Which nitrogenous base is present in ribonucleic acid (RNA) but absent from deoxyribonucleic acid (DNA)?

Introduction / Context:
This question tests your knowledge of the chemical differences between DNA and RNA, two fundamental nucleic acids. One of the most important differences is the type of nitrogenous bases they contain. Recognizing which base is unique to RNA helps in understanding how these molecules store and transmit genetic information.

Given Data / Assumptions:

• DNA stands for deoxyribonucleic acid and RNA stands for ribonucleic acid.
• Both DNA and RNA are composed of nucleotides containing a sugar, a phosphate group, and a nitrogenous base.
• The question asks specifically for a base that is present in RNA but not in DNA.
• The options include uracil, cytosine, adenine, and guanine.

Concept / Approach:
DNA and RNA share several nitrogenous bases but differ in one key base. DNA uses the bases adenine, guanine, cytosine, and thymine. RNA uses adenine, guanine, cytosine, and uracil. By listing which bases are common to both and which are unique, we can identify the correct answer. The special substitution of uracil for thymine in RNA is a classic exam point.

Step-by-Step Solution:
Step 1: Recall that DNA contains adenine, thymine, guanine, and cytosine as its nitrogenous bases.Step 2: Recall that RNA contains adenine, uracil, guanine, and cytosine as its nitrogenous bases.Step 3: Compare the lists: adenine, guanine, and cytosine are common to both DNA and RNA.Step 4: DNA has thymine, whereas RNA has uracil in place of thymine.Step 5: Therefore, the base that is present in RNA but not in DNA is uracil.

Verification / Alternative check:
A quick mnemonic that many learners use is that uracil and ribose are both associated with RNA. Remember that thymine pairs with adenine in DNA, but in RNA, adenine pairs with uracil. Revisiting simple base pairing diagrams reinforces that uracil is unique to RNA in this context.

Why Other Options Are Wrong:
Cytosine: This base is present in both DNA and RNA, so it is not unique to RNA.
Adenine: Adenine also occurs in both DNA and RNA, so it cannot be the base found only in RNA.
Guanine: Like cytosine and adenine, guanine is shared between DNA and RNA and is not distinctive for RNA.

Common Pitfalls:
Some students confuse thymine and uracil or assume that thymine is in RNA and uracil in DNA, which is the reverse of the truth. Others might think multiple bases are exclusive to RNA, but only uracil is uniquely present in standard RNA when compared to DNA. Avoid mixing up base names and focus on the pattern that uracil belongs to RNA and thymine belongs to DNA.

Final Answer:
The nitrogenous base that is present in RNA but absent from DNA is Uracil.