Question

Which of the following bases is not present in DNA? (A) Adenine (B) Thymine (C) Cytosine (D) Uracil

Short answer

The base not present in DNA is (D) Uracil. Uracil is a nitrogenous base in RNA and replaces Thymine in RNA structure.

Step-by-step solution

Recall the nitrogenous bases present in DNA

DNA (Deoxyribonucleic Acid) is a molecule that carries genetic information in living organisms. The DNA structure consists of a double helix, which is composed of two chains of nucleotides. Each nucleotide consists of a phosphate group, a sugar molecule (deoxyribose), and a nitrogenous base. There are four nitrogenous bases in DNA: Adenine (A), Thymine (T), Cytosine (C), and Guanine (G). These bases form complementary base pairs (A-T and C-G) that hold the two strands of DNA together.

Identify the base not present in DNA

Given the four options: (A) Adenine (B) Thymine (C) Cytosine (D) Uracil Adenine (A), Thymine (T), and Cytosine (C) are present in DNA, as discussed in Step 1. So the base that is not present in DNA is Uracil (D). It is worth noting that Uracil (U) is a nitrogenous base in RNA (Ribonucleic Acid) and replaces Thymine (T) in RNA structure.

Key concepts

DNA Structure

DNA, short for deoxyribonucleic acid, is often imagined as a twisting ladder or double helix. This iconic form is not just for aesthetics; it is crucial for the biological functions of DNA. Each rung of this ladder is made up of paired nitrogenous bases, which are the building blocks for DNA structure. Each side of the ladder is a chain of nucleotides. A nucleotide consists of three parts: a phosphate group, a sugar molecule (deoxyribose), and a nitrogenous base. These components assemble to form the long strands of DNA.

The structural integrity and flexibility of the DNA double helix allow for the replication and encoding of genetic instructions. The base pairs—adenine paired with thymine, and cytosine paired with guanine—are held together by hydrogen bonds. This pairing is specific and follows Chargaff's rules, ensuring accurate replication and transmission of genetic material during cell division.

Genetic Information

The true wonder of DNA is its ability to store and transmit genetic information essential for life. This information is encoded in the sequence of the nitrogenous bases—adenine, thymine, cytosine, and guanine. Each sequence forms genes that act as blueprints for making proteins, which perform a variety of functions within organisms.

During transcription and translation, these sequences are read and translated into functional proteins. This meticulous process is the foundation of gene expression. Errors in this sequence could lead to mutations, which can sometimes cause disease, but they can also contribute to diversity and evolution in populations.
The ability of DNA to replicate accurately is essential for genetic inheritance. Through cell division and reproduction, genetic information is passed from parents to offspring, ensuring the continuity of life.

Nucleotide Composition

At the core of DNA's function and structure is its nucleotide composition. A nucleotide is a basic unit of DNA and comprises three components:

• Phosphate group: A phosphorus atom bonded to four oxygen atoms, contributing to the backbone of DNA.
• Deoxyribose sugar: A five-carbon sugar that gives DNA its name (deoxyribonucleic acid).
• Nitrogenous base: These include adenine (A), thymine (T), cytosine (C), and guanine (G) in DNA.

Each nucleotide plays a critical role in forming the DNA structure and encoding genetic information. The sequence of these nucleotides dictates the genetic instructions carried by a DNA molecule.

While uracil is a nitrogenous base, it is not found in DNA. Instead, uracil appears in RNA (ribonucleic acid), replacing thymine as a base. This difference is one of the key distinctions between DNA and RNA, influencing their respective roles in genetic coding and protein synthesis.