Assertion (A): The newly formed mRNA has same sequence as the coding strand of transcriptional unit with uracil present at place of thymine.
Reason (R): The rule of complementarity guides the formation of DNA and RNA.

Options

A. Both assertion (A) and reason (R) are true and reason (R) is the correct explanation of assertion (A)
B. Both assertion (A) and reason (R) are true but reason (R) is not the correct explanation of assertion (A).
C. Assertion (A) is true but reason (R) is false.
D. Assertion (A) is false but reason (R) is true.

Explanation

To solve the assertion and reason question, we will analyze both statements step by step.

### Step 1: Analyze the Assertion (A)
The assertion states: "The newly formed mRNA has the same sequence as the coding strand of the transcriptional unit with uracil present at the place of thymine."

- **Explanation**: During transcription, the mRNA is synthesized based on the DNA template. The coding strand of DNA (which has the same sequence as the mRNA except for the substitution of uracil for thymine) serves as a reference for the mRNA sequence. Therefore, if the coding strand has a sequence of DNA, the corresponding mRNA will have the same sequence but with uracil (U) replacing thymine (T).

### Step 2: Analyze the Reason (R)
The reason states: "The rule of complementarity guides the formation of DNA and RNA."

- **Explanation**: The rule of complementarity refers to the pairing of nitrogenous bases in nucleic acids. In DNA, adenine pairs with thymine (A-T) and cytosine pairs with guanine (C-G). In RNA, adenine pairs with uracil (A-U) instead of thymine. This rule is fundamental for the accurate synthesis of RNA during transcription, as it ensures that the correct bases are paired.

### Step 3: Determine the Truth of Each Statement
- **Assertion (A)**: True. The newly formed mRNA indeed has the same sequence as the coding strand of DNA, with uracil replacing thymine.
- **Reason (R)**: True. The rule of complementarity is essential for the formation of both DNA and RNA, guiding the pairing of bases.

### Step 4: Check if the Reason Explains the Assertion
The reason explains the assertion because the rule of complementarity is what allows the mRNA to be synthesized correctly from the DNA template. The substitution of uracil for thymine is a direct consequence of this rule.

### Conclusion
Both the assertion and the reason are true, and the reason correctly explains the assertion. Therefore, the correct option is:
**Option A: Both A and R are true, and R is the correct explanation of A.**

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