Number of ester bonds established when a desired gene fragment cut with EcoRI is ligated with PBR322 cut with the same enzyme is

Options

A. 6
B. 4
C. 8
D. 16

Answer

To determine the number of ester bonds established when a desired gene fragment cut with EcoRI is ligated with PBR322 cut with the same enzyme, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Cutting Process**:
- Both the desired gene fragment and the PBR322 plasmid are cut using the restriction enzyme EcoRI. This enzyme creates sticky ends on both the gene fragment and the plasmid.

2. **Identify Sticky Ends**:
- When EcoRI cuts DNA, it leaves behind complementary sticky ends. This means that the ends of the desired gene fragment will be able to base-pair with the sticky ends of the PBR322 plasmid.

3. **Ligation Process**:
- During the ligation process, the DNA ligase enzyme will join the desired gene fragment to the PBR322 plasmid. The joining occurs at the sticky ends, where the complementary bases align.

4. **Formation of Ester Bonds**:
- Ester bonds are formed between the phosphate group of one nucleotide and the hydroxyl group of the sugar of the adjacent nucleotide. In this case, when the desired gene fragment is ligated to the plasmid, two ester bonds will be formed at each of the two sticky ends.

5. **Count the Ester Bonds**:
- Since there are two sticky ends on the desired gene fragment and two corresponding sticky ends on the PBR322 plasmid, a total of 4 ester bonds will be formed (2 from the gene fragment and 2 from the plasmid).

6. **Final Answer**:
- Therefore, the total number of ester bonds established when the desired gene fragment is ligated with PBR322 is **4**.