DSM1 Dugga HT06

15. Q: What is the difference between a vector and an intermediate host for a parasite? (1p)

Answer: The vector, but not the intermediate host, actively transfers the parasite to the host.

4. Q: Describe two of the three major mechanisms for horizontal transfer of genes between bacteria. (1p)

Answer: Transformation, i. e., free DNA liberated by one bacterium is taken up by another bacterium. Transduction, i. e., bacterial genes are carried from one bacterium to another via a bacterial virus (bacteriophage) which after packaging some DNA from the donor bacterium infects the recipient bacterium and delivers this DNA. Conjugation, where genetic material is transferred between bacterial cells in direct contact. This contact is facilitated by F-pili.

10. Q: Describe the major steps of retroviral replication to the point of integration in the cellular chromosome. (3p)

Answer: - Attachment to receptors (CD4, chemokine receptors) - Entry (through fusion) - RNA copied to DNA through the RNA dependent DNA polymerase - DNA copied to double stranded (ds) DNA - Integration in the host chromosome

21. Q: Which antibody isotype is connected to a) mucosal immunity (0.5p) b) atopic allergy (0.5p)

Answer: a) IgA b) IgE

11. Q: Define the term "permissive cell" in relation to viral infection. (1p)

Answer: A permissive cell is a cell that can sustain viral replication, leading to production of new virus particles.

18. Q: Which cells in the body express MHC class I molecules (1p)? Suggest an explanation for this expression pattern in the body, taking into account the function of MHC class I molecules. (2p)

Answer: All nucleated cells can express MHC class I molecules. The function of these molecules is to present peptides from proteins that the cell has produced. This makes it possible for CD8 cytotoxic T-cells to detect peptides from foreign proteins, as a sign that the cell´s protein synthesis machinery is used by a foreign microorganism. This leads the cytotoxic T-cell to kill the cell, thereby limiting the replication of microorganisms. Since all cells with a nucleus also have a protein synthesis that can be exploited by an infecting microorganism, it makes sense that all nucleated cells should have MHC class I molecules to sample their peptides and show them to cytotoxic T-cells. (Shorter explanations OK: e g all nucleated cells can be infected by and replicate viruses, whose peptides are displayed by MHC class I molecules to the immune system)

14. Q: How can Trypanosoma brucei gambiense survive in blood, avoid the humoral immune system and give chronic infections? (1p)

Answer: Antigenic variation of variant surface glycoproteins.

6. Q: Why is it advantageous for facultative anaerobic bacteria to grow aerobically? (1p)

Answer: Because aerobic respiration generates energy (ATP) far more efficiently (19 times) than fermentation or anaerobic respiration from carbon sources.

1. Q: Why should the skin be cleaned before using a disinfectant such as alcohol? (1p)

Answer: Because the disinfecting activity of alcohol is reduced by organic matter.

3. Q: Describe some bacterial virulence factors. The following words/terms should be included: Fimbriae, capsule, cytotoxicity, leucocidin, septic shock, cytokines (6p)

Answer: Fimbriae are proteinaceous hair-like structures which protrude from the surface of certain bacteria and help them to adhere to target cells. Some bacteria have a polysaccharide capsule which usually has antiphagocytic properties and in some cases also facilitates adhesion. Leucocidin is an example of a bacterial protein toxin which has specific cytotoxicity towards leucocytes (in contrast to several other cytotoxins which have a broader spectrum of target cells). Septic shock may be induced by bacterial cell wall components, such as teichoic acid and endotoxin, or by superantigenic protein toxins circulating in the body upon systemic infection. These toxic factors overactivate the constitutive immune response. Cytokines like interleukin-1 or TNF-alpha are produced in large amounts and their presence in the circulation triggers increased vascular permeability, hypotension and shock.

20. Q: Name three proinflammatory cytokines secreted by cells of the innate immune system. (1p)

Answer: IL-1, IL-6, IL-12, IL-18, TNF, IFN-alpha or beta, IL-8 (Two correct = 0.5 p; only one correct = 0p)

19. Q: Where does the first activation of a mature but naïve T-cell take place in an immune response? (1p)

Answer: In a lymph node. (OK also for spleen, or Peyers Patch, or tonsil).

