Problem 51 - A Complete Resource Book in Chemistry for JEE Main

Question

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

Step-by-Step Solution

Answer

Tyrosine is not present in a nucleotide.

Step 1: Identifying Nucleotide Components

A nucleotide is composed of three main components: a phosphate group, a sugar molecule (deoxyribose in DNA and ribose in RNA), and one of four nitrogenous bases. These bases typically include adenine, guanine, cytosine, and thymine in DNA or uracil in RNA, replacing thymine.

Step 2: Reviewing Given Options

Review the given options: (a) cytosine, (b) guanine, (c) adenine, and (d) tyrosine. Three of these should match common nucleotide bases, while one does not, based on the components identified earlier.

Step 3: Evaluating Each Option

Cytosine is one of the four main bases in DNA and RNA. Guanine is also one of these bases. Adenine is another base commonly found in nucleotides. Tyrosine, however, is an amino acid and not a nucleotide base.

Step 4: Conclusion

Based on the evaluation, tyrosine is not one of the components typically found in a nucleotide, thus it is the correct answer to the question posed.

Key Concepts

Nitrogenous Bases

Nitrogenous bases are an essential part of nucleotide structure, which in turn make up nucleic acids like DNA and RNA. These bases are organic molecules grouped into two main categories: purines and pyrimidines.
Purines consist of adenine (A) and guanine (G), which are larger structures made up of two carbon-nitrogen rings. Pyrimidines, on the other hand, include thymine (T), cytosine (C), and uracil (U), and are smaller with a single carbon-nitrogen ring.

• Adenine and guanine are present in both DNA and RNA, providing the necessary chemical diversity and structural stability.
• Cytosine is a versatile player found in both DNA and RNA.
• Thymine, specific to DNA, is replaced by uracil in RNA.

These bases pair specifically through hydrogen bonding, crucial for the double-helix structure of DNA and the single-stranded structure of RNA. Understanding nitrogenous bases helps clarify the fundamental differences between DNA and RNA.

Nucleic Acids

Nucleic acids are long chains of repeating units called nucleotides. They store and transmit genetic information. DNA (deoxyribonucleic acid) and RNA (ribonucleic acid) are the two primary types of nucleic acids.
Each nucleotide within a nucleic acid is composed of:

• a phosphate group,
• a sugar molecule (deoxyribose in DNA and ribose in RNA), and
• a nitrogenous base.

These three components work together to form the backbone and functional element of nucleic acids.
In essence, nucleic acids are the blueprints of life, holding instructions for everything from cell structure to protein synthesis. Their chains are incredibly long and complex, but they rely on simple units—nucleotides—to convey a vast amount of information.

DNA and RNA Components

The components of DNA and RNA play critical roles in the cell's genetic machinery. Both DNA and RNA are made up of nucleotides; however, they have distinct structural differences based on their sugar components and the nitrogenous bases they use.
DNA features a deoxyribose sugar, and its nitrogenous bases include adenine, thymine, cytosine, and guanine. It forms a double-helix structure by pairing adenine with thymine and guanine with cytosine.
RNA, in contrast, contains a ribose sugar and substitutes uracil for thymine. It is usually single-stranded, which allows it to fold into a variety of structures necessary for its diverse functions.
These structural variances give DNA its stability and replication abilities, vital for long-term genetic information storage. RNA's versatility makes it indispensable for protein synthesis and regulation. Understanding these components highlights the balance between stability in DNA and functionality in RNA.