Question

Which of the following is not present in a nucleotide? (a) cytosine (b) guanine (c) adenine (d) tyrosine

Short answer

Tyrosine is not present in a nucleotide.

Step-by-step solution

Understanding a Nucleotide

A nucleotide is a basic building block of DNA and RNA. It consists of three components: a phosphate group, a sugar molecule (deoxyribose in DNA and ribose in RNA), and a nitrogenous base.

Identifying Nitrogenous Bases

The common nitrogenous bases present in nucleotides are adenine (A), thymine (T), cytosine (C), guanine (G) in DNA, and uracil (U) replaces thymine in RNA. Notice that adenine, cytosine, and guanine are part of this group.

Analyzing the Options

Given the options (a) cytosine, (b) guanine, (c) adenine, and (d) tyrosine, we see that options (a), (b), and (c) are consistent with the usual nitrogenous bases found in nucleotides.

Recognizing the Odd One Out

Tyrosine is an amino acid, not a nitrogenous base. Therefore, it is not a component of a nucleotide.

Key concepts

Nitrogenous Bases in Nucleotides

Nitrogenous bases are one of the key components of nucleotides. They are organic molecules that serve as the foundation for the genetic information carried by DNA and RNA. These bases can be categorized into two main types: purines and pyrimidines.

• Purines: Adenine (A) and Guanine (G) belong to this group. They are characterized by a double-ring structure.
• Pyrimidines: Cytosine (C), Thymine (T), and Uracil (U) belong to this category. They are identified by their single-ring structure.

In DNA, the nitrogenous bases pair specifically: adenine pairs with thymine, and cytosine pairs with guanine. In RNA, the pairing is similar, but uracil replaces thymine. This specific pairing is crucial as it enables the accurate duplication of genetic information and its transcription for protein synthesis. It is important to note that nitrogenous bases form hydrogen bonds, which facilitate the formation of the double helix structure in DNA.

DNA and RNA Structure

DNA and RNA are essential biomolecules in all living organisms, responsible for storing and transmitting genetic information and playing a crucial role in protein synthesis.
DNA, or deoxyribonucleic acid, consists of two strands that form a double helix. Each nucleotide in DNA contains a deoxyribose sugar, a phosphate group, and one of four nitrogenous bases: adenine, thymine, cytosine, or guanine. The sequence of these bases along the DNA strand encodes genetic information.
RNA, or ribonucleic acid, is usually single-stranded and contains ribose sugar instead of deoxyribose. It also has a phosphate group and complementary nitrogenous bases, with uracil replacing thymine. RNA is essential for various biological roles, including coding, decoding, regulation, and expression of genes. It acts as a messenger carrying genetic instructions from DNA for controlling the synthesis of proteins.
The structural differences between DNA and RNA allow them to perform specialized functions, with DNA being more stable and suited for long-term information storage, while RNA is versatile and can interact with proteins to fulfill its roles in the cell.

Amino Acids and Their Role in Proteins

Amino acids are organic compounds that serve as the building blocks of proteins, which perform a wide range of functions in living organisms. There are 20 different amino acids that combine in various sequences to form proteins. Unlike nucleotides, which are solely involved in the genetic code, amino acids are directly involved in cellular activities through proteins.

• Structure: Each amino acid features an amino group, a carboxyl group, and a unique side chain (R group) that determines its properties and function.
• Linking: Amino acids are linked together by peptide bonds, forming long chains known as polypeptides, which then fold into specific shapes to become functional proteins.
• Function: Proteins made from amino acids can act as enzymes, signaling molecules, structural components of cells, and more.

Tyrosine is an amino acid, which is distinct from nucleotides, and it plays a role in protein structure and function rather than in forming genetic material like nitrogenous bases.