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Chapter 17: Problem 80

Why are there no base pairs in DNA between adenine and guanine, or thymine and cytosine?

Short Answer

Expert verified
Base pairing rules and structural differences prevent stable pairs between adenine-guanine and thymine-cytosine.

Step by step solution

01

Understand DNA Base Pairing Rules

In DNA, base pairs are formed between specific nitrogenous bases. Adenine (A) pairs with thymine (T), and cytosine (C) pairs with guanine (G).
02

Base Pairing and Hydrogen Bonds

Adenine and thymine form two hydrogen bonds, whereas cytosine and guanine form three hydrogen bonds. These bonds are critical for the stability of the DNA double helix.
03

Examine Size and Structure

The molecular structure of the bases plays a role in pairing. Adenine and guanine are both purines (larger, double-ring structures), while cytosine and thymine are pyrimidines (smaller, single-ring structures). Proper base pairing ensures uniform spacing in the DNA double helix.
04

Stability of Base Pairs

Pairs like adenine and guanine, or thymine and cytosine, do not form stable hydrogen bonds and would disrupt the uniformity and proper spacing of the DNA double helix. Thus, they do not pair with each other.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

adenine-thymine pairing
In the structure of DNA, one of the most crucial aspects is the specific pairing between the nitrogenous bases. Adenine (A) pairs with thymine (T), creating what is known as an adenine-thymine pair. This pairing occurs due to the formation of hydrogen bonds between these two bases.
Adenine and thymine form exactly two hydrogen bonds, which contribute to the stability of the DNA molecule. This specific number of bonds ensures that the distance between the two strands of the DNA double helix remains consistent.
This is vital because any irregularity in the spacing could cause structural issues, making it difficult for the genetic material to function correctly.
cytosine-guanine pairing
Similar to the adenine-thymine pairing, cytosine (C) pairs exclusively with guanine (G). The cytosine-guanine pairing occurs due to the formation of three hydrogen bonds between these bases.
These three bonds make the cytosine-guanine pair more stable compared to the adenine-thymine pair. The additional hydrogen bond provides extra strength and stability to the DNA structure.
Just like the adenine-thymine pairing, the specific hydrogen bonding between cytosine and guanine maintains the consistent spacing and structural integrity of the DNA double helix.
hydrogen bonds in DNA
Hydrogen bonds are weak chemical bonds that play a critical role in binding the DNA strands together. Each base pair, whether adenine-thymine or cytosine-guanine, is linked by these hydrogen bonds.
Although hydrogen bonds are individually weak compared to covalent bonds, the large number of them in the DNA molecule collectively provides significant stability.
The integrity of these hydrogen bonds is crucial for the replication and transcription processes of DNA, as they ensure the strands can come apart and rebind efficiently.
purines and pyrimidines
To understand base pairing, it's essential to know the structure of the bases involved. Adenine (A) and guanine (G) are purines, while cytosine (C) and thymine (T) are pyrimidines.
Purines have a larger, double-ring structure, making them bulkier than pyrimidines, which have a smaller, single-ring structure.
This difference in size is critical for base pairing. A purine must pair with a pyrimidine to keep the DNA double helix structure uniform and stable. Pairing either two purines or two pyrimidines would disrupt the consistent spacing of the DNA strands, leading to instability.

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Most popular questions from this chapter

What is the difference between a codon and an anticodon?

How is protein synthesis affected if the normal base sequence CCC in the DNA template strand is changed to ACC?

How do the bases cytosine and uracil differ?

a. If the DNA double helix in salmon contains \(28 \%\) adenine, what is the percent of thymine, guanine, and cytosine? b. If the DNA double helix in humans contains \(20 \%\) cytosine, what is the percent of guanine, adenine, and thymine?

How are the two strands of nucleic acid in DNA held together?
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