All of the following statements are correct about plasmids except :

Options

A. they are extrachromosomal DNA
B. They are smaller, circular, double stranded naked DNA that confer certain unique phenotypic characters to some bacteria like resistance to antibiotics
C. They are used in genetic engineering
D. It helps in the replication of nucleoid

Answer

To solve the question "All of the following statements are correct about plasmids except," we will analyze each statement provided in the options.

### Step-by-Step Solution:

1. **Identify the Nature of Plasmids**:
- Plasmids are known to be extra chromosomal DNA. They exist independently of the chromosomal DNA within a cell.
- **Hint**: Remember that plasmids are not part of the main chromosome.

2. **Evaluate the Structure of Plasmids**:
- Plasmids are typically small, circular, double-stranded DNA molecules. They do not have a protein coat and are often referred to as "naked" DNA.
- **Hint**: Think about the shape and structure of plasmids when considering their characteristics.

3. **Function of Plasmids**:
- Plasmids can confer specific traits to bacteria, such as antibiotic resistance. This is a key feature that allows bacteria to survive in hostile environments.
- **Hint**: Consider how plasmids contribute to the survival and adaptability of bacteria.

4. **Role in Genetic Engineering**:
- Plasmids are widely used in genetic engineering as vectors to transfer genetic material into cells. They can carry genes that provide antibiotic resistance, among other traits.
- **Hint**: Recall the applications of plasmids in biotechnology and genetic manipulation.

5. **Analyze the Incorrect Statement**:
- The last statement claims that plasmids help in the replication of nucleoid. This is incorrect because plasmids replicate independently of the chromosomal DNA. They have their own origin of replication (replicon) and do not assist in the replication of the nucleoid.
- **Hint**: Focus on the independence of plasmid replication compared to chromosomal DNA.

### Conclusion:
Based on the analysis, the statement that is incorrect about plasmids is: "It helps in replication of nucleoid."

### Final Answer:
The correct answer is the statement that says "it helps in replication of nucleoid."