Problem 61 - Arihant Objective Approach to Organic Chemistry for IIT JEE and NEET all Medical Entrances Part 4

Question

If one strand of DNA has the sequence ATGCTTGA, the sequence in the complementary strand would be: (a) TACGTAGT (b) TACGAACT (c) TCCGAACT (d) TACGTACT

Step-by-Step Solution

Answer

The complementary DNA strand is (b) TACGAACT.

Step 1: Understand DNA Base Pairing

DNA is made up of four types of nucleotides, each with a different base: Adenine (A), Thymine (T), Cytosine (C), and Guanine (G). In a DNA double helix, base pairs form between the nucleotides: Adenine pairs with Thymine, and Cytosine pairs with Guanine.

Step 2: Find Complementary Strand Bases

To find the complementary DNA strand, each base on the original strand is replaced by its pair: A -> T, T -> A, C -> G, and G -> C. Start with the DNA sequence provided: ATGCTTGA.

Step 3: Calculate Complementary Sequence

Replace each base in the original sequence with its complementary base: - A (1st position) -> T - T (2nd position) -> A - G (3rd position) -> C - C (4th position) -> G - T (5th position) -> A - T (6th position) -> A - G (7th position) -> C - A (8th position) -> T.

Step 4: Write the Complete Complementary Strand

Once you have replaced each base with its complement, write out the new sequence: TACGAACT. This is the sequence of the complementary DNA strand.

Step 5: Select the Correct Option

Compare the sequence you calculated (TACGAACT) with the given options. The correct answer is (b) TACGAACT.

Key Concepts

Complementary DNA Strand

Complementary DNA strands are crucial to the structure and function of DNA. DNA molecules have two strands that form a double helix by winding around each other. These strands are not identical but are complementary. This means that each base on one strand pairs with a specific base on the opposite strand.
This pairing rule is guided by hydrogen bonding and ensures that the genetic information can be accurately copied and transmitted during cell division. The rules of base pairing are simple:

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

These pairs are like puzzle pieces fitting perfectly together. Understanding how to determine a complementary strand is key in genetics, making these concepts a cornerstone in the study of biology.

Nucleotides

Nucleotides are the building blocks of DNA. They are composed of three parts:

• a sugar molecule called deoxyribose
• a phosphate group
• a nitrogenous base, which can be Adenine (A), Thymine (T), Cytosine (C), or Guanine (G)

The sequence of these nucleotides in DNA encodes genetic instructions.
Each nucleotide is connected to the next via the sugar and phosphate group, forming a backbone for the DNA strands, while the nitrogenous bases stick out. This configuration allows for the complementary base pairing that gives DNA its double-stranded shape.
Nucleotides do much more than hold genetic information. They also play roles in cellular energy supply, signaling, and metabolism.

DNA Sequence

A DNA sequence is a specific linear order of nucleotides in a DNA molecule. This sequence determines the genetic instructions used in the growth and functioning of all living organisms.
The order of the nucleotides is exactly like the letters in an alphabet spelling out a meaningful sentence. Each sequence is unique, and even a small change can have significant impacts on an organism's characteristics.
Knowing how to read and understand DNA sequences is essential for identifying genetic variations and mutations.
Applying the base-pairing rules, as seen when finding complementary strands, is a fundamental task in DNA analysis. This ability helps to create complementary strands, necessary for processes such as replication and transcription in cells.