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

Option (d) tyrosine is not present in a nucleotide.

Step 1: Understanding Nucleotides

Nucleotides are the building blocks of nucleic acids like DNA and RNA. Each nucleotide consists of a phosphate group, a sugar (either ribose or deoxyribose), and a nitrogenous base.

Step 2: Identifying the Nitrogenous Bases in Nucleotides

There are four primary nitrogenous bases in DNA nucleotides: adenine, guanine, cytosine, and thymine. In RNA, thymine is replaced by uracil. None of these include tyrosine.

Step 3: Examining the Answer Choices

The given options are (a) cytosine, (b) guanine, (c) adenine, and (d) tyrosine. Cytosine, guanine, and adenine are nitrogenous bases found in nucleotides, while tyrosine is an amino acid and not a component of nucleotides.

Step 4: Determining the Correct Option

Since tyrosine is an amino acid and not a nitrogenous base, it is not part of a nucleotide structure.

Key Concepts

Nitrogenous Bases

Nitrogenous bases are the fundamental components of nucleotides, which in turn are the building blocks of nucleic acids like DNA and RNA. These bases are categorized as either purines or pyrimidines. Purines, which have a double ring structure, include adenine (A) and guanine (G). Pyrimidines, with a single ring structure, consist of cytosine (C), thymine (T), and uracil (U).

• In DNA, the nitrogenous bases are adenine, guanine, cytosine, and thymine.
• For RNA, the bases are adenine, guanine, cytosine, and uracil (replacing thymine).

These bases pair with each other to form the rungs of the DNA ladder: adenine pairs with thymine (or uracil in RNA), and guanine pairs with cytosine. The sequence of these bases encodes genetic information.

DNA and RNA

DNA (Deoxyribonucleic Acid) and RNA (Ribonucleic Acid) are essential biomolecules that carry genetic instructions. Both DNA and RNA are polymers made up of nucleotides. However, they differ in their structure and function.
DNA is double-stranded and resembles a twisted ladder. The sugar in DNA nucleotides is deoxyribose. It contains the bases adenine (A), guanine (G), cytosine (C), and thymine (T). DNA stores genetic information in living organisms and is responsible for transmitting this information during cell division.
RNA, on the other hand, is usually single-stranded and contains ribose sugars. Instead of thymine, RNA incorporates uracil (U). RNA’s main roles include carrying the genetic code from DNA to the ribosomes for protein synthesis (mRNA), forming the ribosomes (rRNA), and bringing amino acids to the ribosomes during protein construction (tRNA).

Amino Acids

Unlike nitrogenous bases in nucleotides, amino acids are the building blocks of proteins. Proteins fulfill essential roles in biological processes by acting as enzymes, structural components, and signaling molecules. Amino acids come in twenty different varieties, each with a unique side chain, or "R group," that determines its properties.
Tyrosine is one example of an amino acid but not a component of nucleotides or nucleic acids. Amino acids link together through peptide bonds, forming polypeptide chains. These chains fold into specific shapes to function as proteins.
While nucleotides encode genetic information, amino acids perform and regulate virtually all other biological activities in the cell.

Nucleotide Structure

Nucleotides are the building blocks of DNA and RNA. A nucleotide consists of three components:

• A phosphate group, contributing to a negative charge and providing the ability to form the backbone structure by bonding to the sugar group.
• A pentose sugar, which can be either ribose (in RNA) or deoxyribose (in DNA), helping to form the backbone of the nucleotide chain.
• A nitrogenous base, which can be a purine or pyrimidine, involved in base pairing that holds the two strands of DNA together.

Nucleotides are linked into a chain through phosphodiester bonds forming a sugar-phosphate backbone. The specific sequence of nitrogenous bases along this backbone encodes genetic instructions. Such sequences determine how proteins are built, influencing virtually every feature of an organism.