Problem 86 - Chemistry The Molecular Nature of Matter

Question

Which base pairs with (a) \(\mathrm{A}\) in DNA, (b) \(A\) in \(\mathrm{RNA},\) and (c) C in DNA or RNA?

Step-by-Step Solution

Answer

In DNA, (a) Adenine pairs with Thymine and (c) Cytosine pairs with Guanine. In RNA, (b) Adenine pairs with Uracil.

Step 1: Determine the Base Pair for Adenine in DNA

In DNA, the nucleotide Adenine (A) pairs with Thymine (T). This is because of the hydrogen bonding between the bases: Adenine forms two hydrogen bonds with Thymine.

Step 2: Determine the Base Pair for Adenine in RNA

In RNA, the nucleotide Adenine (A) pairs with Uracil (U). Like Thymine, Uracil pairs with Adenine through hydrogen bonding. However, Uracil replaces Thymine in RNA.

Step 3: Determine the Base Pair for Cytosine in DNA and RNA

In both DNA and RNA, the nucleotide Cytosine (C) pairs with Guanine (G). Cytosine and Guanine are complementary because they are able to form three hydrogen bonds with each other.

Key Concepts

DNA Base Pairing

Understanding the mechanism of DNA base pairing is crucial for grasping the fundamentals of genetics. DNA, or deoxyribonucleic acid, is the molecule that carries the genetic instructions in all living organisms. At the heart of DNA's double helix structure are the base pairing rules, often referred to as Chargaff's rules.

In DNA molecules, the nucleobase adenine (A) always pairs with thymine (T), and this partnership is maintained through the establishment of two hydrogen bonds. These specific pairings are crucial for the accurate replication of DNA during cell division and for maintaining the stability of the DNA structure. It's the consistency of these pairings that allows DNA to be copied faithfully from one cell to another during the process of replication.

Importance of Complementarity

The idea that each base can only pair with one specific partner ensures the correct sequence of bases is preserved. Adenine's affinity for thymine, facilitated by hydrogen bonding, is a perfect example of this molecular complementarity. The precise dimensions of the DNA double helix are also due to the way A-T and C-G pairings fit together, maintaining a uniform shape critical for the functioning of the molecule.

RNA Base Pairing

RNA base pairing is slightly different from DNA base pairing due to the presence of uracil instead of thymine. RNA, or ribonucleic acid, plays a central role in the process of translating genetic information from DNA into proteins. The rules of RNA base pairing are essential to ensure the fidelity of this genetic translation.

In RNA, adenine (A) does not pair with thymine but instead pairs with uracil (U). Adenine and uracil are capable of forming two hydrogen bonds, similar to the A-T pairing in DNA. This small change from thymine to uracil has significant biochemical implications and is one of the key differences between RNA and DNA.

RNA's Versatile Roles

RNA molecules are not only pivotal in protein synthesis but also play a role in regulatory functions within the cell. They can fold into complex three-dimensional shapes that allow them to bind with other molecules, an attribute highly dependent on base pairing. RNA's ability to pair with complementary bases is also vital for its role in various biological processes, such as the regulation of gene expression and the catalysis of biochemical reactions.

Hydrogen Bonds in Nucleotides

The role of hydrogen bonds in the stability and function of nucleic acids cannot be overstated. Hydrogen bonds are a type of non-covalent molecular interaction that stabilize the complex structures of DNA and RNA.

In the context of nucleic acids, these bonds form between the nitrogenous bases and are responsible for the specificity of base pairing. For instance, in DNA, cytosine (C) pairs with guanine (G) forming three hydrogen bonds, which is one more than the A-T pairing. This additional bond makes the C-G pairing slightly stronger than the A-T pairing.

Hydrogen Bonding Patterns

Hydrogen bonds occur when a hydrogen atom, already covalently linked to a strongly electronegative atom such as nitrogen or oxygen, experiences attraction towards another electronegative atom in close proximity. The distinctive hydrogen bonding patterns of nucleotide base pairs contribute to the double helix's resistant yet flexible nature, a property essential for DNA's storage and RNA's catalytic functionality. These bonds also play a key role during DNA replication and transcription, as they need to be broken and reformed to allow these processes to occur.