BECTION AI ANSWER ALL QUESTIONS (40 MARKB) 1. List four differences in the chemical nature and function of cell wall structures between Gram positive and Gram-negative bacteria [4 Marka] 2. Briefly explain generation time (2 Marka) b) Briefly describe the bacterial growth cycles. (4 Marko) 3. a) Differentiate between DNA and RNA viruses. [2 Marks] b) Enumerate the basis for Viral classification (3) Marks) 4. Briefly explain how a virus causes disease in humans. (5) Marks) 5. State five human and ecological relevance of the Asperpillus spp and penicillin spp. type of fungi (5) Marks

1. Four differences in the chemical nature and function of cell wall structures between Gram-positive and Gram-negative bacteria:<br /><br />* **Peptidoglycan Layer:** Gram-positive bacteria have a thick (20-80 nm) peptidoglycan layer comprising multiple layers cross-linked by teichoic acids, while Gram-negative bacteria have a thin (5-10 nm) peptidoglycan layer with a single layer and no teichoic acids. This thick peptidoglycan layer in Gram-positive bacteria provides greater rigidity and protection from osmotic lysis.<br /><br />* **Outer Membrane:** Gram-negative bacteria possess an outer membrane external to the peptidoglycan layer, containing lipopolysaccharide (LPS), which acts as an endotoxin and contributes to antibiotic resistance. Gram-positive bacteria lack this outer membrane.<br /><br />* **Periplasmic Space:** Gram-negative bacteria have a distinct periplasmic space between the inner and outer membranes, containing enzymes and transport proteins. Gram-positive bacteria have a much smaller periplasmic space, if any.<br /><br />* **Teichoic Acids:** Present in Gram-positive bacteria, teichoic acids are anchored to the peptidoglycan layer and play roles in cell wall maintenance, ion binding, and antigenic properties. They are absent in Gram-negative bacteria.<br /><br /><br />2. a) **Generation Time:** Generation time is the time it takes for a bacterial population to double in number under specific growth conditions.<br /><br />b) **Bacterial Growth Cycles:** Bacteria in a closed system (like a culture flask) typically exhibit four growth phases:<br /> * **Lag Phase:** Bacteria adapt to their environment; there's little to no increase in cell number.<br /> * **Log (Exponential) Phase:** Bacteria reproduce rapidly at a constant rate; the population doubles at regular intervals (generation time).<br /> * **Stationary Phase:** Nutrient depletion and waste accumulation slow growth; the number of new cells equals the number of dying cells.<br /> * **Death (Decline) Phase:** Nutrients are exhausted, and toxic waste products accumulate, leading to a decline in the viable cell population.<br /><br /><br />3. a) **Difference between DNA and RNA viruses:** DNA viruses generally have double-stranded DNA genomes and replicate in the host cell's nucleus, while RNA viruses typically have single-stranded RNA genomes and replicate in the host cell's cytoplasm. DNA viruses tend to have lower mutation rates than RNA viruses.<br /><br />b) **Basis for Viral Classification:** Viruses are classified based on several factors:<br /> * **Nucleic acid type (DNA or RNA)**<br /> * **Genome structure (single-stranded or double-stranded)**<br /> * **Presence or absence of an envelope**<br /> * **Morphology (shape and size)**<br /> * **Mode of replication**<br /> * **Host range**<br /><br /><br />4. **How a virus causes disease in humans:**<br /> 1. **Attachment:** The virus attaches to specific receptors on the surface of host cells.<br /> 2. **Entry:** The virus enters the host cell through various mechanisms (e.g., endocytosis, fusion).<br /> 3. **Replication:** The viral genome is replicated using the host cell's machinery.<br /> 4. **Assembly:** New viral particles are assembled from the replicated components.<br /> 5. **Release:** New viruses are released from the host cell, often causing cell lysis (destruction) and spreading the infection. The released viruses can then infect other cells.<br /><br /><br />5. **Human and ecological relevance of *Aspergillus spp.* and *Penicillium spp.*:**<br /> * **Antibiotic Production (*Penicillium*):** *Penicillium chrysogenum* is the source of penicillin, a crucial antibiotic.<br /> * **Food Production:** Some species are used in cheese making (e.g., *Penicillium roqueforti* in blue cheese).<br /> * **Decomposition:** Both genera play important roles in nutrient cycling by decomposing organic matter.<br /> * **Biotechnology:** Used in the production of various enzymes and organic acids.<br /> * **Plant Pathogens:** Some species can cause diseases in plants, impacting agriculture. *Aspergillus* species can also cause opportunistic infections in humans (aspergillosis), particularly in immunocompromised individuals.<br />