Properties of Single-Stranded DNA in Viruses

Steps:

• Statement 1 is correct: ssDNA lacks a complementary strand, so it cannot undergo semi-conservative replication like dsDNA, where each strand templates the other.
• Statement 2 is incorrect: ssDNA has unpaired bases, so %C does not pair with %G (unlike Chargaff's rules for dsDNA), preventing calculation of %G, %A, or %T.
• Statement 3 is correct: ssDNA can form intramolecular folds like hairpins, stabilized by hydrogen bonds between bases.
• Valid statements are 1 and 3, corresponding to option C.

Why C is correct:

• C identifies 1 and 3 only, aligning with ssDNA's lack of complementary pairing (no semi-conservative replication) and ability to form internal H-bonds, per molecular biology principles.

Why the others are wrong:

• A includes 2, but ssDNA violates base-pairing rules, so %C alone yields no other base percentages.
• B includes 2 (false, as above) and omits 3's valid intramolecular bonding.
• D omits 1, ignoring ssDNA's inability to replicate semi-conservatively without first forming dsDNA.

Final answer: C