Self-Replication of Nucleic Acids — NEET Biology
📚 Practice ConceptIf one examines each requirement one by one, because of rule of base pairing and complementarity, both the nucleic acids (DNA and RNA) have the ability to direct their duplications. The other molecules in the living system, such as proteins fail to fulfill first criteria itself.
Which of the following statements are correct regarding the chemical basis of labeling nucleic acids and proteins in molecular biology experiments? S1: The primary reason for using 35S in bacteriophage experiments is to exclusively label the DNA component of the virus. S2: The DNA backbone is characterized by deoxyribose sugars, phosphate groups, and nitrogenous bases, but it notably lacks sulfur atoms. S3: Proteins contain sulfur atoms primarily due to the presence of amino acids such as methionine and cysteine, allowing for their radioactive labeling with 35S. S4: For successful differential labeling of DNA and proteins, it is crucial that one type of macromolecule contains the specific element used for labeling while the other completely lacks it. S5: All nucleic acids, whether DNA or RNA, invariably possess phosphate groups in their backbone, thus making them amenable to labeling with 32P.
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Correct answer: C — S2, S3, S4 and S5
The correct understanding of chemical composition is key to understanding differential labeling in molecular biology. S1 is incorrect. 35S (radioactive sulfur) is used to label proteins, not DNA, because DNA does not contain sulfur. S2 is correct. The DNA backbone is indeed composed of deoxyribose sugar and phosphate groups, with nitrogenous bases projecting from it. Sulfur is not a component of DNA. S3 is correct. Sulfur is found in the amino acids methionine and cysteine, which are common components of proteins, thus enabling proteins to be labeled with 35S. S4 is correct. The principle of differential labeling, as used in the Hershey-Chase experiment, relies on the selective incorporation of a radioactive isotope into one type of macromolecule (e.g., 32P into DNA) while being absent from another (e.g., proteins lack P), and vice versa for the other isotope (e.g., 35S into proteins, absent from DNA). S5 is correct. Both DNA and RNA are polymers of nucleotides, and each nucleotide contains a phosphate group. Therefore, both types of nucleic acids can be labeled with radioactive phosphorus (32P). Therefore, statements S2, S3, S4, and S5 are correct.
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