Problem 4 - Biology Today and Tomorrow with Physiology

Question

What are the base-pairing rules for DNA? a. \(A-G, T-C\) b. \(\mathrm{A}-\mathrm{C}, \mathrm{T}-\mathrm{G}\) c. \(A-T, G-C\) d. \(\mathrm{A}-\mathrm{A}, \mathrm{G}-\mathrm{G}, \mathrm{C}-\mathrm{C}, \mathrm{T}-\mathrm{T}\)

Step-by-Step Solution

Answer

Option c (A-T, G-C) is correct.

Step 1: Understand DNA Base-Pairing

DNA (Deoxyribonucleic Acid) is composed of two strands forming a double helix. The strands are connected by pairs of nucleotides. The four types of nucleotides in DNA are adenine (A), thymine (T), guanine (G), and cytosine (C). Understanding the rules for which nucleotide pairs with which is essential for DNA structure.

Step 2: Review Base-Pair Rule

In DNA, adenine (A) always pairs with thymine (T), and guanine (G) always pairs with cytosine (C). This specific pairing is known as complementary base pairing, and it is critical for the structure and function of DNA.

Step 3: Apply the Base-Pair Rule to Options

Examine each given option to determine which follows the correct base-pairing rules: a. (A-G, T-C) - Incorrect b. (A-C, T-G) - Incorrect c. (A-T, G-C) - Correct d. (A-A, G-G, C-C, T-T) - Incorrect

Step 4: Select the Correct Option

Based on our understanding of base-pairing rules, the correct option where adenine pairs with thymine and guanine pairs with cytosine is option "c" (A-T, G-C).

Key Concepts

Nucleotide Pairing

In the realm of DNA, nucleotide pairing is the fundamental concept governing its structure. Nucleotides are the building blocks of DNA, and each consists of a phosphate group, a sugar molecule, and a nitrogenous base. There are four different nitrogenous bases in DNA: adenine (A), thymine (T), guanine (G), and cytosine (C). Each base pairs with a specific partner to form the rungs of the DNA ladder.

The specific partnerships are as follows:

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

This pairing is crucial because it ensures the DNA’s stability and functionality. By adhering to these pairings, DNA maintains a consistent structure, allowing it to effectively replicate and carry genetic information.

Complementary Base Pairing

Complementary base pairing is a central concept in the structure and replication of DNA. The term "complementary" refers to the specific pairing rules that dictate which nitrogenous bases can pair together. This pairing is not arbitrary but is determined by the molecular structure of the bases themselves, enabling hydrogen bonds to form between specific pairs.

These are the rules:

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

These specific hydrogen bonds are what make the pairing "complementary." This precise pairing is not only essential for structural reasons but also for DNA replication and genetic heredity. During replication, each strand of the DNA double helix serves as a template for the creation of a new complementary strand, ensuring the genetic code is accurately transmitted.

DNA Double Helix

The DNA double helix is one of the most iconic structures in biology. Imagine a twisted ladder, where the sugar-phosphate backbones of DNA form the sides, and the complementary base pairs form the rungs. This twisting structure is what we refer to as the "double helix," and it was first described by James Watson and Francis Crick in 1953.

The structure is stable due to:

• The sugar-phosphate backbones running in opposite directions, known as antiparallel strands
• The strong covalent bonds along the backbone
• The weak hydrogen bonds between bases, which make the strand flexible yet stable

This architecture is essential for DNA function. It allows the compact packaging of DNA within the cell nucleus and plays a pivotal role in replication, as each strand can be used as a template for creating an exact copy. Understanding the DNA double helix is crucial for comprehending how genetic information is stored and transmitted across generations.