If an isolated strain of DNA is kept at 82- `90^(@)` C , then

Options

A. The two strands uncoil and separate
B. Fragmentation occurs
C. Thymine is replaced by uracil
D. The structure is stabilised.

Answer

To solve the question regarding the effects of high temperatures on an isolated strain of DNA, we can follow these steps:

### Step 1: Understand the Structure of DNA
DNA is composed of two strands that are held together by hydrogen bonds between nitrogenous bases (adenine, thymine, cytosine, and guanine). Additionally, the backbone of each strand is formed by phosphodiester bonds between the sugar and phosphate groups.

**Hint:** Recall the components of DNA and how the strands are held together.

### Step 2: Identify the Temperature Range
The question specifies that the DNA is kept at a temperature range of 82 to 90 degrees Celsius. This is significantly higher than physiological temperatures (around 37 degrees Celsius).

**Hint:** Consider how temperature affects molecular stability, particularly in biological molecules.

### Step 3: Understand Denaturation
At high temperatures, the hydrogen bonds between the nitrogenous bases can break. This process is known as denaturation. Denaturation leads to the separation of the two strands of DNA.

**Hint:** Think about what happens to proteins when they are heated—similar principles apply to DNA.

### Step 4: Analyze the Options
1. **Two strands uncoil and separate**: This is correct because denaturation causes the strands to separate.
2. **Fragmentation occurs**: This is incorrect; fragmentation would imply breaking of the phosphodiester bonds, which does not happen at these temperatures.
3. **Thymine is replaced by uracil**: This is incorrect; this process occurs in RNA synthesis, not in DNA under these conditions.
4. **Structure is stabilized**: This is incorrect; high temperatures destabilize the structure of DNA.

**Hint:** Evaluate each option based on the effects of high temperatures on DNA structure.

### Step 5: Conclusion
Based on the analysis, the correct answer is that the two strands of DNA will uncoil and separate due to denaturation caused by the high temperature.

**Final Answer:** The correct option is that the two strands uncoil and separate.