Question

5’ end of DNA is characterized by

• A. Hydroxyl group
• B. Peptide bond
• C. Nitrous group
• D. Phosphate group

Hint:

To chemically prevent a vector DNA strand from self-ligating during cloning, the enzyme alkaline phosphatase is used to remove the 5'-phosphate groups from the vector ends.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
A single strand of DNA is a polar polymer composed of repeating deoxyribonucleotide units linked together by covalent $3'-5'$ phosphodiester bonds.
This asymmetric linkage gives the DNA strand a distinct directional polarity, with defined 5'-ends and 3'-ends.

Step 2: Detailed Explanation:
Let us examine the chemical structure of the nucleotide building blocks:
Each nucleotide consists of a nitrogenous base, a deoxyribose sugar, and a phosphate group.
The carbon atoms of the deoxyribose sugar are numbered 1' through 5'.
- At the 5' end of a DNA strand, the terminal nucleotide has a free, unlinked phosphate group covalently attached to the 5' carbon of its deoxyribose ring.
This phosphate group often carries a negative charge under physiological pH conditions.
- At the 3' end of the DNA strand, the terminal nucleotide has a free, unlinked hydroxyl group ($-\text{OH}$) attached to the 3' carbon of its deoxyribose ring.
During DNA replication and transcription, DNA and RNA polymerases synthesize new strands exclusively in the $5'\text{ to }3'$ direction by attaching the 5'-phosphate group of an incoming nucleotide to the free 3'-OH group of the growing strand.
Therefore, the 5' end is chemically characterized by a phosphate group.

Step 3: Final Answer:
The 5' end of DNA is characterized by a phosphate group, corresponding to option (D).