Polynucleotide chain of DNA contains
A nitrogenous base, deoxyribose sugar and phosphate group
Understanding the DNA Polynucleotide Chain Structure
The question asks about the composition of a polynucleotide chain specifically for Deoxyribonucleic Acid (DNA). DNA is a crucial molecule in biology, carrying genetic information. Its structure is a polymer, meaning it's made up of repeating smaller units.
What Makes Up a DNA Polynucleotide Chain?
The building blocks of a DNA polynucleotide chain are called nucleotides. Each nucleotide in a DNA strand consists of three main parts chemically linked together:
- A Nitrogenous Base: These are molecules containing nitrogen. In DNA, there are four types: Adenine (A), Guanine (G), Cytosine (C), and Thymine (T). These bases follow specific pairing rules (A pairs with T, C pairs with G).
- A Deoxyribose Sugar: This is a five-carbon sugar molecule. The term "deoxyribose" indicates that it lacks an oxygen atom at the 2' carbon position compared to ribose sugar found in RNA.
- A Phosphate Group: This is a molecule containing phosphorus and oxygen atoms (\(\text{PO}_4^{3-}\)). It is attached to the 5' carbon of the deoxyribose sugar.
These individual nucleotides link together to form the long polynucleotide chain. The linkage occurs between the phosphate group of one nucleotide and the 3' carbon of the deoxyribose sugar of the next nucleotide through a phosphodiester bond, creating the sugar-phosphate backbone of the DNA strand.
Analyzing the Given Options
Let's evaluate each option based on our understanding of DNA composition:
- Option 1: A nitrogenous base, deoxyribose sugar and phosphate group
This option lists the three correct components of a single DNA nucleotide, which are the repeating units that form the DNA polynucleotide chain. This aligns perfectly with the structure of DNA. - Option 2: A nitrogenous base, ribose sugar and phosphate group
This option describes the components of an RNA nucleotide. DNA contains deoxyribose sugar, not ribose sugar. Therefore, this option is incorrect for DNA. - Option 3: Deoxyribose sugar, ribose sugar and phosphate group
This option incorrectly mixes the sugars found in DNA (deoxyribose) and RNA (ribose). A single DNA polynucleotide chain does not contain both types of sugar. - Option 4: A nitrogenous base and phosphate group only
This option is incomplete. While a nitrogenous base and phosphate group are part of a DNA nucleotide, the deoxyribose sugar is missing. The sugar is essential for linking the base to the phosphate and for connecting nucleotides together.
Based on the analysis, Option 1 correctly identifies the fundamental components found in the repeating unit (nucleotide) that constitutes a DNA polynucleotide chain.
| Component Type | Found in DNA Nucleotide | Found in RNA Nucleotide |
|---|---|---|
| Nitrogenous Base | Adenine (A), Guanine (G), Cytosine (C), Thymine (T) | Adenine (A), Guanine (G), Cytosine (C), Uracil (U) |
| Sugar | Deoxyribose | Ribose |
| Phosphate Group | Yes (\(\text{PO}_4^{3-}\)) | Yes (\(\text{PO}_4^{3-}\)) |
Conclusion
The polynucleotide chain of DNA is built from repeating units, each containing a nitrogenous base, a deoxyribose sugar, and a phosphate group. These components together form a DNA nucleotide.
Revision Table: DNA Polynucleotide Chain Components
| Component | Description | Significance |
|---|---|---|
| Nitrogenous Base | A, T, C, or G | Carries the genetic code information |
| Deoxyribose Sugar | Five-carbon sugar (lacking oxygen at 2') | Part of the backbone, links base to phosphate |
| Phosphate Group | \(\text{PO}_4^{3-}\) | Part of the backbone, links nucleotides together |
| Nucleotide | Base + Sugar + Phosphate | Basic building block of DNA |
| Polynucleotide Chain | Many nucleotides linked by phosphodiester bonds | Forms a single strand of DNA |
Additional Information: DNA and RNA Comparison
While DNA contains deoxyribose sugar and the base Thymine (T), RNA contains ribose sugar and the base Uracil (U) instead of Thymine. Both are polynucleotides, but they have different sugars and one different base, leading to differences in structure and function. DNA typically exists as a double helix, while RNA is usually single-stranded. The phosphate group is present in both DNA and RNA nucleotides and plays the same structural role in linking the sugar molecules.
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