Final answer:
The PO4 group is connected to the 3rd carbon on the pentose sugar of the next nucleotide by a covalent bond known as a phosphodiester bond, which is essential for forming the nucleic acid's sugar-phosphate backbone.
Step-by-step explanation:
The bond that connects the PO4 group to the 3rd carbon on the pentose sugar of the next nucleotide in the chain is a covalent bond called a phosphodiester bond. The phosphate residue attached to the 5' carbon of the sugar of one nucleotide forms an ester linkage with the hydroxyl group of the 3' carbon of the sugar of the next nucleotide through a dehydration synthesis reaction, thus forming this 5'-3' phosphodiester bond. These bonds are key in forming the sugar-phosphate backbone of the nucleic acid strand, providing the structure for DNA and RNA.