T-cell begins to divide, producing numerous daughter cells, all identical copies of the parent cell. This produces numerous cells that will only attack a particular antigen Clonal Selection: only the B or Tcell that bears the particular effective receptor is “selected” and reproduces to make clones, or identical copies of itself.

The Cell. Mediated Immune Response: The Cell-Mediated Immune Response uses mostly T-cells and responds to any non-self cell, including cells invaded by pathogens. When a non-self cell binds to a T-cell, the T -cell undergoes clonal selection.

The Humoral Response (Or Antibody-Mediated Response) This response involves most cells and responds to antigens or pathogens that are circulating in the lymph or blood.

The Humoral (Antibody-Mediated) Response: 1. B-cells produce plasma cells. The plasma cells release antibodies that bind with antigens or antigenbearing pathogens. 2. B-cells produce memory cells which provide future immunity. 3. Macrophage and Helper T-cells stimulate B-cell production. Most often, the antigen or pathogen is engulfed by a macrophage. T-cells then bind to the macrophage in a cellmediated response. Interleukins secreted by the helper T-cells stimulate the production of B-cells.

The Humoral (Antibody-Mediated) Response

Supplementing the Immune System’s Defenses: 1. Antibiotics—chemical derived from bacteria or fungi that are harmful to other microorganisms 2. Vaccines—substances (usually inactivated viruses or fragments of viruses or bacteria) that stimulate the production of memory cells. Passive Immunity—obtained by transferring antibodies from an individual who previously had a disease to a newly infected individual. Newborn infants are protected by passive immunity through the transfer of antibodies across the placenta and in breast milk.