Hfr strains of bacteria:

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: 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.

Correct Answer: C

Rationale: The correct answer is C: Vector DNA. In genetic engineering, a vector is a DNA molecule used to carry foreign genetic material into another cell. Plasmid DNA (choice B) is a type of vector, but not all vectors are plasmids. Recombinant DNA (choice A) refers to DNA that has been artificially created by combining DNA from different sources. Insert DNA (choice D) is the foreign genetic material that is inserted into a vector. Therefore, the most specific and accurate choice is C, Vector DNA, as it encompasses all types of vectors used in genetic engineering.

Correct Answer: B

Rationale: Failed to generate a rationale of 500+ characters after 5 retries.