Question

How many nucleotides will be present in a double stranded DNA fragment of 2 kb ?

• A. 1000
• B. 2000
• C. 4000
• D. 500

Hint:

For single-stranded RNA (ssRNA) or single-stranded DNA (ssDNA), a fragment of 2 kb contains only 2,000 nucleotides because there is no complementary strand.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Double-stranded DNA (dsDNA) is composed of two complementary polynucleotide chains held together by hydrogen bonds between base pairs.
The length of a dsDNA fragment is typically measured in kilobases (kb).
Key Formula or Approach:
The structural calculations are based on the following relationships:
- $1 \text{ kb} = 1,000 \text{ base pairs (bp)}$.
- Each base pair is composed of exactly $2$ nucleotides (one on each of the two complementary strands).
\[ \text{Total Nucleotides} = \text{Length in kb} \times 1,000 \times 2 \]

Step 2: Detailed Explanation:
We are given a double-stranded DNA fragment with a length of:
\[ \text{Length} = 2 \text{ kb} \]
First, convert the length from kilobases to base pairs:
\[ 2 \text{ kb} = 2 \times 1000 = 2000 \text{ base pairs (bp)} \]
Since each base pair consists of two nucleotides facing each other, the total number of individual nucleotides in the double-stranded fragment is:
\[ \text{Total Nucleotides} = 2000 \text{ base pairs} \times 2 \text{ nucleotides/bp} = 4000 \text{ nucleotides} \]

Step 3: Final Answer:
The total number of nucleotides present in a 2 kb double-stranded DNA fragment is 4000, which corresponds to option (C).