Problem 70 - A Complete Resource Book in Chemistry for JEE Main

Question

RNA and DNA are chiral molecules, their chirality is due to (a) L-sugar component (b) Chiral bases (c) Chiral phosphate ester units (d) D-sugar component

Step-by-Step Solution

Answer

The chirality of RNA and DNA is due to the D-sugar component (option d).

Step 1: Understanding Chirality

Chirality is a property of a molecule having non-superimposable mirror images. In biochemistry, this often refers to the specific arrangement of atoms, particularly at carbon centers.

Step 2: Identify Components of RNA and DNA

RNA and DNA are composed of sugar, phosphate, and nitrogenous bases. DNA contains a deoxyribose sugar, and RNA has ribose. Both sugars are linked to phosphate groups and nitrogenous bases.

Step 3: Determine Chirality in Each Component

In nucleic acids, the sugar component has a chiral center. The predominant form of the sugar used in nucleic acids is a D-sugar. This chirality contributes to the overall structure of RNA and DNA.

Step 4: Eliminate Inapplicable Options

The bases in RNA and DNA (adenine, guanine, cytosine, thymine, uracil) are not chiral. Phosphate groups do not have chiral centers. Thus, options (b) and (c) do not contribute to chirality.

Step 5: Conclusion: Correct Answer

From the analysis, D-sugar components in RNA and DNA are responsible for the chirality, making option (d) the correct choice.

Key Concepts

RNA and DNA structure

Both RNA and DNA are fundamental molecules that carry genetic information. They are made up of a sequence of nucleotides that form a long chain. These nucleotides are composed of three subunits: a phosphate group, a five-carbon sugar, and a nitrogenous base.

While both RNA and DNA share a similar backbone structure, there are key differences between them. DNA contains a deoxyribose sugar, which lacks one oxygen atom, making it more stable and less reactive than RNA's ribose sugar. This structural difference is crucial as it allows DNA to be more suitable for long-term storage of genetic information. In contrast, RNA is more flexible and able to form many structures, which is essential for its roles in protein synthesis and regulation.

The arrangement and organization of these nucleotides in RNA and DNA ensure that genetic information is accurately stored and transmitted, playing vital roles in the functioning of living organisms.

Nucleic acid components

Nucleic acids, such as RNA and DNA, are built from specific components that determine their functions and properties. These components are:

• Sugar: In DNA, we find deoxyribose, whereas in RNA, ribose is present. The sugar forms part of the nucleic acid backbone.
• Phosphate group: This group links the sugars of adjacent nucleotides, forming a strong and stable backbone.
• Nitrogenous bases: The bases include adenine (A), thymine (T) in DNA, uracil (U) in RNA, cytosine (C), and guanine (G). These bases pair specifically (A with T, and C with G in DNA) to encode genetic information.

Each of these components contributes to the characteristic properties of nucleic acids.

The uniqueness of nucleic acids lies in the specific sequence of bases, which forms the genetic code. This sequence of nucleotides conveys genetic instructions essential for the development and functioning of living cells.

Chiral centers in biochemistry

Chirality in biochemistry refers to molecules that cannot be superimposed onto their mirror images. This property is crucial for the structure and function of many biomolecules, including nucleic acids like RNA and DNA. In these molecules, chirality primarily arises from the sugar component, specifically the D-sugar.

In RNA and DNA, the five-carbon sugar, either ribose in RNA or deoxyribose in DNA, contains several chiral centers. The presence of these chiral centers determines the molecule's three-dimensional shape and affects how it interacts with other biomolecules. In nucleic acids, this three-dimensionality is vital for the double helix structure of DNA and the varied folding patterns of RNA.

Understanding chirality helps explain how biological systems function, as the shape and orientation of molecules are often critical in biochemical reactions and interactions. In particular, the chirality of the sugar component in nucleic acids is essential for the proper storage and expression of genetic information.