Write the main structural difference between `DNA` and `RNA`. Of the four bases, common to both `DNA` and `RNA`.

Answer

To address the question regarding the main structural differences between DNA and RNA, as well as the common bases found in both, we can follow these steps:

### Step 1: Identify the Structure of DNA
- DNA (Deoxyribonucleic Acid) is characterized by its double-stranded helical structure. It consists of two strands that wind around each other, forming a double helix. Each strand is made up of a backbone of sugar (deoxyribose) and phosphate groups, with nitrogenous bases attached.

### Step 2: Identify the Structure of RNA
- RNA (Ribonucleic Acid) is typically single-stranded. It consists of a single chain of nucleotides, which includes a backbone of sugar (ribose) and phosphate groups, with nitrogenous bases attached.

### Step 3: Compare the Strands
- The main structural difference is that DNA is double-stranded, while RNA is single-stranded. This fundamental difference in structure affects their functions in biological systems.

### Step 4: Identify the Common Nitrogenous Bases
- Both DNA and RNA share three common nitrogenous bases: adenine (A), guanine (G), and cytosine (C).

### Step 5: Identify the Unique Bases
- The fourth base in DNA is thymine (T), while in RNA, it is uracil (U). This means that thymine is present in DNA but not in RNA, and uracil is present in RNA but not in DNA.

### Final Summary
- **Main Structural Difference**: DNA is double-stranded, while RNA is single-stranded.
- **Common Nitrogenous Bases**: Adenine, guanine, and cytosine are common to both DNA and RNA, while thymine is unique to DNA and uracil is unique to RNA.