Problem 50 - Advanced Problems in Organic Chemistry for Competitive Examinations

Question

In DNA replication the complementary nucleotide sequence for \(5^{\prime}-\mathrm{ACGT}-3^{\prime}\) is which of the following? (a) \(3^{\prime}-\mathrm{ACGT}-5^{\prime}\) (b) \(3^{\prime}-\mathrm{TGCA}-5^{\prime}\) (c) \(5^{\prime}-\mathrm{AGCT}-3^{\prime}\) (d) \(5^{\prime}\) -TCGA-3 \(^{\prime}\)

Step-by-Step Solution

Answer

The complementary sequence is (b) 3'-TGCA-5'.

Step 1: Understanding DNA Base Pairing

In DNA, adenine (A) pairs with thymine (T), and cytosine (C) pairs with guanine (G). These pairings are complementary, meaning each base on one side of the DNA ladder pairs with a specific base on the other side.

Step 2: Identifying the Base Pairs for Each Nucleotide

For the nucleotide sequence given, 5'-ACGT-3', we need to identify the complementary base for each: - A (adenine) pairs with T (thymine). - C (cytosine) pairs with G (guanine). - G (guanine) pairs with C (cytosine). - T (thymine) pairs with A (adenine).

Step 3: Forming the Complementary Sequence

Align the complementary base pairs in sequence opposite the original: - Starting with the original 5'-ACGT-3' sequence: 1. A pairs with T 2. C pairs with G 3. G pairs with C 4. T pairs with A Thus, our complementary sequence is 3'-TGCA-5'.

Step 4: Selecting the Proper Answer Format

The complementary sequence must be written from 3' to 5' to correctly match the antiparallel nature of DNA strands. The answer 3'-TGCA-5' matches these conditions. - Check the provided answer choices: Option (b) matches exactly: 3'-TGCA-5'.

Key Concepts

Nucleotide Base Pairing

In the DNA structure, nucleotide bases form the rungs of the ladder-like double helix. These bases pair in a specific manner, known as nucleotide base pairing. This pairing is the backbone of DNA's ability to store and transmit genetic information. There are two main pairs:

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

The pairs are held together by hydrogen bonds, with A and T connected by two hydrogen bonds, while C and G have three. These pairings are complementary, meaning each base on one strand of the DNA fits precisely with a base on the opposite strand.
If you learn these base pair rules, reading and understanding DNA sequences becomes a lot easier. It's like matching puzzle pieces that only fit in one perfect way.

Complementary Sequences

In the DNA double helix, each strand is made up of sequences of nucleotides, which need to be complementary to form the stable DNA structure. If you have one DNA strand, you can determine the sequence of its complementary strand by following the base pairing rules.
For example, if the original sequence is 5'-ACGT-3', the complementary sequence will be 3'-TGCA-5'. Each nucleotide on one strand finds its complementary counterpart from the other strand:

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

Identifying these sequences helps in various genetic processes, such as DNA replication and repair. It's essential to ensure no errors occur during the copying of DNA for cell division, making complementary sequences critical for accuracy.

Antiparallel DNA Strands

DNA strands run in opposite directions, which is known as being antiparallel. This concept is central to understanding the double helix structure of DNA. Each strand has a directionality, defined by the sugar-phosphate backbone on which the nucleotides are attached.
One end of the DNA strand is designated as the 5' end, and the other as the 3' end. In the typical helix structure, one strand runs 5' to 3', and the other runs 3' to 5'. This opposite directionality is essential because:

• It enables the proper alignment of nucleotide base pairs.
• It is necessary for the enzymes involved in DNA replication.

When DNA replicates, it does so by unwinding and creating two new strands, each complementary to one of the original strands. Understanding the antiparallel nature and how the sequences align allows scientists to predict the complementary strand of any DNA sequence correctly.