Final answer:
The phrase 'double-stranded nucleic acids are anti-parallel' means that the two strands run in opposite directions to one another, with one strand oriented from the 5' end to the 3' end and the other from 3' end to 5' end. This structure is crucial for the stabilization of the DNA molecule and for the proper functioning of biological processes such as replication and transcription.
Step-by-step explanation:
To answer the student's question, when we say that double-stranded nucleic acids are anti-parallel, we are referring to the orientation of the two strands in relation to each other. In the structure of DNA, the two strands run in opposite directions, making choice (b) the correct answer. Each strand has its own directionality, indicated by a 5' end and a 3' end. This means that one strand runs from the 5' end to the 3' end, while the other strand runs from 3' end to 5' end.
Each strand of the DNA molecule has a backbone composed of a repeating sugar-phosphate structure, with the sugar being deoxyribose. The two backbones are like the rails of a ladder, and the pairs of bases—adenine (A) pairing with thymine (T), and guanine (G) pairing with cytosine (C)—are like the rungs. This anti-parallel arrangement allows the nitrogenous bases to be opposite each other and form hydrogen bonds, stabilizing the DNA structure into its characteristic double helix shape.
The anti-parallel nature of DNA is essential for many biological processes, such as DNA replication and transcription, because it ensures that the enzymes involved in these processes can correctly identify and interact with the DNA strands.