Here's why:

* Structure: Both RNA and DNA are made up of nucleotides, which consist of a sugar molecule (ribose in RNA, deoxyribose in DNA), a nitrogenous base, and a phosphate group.

* Phosphate Group: The phosphate group (PO4) contains a phosphorus atom bonded to four oxygen atoms. One of these oxygen atoms has a negative charge, making the phosphate group acidic.

* Acidic Properties: Acids are substances that donate protons (H+). The negative charge on the phosphate group can readily accept a proton, making it acidic.

This acidic nature plays a crucial role in the structure and function of RNA and DNA:

* Negatively Charged Backbone: The phosphate groups contribute to the overall negative charge of the nucleic acid backbone.

* Interactions: This negative charge allows for electrostatic interactions with positively charged molecules, such as proteins, which are essential for DNA replication, transcription, and other cellular processes.