13. Q: Explain how Plasmodium falciparum, in contrast to other Plasmodium species, can cause severe (malignant) malaria. (3p)

Answer: P. falciparum invades red blood cells (RBC) of any age (in contrast to for example P. vivax that only invades reticulocytes). Multiple merozoites may infect a single RBC. P. falciparum has the ability to express its own proteins on the RBC surface and these enable the RBC to bind both uninfected RBC and the capillary walls, particularly in the brain and the lungs. This may lead to a blockage of the blood-flow, and hamper the oxygen transport, and may cause respiratory distress and coma. P. falciparum furthermore suppresses bone-marrow development and both infected and unifected erythrocytes are lysed during an infection which may lead to very severe anemia. Hemoglobinuria may damage the kidneys.

5. Q: Give two examples each of phenotypic and genotypic classification methods/criteria used in bacterial identification. (2p)

Answer: Phenotypic: morphology, Gram stain, appearance of colonies, fermentation patterns, presence or absence of specific biochemical markers, phage typing, etc Genotypic: ratio of guanine to cytosine, DNA hybridization, nucleic acid sequence analysis, plasmid analysis, ribotyping, analysis of chromosomal DNA fragments etc

8. Q: Mention two of the functions of the "capsid" in non-enveloped virus particles. (2p)

Answer: Protection of nucleic acid, cell binding, maturation of the virus.

7. Q: Which are the standard criteria for classifying viruses? (2p)

Answer: RNA/DNA; helical/icosahedral capsid; envelope +/- .

12. Q: Describe the following outcomes of a viral infection? (4p) a. Abortive infection b. Acute infection c. Chronic infection d. Latent infection

Answer: a. The virus enters the cell, but either cannot initiate or cannot complete its replication (defective virus or infection of a non permissive cell) b. The virus enters the cell and completes its replicative cycle (permissive cell), new virions are produced (productive infection) and then the host immune response clears the virus (e. g. HAV) c. The virus enters the cell and completes its replicative cycle (permissive cell), new virions are produced (productive infection), but virus is not cleared by the host immune system (e. g. HBV and HCV). d. The virus establishes itself within the host cells without ongoing replication (no production of virions). During latent infection there is a limited viral gene expression. Latent infection is life long, no clearance (e. g. Herpes viruses).

22. Q: Name one similarity and one difference between the antigen receptors of T- cells and the immunoglobulin molecules of B-cells (2p)

Similarities: Only one receptor specificity per cell (clonal distribution of receptors), both are encoded by genes that rearrange at the DNA level during cellular development, both have a variable, antigen binding part and a more constant part Differences: Immunoglobulin (Ig) can be membrane bound or secreted, T-cell receptors (TcR) are always membrane bound. Ig genes undergo somatic /mutation/affinity maturation, TcR genes do not. Ig molecules are built by 4 polypeptide chains, TcR by 2. TcR can only bind to MHC-molecules (with or without peptides), B-cells can bind a variety of molecules.

16. Q: Name two general types of receptors on leucocytes that will guide them to migrate to an inflammatory lesion, and the principal function of the receptor in that process. (4p)

a) Adhesion molecules (that make the cell adhere to endothelium at the inflamed site) b) Chemokine receptors (that sense chemokine concentrations and control directed extravasation and further migration towards the inflammation in the tissue) (One point for the name of the receptor group and one point for its principal function. OK also if they give the name of a specific receptor within the correct family. Selectins as a group is also OK on a))

17. Q: a)What do we mean when we say that a B-cell "switches"? (1p) b) What happens to the antibody specificity for antigen during this process? (1p) c) Explain in simple terms (one sentence, 3-30 words) the molecular mechanism behind the switch. (1p)

a) The B-cell changes the isotype (Fc-part, tail) of its antibody (receptor) b) The specificity remains the same c) The DNA coding for the original isotype is excised, placing the remaining part of the antibody encoding gene adjacent to DNA coding for another isotype (OK to answer " recombination/excision/rearrangement of DNA")

9. Q: List the 7 basic steps of viral replication at the cellular level? (3p)

a. Attachment (recognition of a specific receptor on the surface of the host cell: tropism) b. Penetration (receptor mediated endocytosis for non-enveloped and some enveloped viruses, or fusion for some enveloped viruses) c. Uncoating d. Replication of viral genome and production of proteins e. Assembly of the new virus particles f. Maturation (e. g. HIV: cleavage of the gag-pol polypeptide) g. Exit (cell lysis for non enveloped viruses or budding for enveloped viruses)