Problem 115 - A Complete Resource Book in Chemistry for JEE Main

Question

The number of hydrogen bonds formed between in the sequence of a stretch of a double helical DNA 5'ATGCCTAA3' is (a) 15 (b) 19 (c) 29 (d) 12

Step-by-Step Solution

Answer

19 hydrogen bonds (19 is the correct answer).

Step 1: Identify DNA Base Pairing Rules

In a double-stranded DNA, adenine (A) pairs with thymine (T) using two hydrogen bonds, and guanine (G) pairs with cytosine (C) using three hydrogen bonds.

Step 2: Count Hydrogen Bonds for Each Base Pair

The given sequence is 5'-ATGCCTAA-3'. - A-T has 2 hydrogen bonds. - G-C has 3 hydrogen bonds. - Calculate the total hydrogen bonds for each complementary pair: A-T (2), T-A (2), G-C (3), C-G (3), C-G (3), T-A (2), A-T (2), A-T (2).

Step 3: Add Total Hydrogen Bonds

Sum the hydrogen bonds calculated: 2 (A-T) + 2 (T-A) + 3 (G-C) + 3 (C-G) + 3 (C-G) + 2 (T-A) + 2 (A-T) + 2 (A-T) = 19.

Key Concepts

Understanding DNA Base Pairing

DNA base pairing is a fundamental concept in molecular biology, describing how two strands of DNA are linked together. Each DNA strand is composed of nucleotides, which include a sugar, a phosphate group, and a nitrogenous base. The base pairing rules are crucial because they determine the interaction between the two DNA strands.

In DNA, the bases pair in specific ways:

• Adenine (A) pairs with Thymine (T) through two hydrogen bonds.
• Guanine (G) pairs with Cytosine (C) through three hydrogen bonds.

This specific pairing ensures that the helical structure of DNA is stable and can store genetic information accurately.
DNA base pairing allows the double helix to be complementary. Each strand acts as a template during DNA replication, ensuring that genetic information is conserved and transmitted accurately during cell division.

Calculating Hydrogen Bonds in DNA

Hydrogen bonds play a significant role in maintaining the integrity of DNA's structure. By calculating hydrogen bonds, we can understand the strength and stability of DNA interaction.

To calculate the hydrogen bonds in a DNA sequence, follow these steps:

• Recognize each base pair and know their specific bonds: A-T pairs have 2 hydrogen bonds, and G-C pairs have 3 hydrogen bonds.
• Identify the sequence: For example, 5'-ATGCCTAA-3' is a specific strand in a DNA helix.
• Match each base on the strand with its complement: A with T, G with C, and so on.

Once you have the pairs, add the hydrogen bonds according to the type of pairs identified. For instance, with the sequence 5'-ATGCCTAA-3', add:

• 2 bonds for each A-T pair.
• 3 bonds for each G-C pair.

With this procedure, you can accurately calculate the number of hydrogen bonds contributing to the DNA's structural stability.

Exploring Nucleotide Sequences

Nucleotide sequences are the blueprint for genetic information, as they determine the sequence of amino acids in proteins through the genetic code. A nucleotide consists of three parts: a phosphate group, a sugar molecule (deoxyribose in DNA), and a nitrogenous base.

Each sequence of nucleotides is unique, defining specific genetic instructions. The sequence is read in a 5' to 3' direction; this orientation is critical in many biological processes, including DNA replication and transcription.
For instance, the sequence 5'-ATGCCTAA-3' follows the 5' to 3' rule, where each base in the sequence forms pairs with complementary bases on a matching DNA strand, creating a stable double helix.

• A-T pairs maintain structural stability with 2 hydrogen bonds.
• G-C pairs are stronger with 3 hydrogen bonds.

Understanding nucleotide sequences helps scientists predict the physical and chemical properties of DNA, elucidating its role in heredity and cellular function.