What is the difference between a nucleoside and a nucleotide?

The question asks for the difference between nucleosides and nucleotides.

Let's break down the components:

• A nucleoside consists of a nitrogenous base (like adenine, guanine, cytosine, thymine, or uracil) covalently bonded to a pentose sugar (either ribose or deoxyribose).
• A nucleotide consists of a nucleoside with one or more phosphate groups attached to the sugar.

Now let's evaluate the given options:

• "only nucleotides have nitrogenous bases": This is incorrect. Both nucleosides and nucleotides contain nitrogenous bases.
• "a phosphate group is found in nucleotides but not in nucleosides": This is correct. The presence of a phosphate group is the key difference that distinguishes a nucleotide from a nucleoside.
• "nucleotides have the pentose sugar ribose while nucleosides have the pentose sugar deoxyribose": This is incorrect. Both nucleosides and nucleotides can contain either ribose (in RNA) or deoxyribose (in DNA). The type of sugar depends on the nucleic acid, not on whether it's a nucleoside or nucleotide.
• "in nucleosides the monomers are joined by phosphodiester bonds; in nucleotides the monomers are joined by hydrogen bonds": This is incorrect. Nucleotides are joined by phosphodiester bonds to form nucleic acid polymers (DNA or RNA). Hydrogen bonds form between complementary bases in a DNA double helix, not between monomers of nucleosides or nucleotides.
• "nucleosides are the monomers of DNA; nucleotides are the monomers of RNA": This is incorrect. Nucleotides are the monomers of both DNA and RNA.

The correct option is:

• a phosphate group is found in nucleotides but not in nucleosides.