To determine the composition of DNA, we need to analyze the components that make up its structure. DNA, or deoxyribonucleic acid, is a polynucleotide composed of several key elements. Here’s a step-by-step breakdown of the composition of DNA:
### Step 1: Identify the Components of DNA
DNA consists of three main components:
1. **Deoxyribose Sugar**: This is a five-carbon sugar (pentose) that is a crucial part of the DNA structure.
2. **Phosphate Group**: This group links the sugar molecules together, forming the backbone of the DNA strand.
3. **Nitrogenous Bases**: These are the building blocks of the genetic code. The four bases in DNA are:
- Adenine (A)
- Thymine (T)
- Cytosine (C)
- Guanine (G)
### Step 2: Understand the Structure of DNA
DNA is structured as a double helix, where two strands run in opposite directions. The sugar and phosphate groups form the backbone, while the nitrogenous bases pair up in the center:
- Adenine pairs with Thymine (A-T)
- Cytosine pairs with Guanine (C-G)
### Step 3: Eliminate Incorrect Options
Given the options:
- **Option A**: Pentose, phosphoric acid, and adenine, guanine, uracil, thymine.
- **Option B**: Aldopentose, phosphoric acid, adenine, guanine, uracil, thymine.
- **Option C**: Ketopentose, phosphoric acid.
- **Option D**: Aldopentose, phosphoric acid, adenine, guanine.
We can eliminate options A and B because they include uracil, which is not found in DNA (it is found in RNA).
### Step 4: Determine the Correct Option
Now we are left with options C and D:
- **Option C** mentions ketopentose, which is incorrect because DNA contains aldopentose (deoxyribose).
- **Option D** correctly states aldopentose (deoxyribose), phosphoric acid, and includes adenine and guanine.
### Conclusion
The correct answer regarding the composition of DNA is:
**Option D: Aldopentose, phosphoric acid, adenine, guanine.**
