Answer:
a=
Primer RNA is RNA that initiates DNA synthesis. Primers are required for DNA synthesis because no known DNA polymerase is able to initiate polynucleotide synthesis. DNA polymerases are specialized for elongating polynucleotide chains from their available 3′-hydroxyl termini.
b=
The sugar-phosphate backbone forms the structural framework of nucleic acids, including DNA and RNA. This backbone is composed of alternating sugar and phosphate groups, and defines directionality of the molecule.