In 'B' model of DNA,the diameter is 20Å. It is ______in Z DNA.

Options

A. `23Å`
B. `18Å`
C. `21Å`
D. `26Å`

Answer

To solve the question regarding the diameter of Z DNA in comparison to B DNA, we will follow these steps:

### Step-by-Step Solution:

1. **Understand the B DNA Model**: In the B model of DNA, the diameter is given as 20 Å (angstroms). This is a standard measurement for the B form of DNA, which is the most common form found in cells.

2. **Identify the Z DNA Model**: The Z DNA model is another conformation of DNA that differs from B DNA in several structural aspects.

3. **Compare the Diameters**: According to the information provided, the diameter of Z DNA is different from that of B DNA. Specifically, while B DNA has a diameter of 20 Å, Z DNA has a smaller diameter.

4. **State the Diameter of Z DNA**: Based on the differences mentioned, the diameter of Z DNA is 18 Å.

5. **Fill in the Blank**: Therefore, the statement can be completed as follows: "In the B model of DNA, the diameter is 20 Å. It is 18 Å in Z DNA."

### Final Answer:
The diameter in Z DNA is **18 Å**.