Problem 84 - Arihant Objective Approach to Organic Chemistry for IIT JEE and NEET all Medical Entrances Part 4

Question

In DNA, the complementary bases are: (a) uracil and adenine, cytosine and guanine (b) adenine and thymine, guanine and cytosine (c) adenine and thymine; guanine and uracil (d) adenine and guanine; thymine and cytosine

Step-by-Step Solution

Answer

(b) adenine and thymine, guanine and cytosine

Step 1: Understanding DNA Base Pairing

In DNA, there are four types of nucleotides that pair up with each other: adenine (A), thymine (T), cytosine (C), and guanine (G). These bases pair according to specific rules known as complementary base pairing. Adenine always pairs with thymine, and cytosine always pairs with guanine.

Step 2: Reviewing Multiple Choice Options

Let's examine each option one by one: - Option (a) mentions uracil and adenine. Uracil is not found in DNA; it's found in RNA. - Option (b) mentions adenine and thymine, guanine and cytosine, which matches the DNA pairing rules. - Option (c) mentions uracil again, which, as noted, is only found in RNA. - Option (d) pairs adenine with guanine and thymine with cytosine, which is incorrect as it does not follow the complementary pairing rules.

Step 3: Choosing the Correct Answer

Based on the discussion in the previous steps, option (b) is the only choice that aligns with the known DNA base pairing rules. Adenine pairs with thymine and guanine pairs with cytosine, which is exactly what option (b) states.

Key Concepts

Nucleotide Pairing

DNA, the building block of life, is made up of units called nucleotides. These tiny units contain a sugar, a phosphate, and a base. The bases are what we talk about when we refer to nucleotide pairing, and they follow a blueprint.
In DNA, there are four bases: adenine (A), thymine (T), cytosine (C), and guanine (G). These bases form the core of nucleotide pairing and are crucial for DNA's structure. The bases are like pieces of a jigsaw puzzle that fit together according to specific rules. These rules are called complementary base pairing rules.

• Adenine pairs with thymine
• Cytosine pairs with guanine

These pairing rules allow DNA to replicate precisely, with each half of the DNA strand perfectly complementary to the other. This means that understanding nucleotide pairing is like understanding the language that tells cells how to function.

Adenine and Thymine

Adenine (A) and thymine (T) are two of the four bases in DNA that follow the complementary base pairing principles. In DNA, adenine and thymine are paired together via two hydrogen bonds. This specific pairing ensures stability in the DNA double helix.
The hydrogen bonds between adenine and thymine help to stabilize the double-stranded DNA, although these bonds are relatively weak compared to the covalent bonds that hold the backbone of each DNA strand together.
Their ability to bind specifically and only to each other plays a key role in DNA's ability to replicate itself. Adenine pairing with thymine allows the DNA to be copied accurately during cell division. Understanding the pairing of adenine and thymine helps us comprehend how genetic information is passed on and inherited across generations.

Cytosine and Guanine

Cytosine (C) and guanine (G) stand as the other half of the important nucleotide pairings in DNA. These two bases create a strong pair through the formation of three hydrogen bonds. This additional bond compared to adenine-thymine makes cytosine-guanine pairing slightly stronger.
This stronger bond is significant when considering the regions of DNA that need to be stable or, conversely, where it might be beneficial for DNA to unwind for replication or transcription.

• Three hydrogen bonds provide extra stability
• Important for DNA protein interactions

The specificity of cytosine's pairing with guanine ensures that genetic information is consistently and accurately transcribed and translated in biological systems. Mastering this pairing concept is essential for delving deeper into gene expression and molecular biology processes.