Question

The double-helix model of DNA resembles a twisted ladder in which the rungs of the ladder are a. complementary base pairs. b. A paired with G and C paired with T. c. A paired with \(\mathrm{T}\) and \(\mathrm{G}\) paired with \(\mathrm{C}\) d. a sugar-phosphate paired with a sugar-phosphate. e. Both a and c are correct.

Short answer

The correct answer is e. Both a and c are correct, as the rungs of the twisted ladder in the double-helix model of DNA are formed by complementary base pairs, with A paired with T and G paired with C.

Step-by-step solution

Base Pairing Rules

To answer this question, it's crucial to understand the base pairing rules in a DNA molecule. In DNA, there are four nitrogenous bases: Adenine (A), Guanine (G), Cytosine (C), and Thymine (T). These bases pair with each other in a specific manner following the complementary base pairing rules, which state that Adenine (A) pairs with Thymine (T) and Guanine (G) pairs with Cytosine (C).

Analyze Each Option

Now, let's analyze each option in the question to see which one aligns with the base pairing rules. a. Complementary base pairs: This statement is consistent with the base pairing rules mentioned above. b. A paired with G and C paired with T: This statement contradicts the base pairing rules. c. A paired with T and G paired with C: This statement matches the base pairing rules. d. A sugar-phosphate paired with a sugar-phosphate: This statement confuses the backbone structure of the DNA with the rungs. The sugar-phosphate structures form the backbone of the DNA molecule, while the base pairs form the rungs. e. Both a and c are correct: This statement combines options a and c, both of which are consistent with the base pairing rules.

Choose the Correct Answer

Based on the analysis, option e, which includes both options a and c, best describes the double-helix model of DNA. Therefore, the correct answer is: e. Both a and c are correct.

Key concepts

Double-Helix Model of DNA

Imagine twirling a ladder as you would a jump rope. This twisting ladder is a vivid representation of the double-helix model of DNA, first described by James Watson and Francis Crick in 1953. This revolutionary model revealed that DNA's structure resembles two intertwined strands, each twist of the helix comprising a pair of nucleotide bases. These strands form the DNA's 'backbone', consisting of alternating sugar (deoxyribose) and phosphate groups.

Within this model, the 'rungs' of the twisted ladder are composed of the base pairs. These nucleotide bases are the critical elements that carry genetic information. The pattern in which these bases are arranged defines the hereditary characteristics passed from parents to offspring. The double-helix structure is not random; it is meticulously organized to facilitate the storage and retrieval of genetic data, as well as the accurate replication of DNA during cell division.

Nitrogenous Bases in DNA

The 'letters' of the genetic code are called nitrogenous bases. There are four such bases in DNA: Adenine (A), Thymine (T), Guanine (G), and Cytosine (C). These are organic molecules containing nitrogen atoms that contribute to DNA's ability to encode information. Their role is essential as they form the internal steps of the DNA ladder, held together by hydrogen bonds.

H4: Structure and Pairing
The nitrogenous bases are categorized into two groups based on their chemical structures: purines and pyrimidines. Adenine and Guanine are purines, which are larger molecules with two carbon-nitrogen rings. Thymine and Cytosine are pyrimidines, smaller molecules with just one carbon-nitrogen ring. The pairing of purines and pyrimidines is size-specific, ensuring that the double helix structure maintains a constant width.

Complementary Base Pairing

The secret behind DNA's code lies in the complementary base pairing rule. This foundational concept postulates that in DNA, Adenine (A) always pairs with Thymine (T) and Guanine (G) always pairs with Cytosine (C). This is due to their specific molecular structures, which enable them to bond with each other perfectly, like puzzle pieces. A matches with T, forming two hydrogen bonds, while G pairs with C, creating three hydrogen bonds, making the G-C pair slightly stronger.

Complementary base pairing is crucial because it serves two significant functions: it ensures the DNA's double helix structure remains stable and uniform in width, and it allows for the precise duplication of DNA during cell division, enabling genetic information to be replicated accurately. Understanding this principle is essential for studying how genes are passed on and how genetic information is expressed in living organisms.