Final answer:
The replicative form of nucleic acid in filamentous phages is single-stranded DNA (ssDNA) with positive polarity, which must be converted into a double-stranded DNA (dsDNA) intermediate before mRNA transcription.
Step-by-step explanation:
The replicative form of nucleic acid in filamentous phages is single-stranded DNA (ssDNA) with positive polarity. Filamentous phages are a group of viruses that infect bacteria like E. coli and are known for their unique filamentous shape. They contain an ssDNA genome that must be converted into a double-stranded DNA (dsDNA) intermediate before transcription into mRNA can occur. This step is necessary for the synthesis of viral proteins that will be packaged into new viral particles. Such mechanisms of replication and transcription are typical of Group II viruses in the Baltimore classification scheme.