Which of the following is NOT a component of a nucleotide?
1. A phosphoryl group
2. A pentose sugar
3. A nitrogenous base
4. A hexose sugar
The Correct Answer and Explanation is:
The correct answer is 4. A hexose sugar.
Explanation:
Nucleotides are the basic building blocks of nucleic acids, such as DNA and RNA. A nucleotide consists of three key components:
- A Phosphoryl Group: This is a phosphate group (PO₄³⁻), which is attached to the 5′ carbon of the sugar in a nucleotide. This group is essential for the formation of the nucleic acid backbone, connecting the nucleotides together through phosphodiester bonds.
- A Pentose Sugar: This is a five-carbon sugar molecule. In DNA, this sugar is deoxyribose, while in RNA, it is ribose. The sugar forms the backbone of the nucleotide and connects to both the phosphate group and the nitrogenous base. The difference between deoxyribose and ribose lies in the presence or absence of an -OH group at the 2′ position of the sugar. Deoxyribose lacks this hydroxyl group, whereas ribose contains it.
- A Nitrogenous Base: This is a nitrogen-containing molecule that is classified into two categories: purines and pyrimidines. Purines include adenine (A) and guanine (G), while pyrimidines include cytosine (C), thymine (T), and uracil (U). The nitrogenous base forms hydrogen bonds with complementary bases in the nucleic acid strand, contributing to the structure of DNA or RNA.
Why “Hexose Sugar” Is Incorrect:
A hexose sugar is a six-carbon sugar (such as glucose or fructose), which is structurally different from the pentose sugar found in nucleotides. Hexoses are not a component of nucleotides; instead, they are involved in other biological processes such as energy metabolism. The presence of a pentose (five-carbon) sugar, not a hexose, is what defines the structure of a nucleotide.
In summary, a nucleotide contains a phosphoryl group, a pentose sugar, and a nitrogenous base, but not a hexose sugar.