What groups listed below have true cell walls?

Correct Answer: B

Rationale: The correct answer is B: mycoplasmas. Mycoplasmas have cell walls, which contain peptidoglycan, making them structurally similar to typical bacteria. Algae (choice A) have cell walls made of cellulose, Gram-positive bacteria (choice C) have cell walls containing thick layers of peptidoglycan, and fungi (choice D) have cell walls made of chitin. Mycoplasmas lack a cell wall, but they have a flexible cell membrane that provides them with shape and protection.

Correct Answer: C

Rationale: The correct answer is C because Hfr strains of bacteria have the F factor integrated into their bacterial chromosome. This integration allows for the transfer of genetic material during conjugation. This integration results in a high frequency of genetic information transfer to recipient cells, which is a characteristic of Hfr strains. Choice A is incorrect because Hfr strains do have the F factor, but it is integrated into their chromosome, not absent. Choice B is incorrect because Hfr strains do not just have an F factor plasmid; it is integrated into the chromosome. Choice D is incorrect because Hfr strains actually transfer genetic information with high frequency, not low frequency.

Correct Answer: B

Rationale: The correct answer is B: free oxygen availability. In sediment environments, free oxygen availability plays a crucial role in determining bacterial population distribution due to oxygen gradients. Bacteria that require oxygen will be found in aerobic zones where oxygen is present, while anaerobic bacteria will thrive in zones with limited or no oxygen. This division is primarily responsible for the spatial organization of bacterial populations within sediments. Choice A: Predation by protozoans is not primarily responsible for division of bacterial populations within sediments as it focuses on interactions between different organisms rather than the environmental factor of oxygen availability. Choice C: While competition for suitable electron acceptors is important for microbial communities, it is not the primary factor responsible for the division of bacterial populations within sediments. Choice D: Cometabolism of organic compounds refers to the metabolism of compounds alongside the primary substrate, and it is not directly related to the spatial distribution of bacterial populations within sediments based on oxygen availability.

Correct Answer: B

Rationale: The correct answer is B: Allergic contact dermatitis. This is classified as a delayed hypersensitivity reaction because it involves activation of T cells, which takes time to develop. Anaphylaxis (A) is an immediate hypersensitivity reaction mediated by IgE. Cytotoxic hypersensitivity (C) involves antibodies attacking cells directly, not delayed. Immune complex hypersensitivity (D) involves immune complexes depositing in tissues, triggering inflammation, not T cell activation.

Correct Answer: C

Rationale: EcoRI recognizes the palindromic sequence GAATTC and cuts between G and A on both strands, producing 5' cohesive ends. Cohesive ends can base-pair with complementary ends, facilitating DNA ligation. Blunt ends (choice B) are produced by enzymes that cut both DNA strands at the same position. Choice A is incorrect as EcoRI produces 5' cohesive ends, not 3' cohesive ends. Choice D is incorrect as EcoRI does produce cohesive ends.