Choose the subunit of a ribosome which encounters an mRNA

Options

A. Larger
B. Smaller
C. Both (a) and (b)
D. Not define

Explanation

To answer the question "Choose the subunit of a ribosome which encounters an mRNA," we can break down the solution into the following steps:

### Step 1: Understand the Structure of Ribosomes
Ribosomes are composed of two subunits: a larger subunit and a smaller subunit. In prokaryotes, these are referred to as the 50S (larger) and 30S (smaller) subunits, while in eukaryotes, they are the 60S (larger) and 40S (smaller) subunits.

**Hint:** Remember that ribosomes consist of two subunits that work together during protein synthesis.

### Step 2: Identify the Role of Each Subunit
The smaller subunit of the ribosome is primarily responsible for binding to the mRNA. It is the first to encounter the mRNA during the initiation of protein synthesis (translation).

**Hint:** Focus on the initiation phase of translation to understand which subunit interacts with mRNA first.

### Step 3: Determine the Correct Subunit
Since the question specifically asks for the subunit that encounters mRNA first, we can conclude that it is the smaller subunit. In prokaryotes, this is the 30S subunit, and in eukaryotes, it is the 40S subunit.

**Hint:** Recall that the smaller subunit is essential for the initial binding of mRNA during the translation process.

### Step 4: Final Answer
Based on the above reasoning, the correct answer to the question is the smaller subunit of the ribosome.

**Final Answer:** The smaller subunit (Option B).