Problem 53 - A Complete Resource Book in Chemistry for JEE Main

Question

Which base is present in RNA but not in DNA? (a) uracil (b) thymine (c) cytosine (d) guanine

Step-by-Step Solution

Answer

Uracil (a) is present in RNA but not in DNA.

Step 1: Identify bases in DNA

DNA (Deoxyribonucleic Acid) is composed of four types of nitrogenous bases: adenine (A), thymine (T), cytosine (C), and guanine (G). These bases pair with each other in specific ways to form the rungs of the DNA ladder.

Step 2: Identify bases in RNA

RNA (Ribonucleic Acid) also consists of four types of nitrogenous bases: adenine (A), uracil (U), cytosine (C), and guanine (G). In RNA, uracil takes the place of thymine, which is found in DNA.

Step 3: Compare bases in RNA and DNA

By comparing the bases present in DNA and RNA, we can see that all the bases are the same except for thymine and uracil. Thymine is found in DNA, while uracil is found in RNA.

Step 4: Select the correct answer

Since uracil is the base present in RNA but not in DNA, the correct answer is option (a) uracil.

Key Concepts

RNA structure

Ribonucleic Acid, or RNA, is one of the key players in the world of genetics and cellular biology. Unlike the double helix structure of DNA, RNA is typically single-stranded, making it more versatile and flexible in its functions. RNA is crucial for various biological roles, particularly coding, decoding, regulation, and expression of genes.
It comprises four types of nitrogenous bases:

• Adenine (A)
• Uracil (U)
• Cytosine (C)
• Guanine (G)

One of the distinct aspects of RNA is the use of uracil in place of thymine, which is found in DNA. This distinction plays a significant role in the different functions and properties of RNA and DNA. RNA is also involved in protein synthesis as it forms the template that guides the sequence of amino acids in a protein.

DNA structure

Deoxyribonucleic Acid, or DNA, serves as the hereditary material in humans and almost all other organisms. It is renowned for its double-helix structure, which resembles a twisted ladder. The sides of this ladder are composed of sugar-phosphate backbones, while the rungs are made up of paired nitrogenous bases.
The four bases found in DNA are:

• Adenine (A)
• Thymine (T)
• Cytosine (C)
• Guanine (G)

These bases pair specifically; adenine pairs with thymine, and cytosine pairs with guanine, holding the two strands of the double helix together with hydrogen bonds. This specific pairing is crucial for the DNA's stability and for the accurate replication of genetic information during cell division.

Base pairing

Base pairing is a fundamental concept that refers to the specific hydrogen bonding between complementary nitrogenous bases on opposite strands of nucleic acids. This concept is crucial for both the structure and function of DNA and RNA.
In DNA:

• Adenine (A) pairs with Thymine (T)
• Cytosine (C) pairs with Guanine (G)

These pairs are held together by hydrogen bonds that stabilize the DNA's double helix structure. In RNA:

• Adenine (A) pairs with Uracil (U)
• Cytosine (C) pairs with Guanine (G)

This pairing is also essential during the process of transcription, where RNA forms a complementary strand to DNA. The specificity in base pairing helps ensure that genetic information is transferred accurately and reliably, playing a vital role in protein synthesis and cellular function.