How many phosphodiester bonds are present in plasmid DNA having 1000 bp?

Options

A. 1000
B. 2000
C. 999
D. 1998

Answer

To determine the number of phosphodiester bonds in a plasmid DNA molecule with 1000 base pairs (bp), we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Structure of DNA**:
- DNA is composed of two strands that are complementary to each other. Each strand consists of a sequence of nucleotides, and each nucleotide contains a phosphate group, a sugar (deoxyribose), and a nitrogenous base.

2. **Identifying Phosphodiester Bonds**:
- Phosphodiester bonds are the connections between the phosphate group of one nucleotide and the sugar of the next nucleotide in a single strand of DNA. Each phosphodiester bond links two nucleotides together.

3. **Calculating Phosphodiester Bonds in One Strand**:
- For a strand of DNA with 1000 base pairs, there are 1000 nucleotides on one strand and 1000 nucleotides on the complementary strand.
- However, the number of phosphodiester bonds in one strand is one less than the number of nucleotides because the last nucleotide does not form a bond with another nucleotide after it.
- Therefore, in one strand, the number of phosphodiester bonds = 1000 - 1 = 999.

4. **Considering Both Strands**:
- Since DNA is double-stranded, we need to consider both strands. Each strand has 999 phosphodiester bonds.
- Thus, the total number of phosphodiester bonds in the plasmid DNA = 999 (from strand 1) + 999 (from strand 2) = 1998.

5. **Final Answer**:
- Therefore, the total number of phosphodiester bonds in plasmid DNA with 1000 base pairs is **1998**.