What are the fundamental building blocks of nucleic acids such as DNA and RNA in living organisms?

Introduction / Context:
Nucleic acids like DNA and RNA are essential biomolecules that store and transmit genetic information in all known forms of life. To understand how these macromolecules are built, it is important to know their basic repeating units. This question tests your knowledge of the fundamental building blocks that join together to form long nucleic acid chains.

Given Data / Assumptions:

• The question asks for the basic units that make up nucleic acids.

• The nucleic acids referred to include DNA and RNA.

• Several related biological terms are provided as options, including sugars and entire molecules.

• Only one option correctly identifies the smallest repeating unit of nucleic acids.

Concept / Approach:
Nucleic acids are polymers, and their monomers are nucleotides. Each nucleotide has three components: a nitrogenous base, a pentose sugar deoxyribose in DNA or ribose in RNA, and one or more phosphate groups. Many nucleotides link together through phosphodiester bonds to form the sugar phosphate backbone of DNA or RNA, with bases projecting from the backbone. Therefore, while pentose sugars are a part of each nucleotide, they are not the entire building block. DNA and RNA themselves are polymers, not the monomers.

Step-by-Step Solution:
Step 1: Recall that a monomer is the smallest repeating unit that can be linked to form a polymer. Step 2: Identify that nucleic acids are polymers of nucleotides, where each nucleotide contains a sugar, a nitrogenous base, and a phosphate group. Step 3: Recognize that a pentose sugar is only part of the nucleotide structure and cannot make nucleic acids alone. Step 4: Understand that DNA and RNA molecules are themselves long chains of nucleotides and therefore represent polymers, not monomeric building blocks. Step 5: Amino acids are the building blocks of proteins, not nucleic acids, so they must be excluded. Step 6: Conclude that nucleotides are the correct fundamental building blocks of nucleic acids.

Verification / Alternative check:
Any standard diagram of DNA shows a sugar phosphate backbone with nitrogenous bases attached. Labels usually mark each repeating unit as a nucleotide. Similarly, descriptions of replication and transcription refer to addition of nucleotides to growing DNA or RNA chains. These explanations consistently treat nucleotides as the monomers, confirming that they are the true building blocks of nucleic acids.

Why Other Options Are Wrong:

Pentose sugars only: These are components of nucleotides but do not include the base and phosphate needed to form a complete nucleotide.

DNA molecules: These are entire nucleic acid strands, which are polymers, not monomeric units.

RNA molecules: Like DNA, these are polynucleotide chains, not the basic units.

Amino acids: These are the monomers of proteins, not nucleic acids.

Common Pitfalls:
Students sometimes confuse the levels of organization in biomolecules and may think of DNA or RNA molecules as the basic units because they are widely mentioned. Others may focus only on the sugar component and forget that bases and phosphate are required to make a nucleotide. Remember that each nucleotide is a complete unit with three parts, and these nucleotides join to form DNA and RNA strands.

Final Answer:
The fundamental building blocks of nucleic acids are Nucleotides.