During electrophoresis of the digested DNA , the DNA fragment closest to the cathode is

Options

A. the heaviest and is positively charged
B. the lightest and is positively charged
C. the heaviest and is negatively charged
D. the lightest and is negatively charge

Answer

To solve the question regarding the behavior of DNA fragments during electrophoresis, we can break it down into the following steps:

### Step 1: Understand the Basics of Electrophoresis
Electrophoresis is a technique used to separate charged particles, such as DNA fragments, based on their size and charge. In this process, an electric field is applied, causing the charged molecules to migrate towards the electrode of opposite charge.

**Hint:** Remember that DNA is negatively charged due to its phosphate backbone.

### Step 2: Identify the Electrodes
In electrophoresis, there are two electrodes: the anode (positive) and the cathode (negative). DNA fragments will migrate towards the anode because they are negatively charged.

**Hint:** The negatively charged DNA moves towards the positively charged anode.

### Step 3: Analyze the Movement of DNA Fragments
The speed at which DNA fragments move through the gel is influenced by their size. Smaller fragments can navigate through the gel matrix more easily than larger ones, allowing them to travel faster.

**Hint:** Smaller fragments move faster than larger fragments due to less resistance in the gel.

### Step 4: Determine the Position of DNA Fragments
Since the smaller (lightest) DNA fragments move faster, they will be located closer to the anode (the positive electrode) after the electrophoresis process is complete. Conversely, the larger (heaviest) fragments will be found closer to the starting point (the cathode).

**Hint:** The lightest fragments will be the furthest from the starting point (cathode) and closest to the anode.

### Step 5: Answer the Question
The question asks which DNA fragment is closest to the cathode. Since the heaviest fragments move slower and remain closer to the starting point (cathode), the correct answer is that the DNA fragment closest to the cathode is the heaviest and negatively charged.

**Final Answer:** The heaviest and negatively charged DNA fragment is closest to the cathode.

### Summary of the Steps:
1. Understand the basics of electrophoresis.
2. Identify the electrodes (anode and cathode).
3. Analyze the movement of DNA fragments based on size.
4. Determine the position of the fragments after electrophoresis.
5. Conclude that the heaviest and negatively charged fragment is closest to the cathode.