DNA base pairing and structure practice exercise with labeled parts for learning.

Let's solve this step-by-step based on the image you've described, which is a DNA structure identification practice worksheet.

We are given a diagram of a DNA segment with labeled parts (1 through 12), and we need to:

1. Fill in the base pairing rules.
2. Identify the labeled parts of the DNA structure.
3. Understand the structure of nitrogenous bases, especially purines and pyrimidines.

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🔹 Part 1: Base Pairing Rules

These are standard rules in DNA:

a. A pairs with T with 2 hydrogen bonds
b. G pairs with C with 3 hydrogen bonds
c. A and G are purines made of a double ring structure
d. T and C are pyrimidines made of a single ring structure

✔ So fill in:
- a. A pairs with T with 2 hydrogen bonds
- b. G pairs with C with 3 hydrogen bonds
- c. A and G are purines...
- d. T and C are pyrimidines...

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🔹 Part 2: Label Identification (Numbers 1–12)

Now let’s analyze the diagram based on typical DNA structure:

In DNA, each nucleotide consists of:
- Phosphate group
- Deoxyribose sugar
- Nitrogenous base

The strand runs from 5' end to 3' end.

Looking at the diagram:

#### Let's identify each label:

1. Phosphate group – attached to the 5' carbon of deoxyribose
2. Deoxyribose sugar – the five-carbon sugar (pentose)
3. Nitrogenous base – the base attached to the sugar (in this case, likely cytosine or thymine)
4. Hydrogen bond – between complementary bases (e.g., between G and C)
5. Nitrogenous base – possibly guanine (since it has a double ring)
6. Phosphate group – part of the backbone
7. Nitrogenous base – likely adenine (double ring, purine)
8. Deoxyribose sugar – connected to adenine
9. Nitrogenous base – likely thymine (single ring, pyrimidine)
10. Deoxyribose sugar – connected to thymine
11. 5' end – phosphate group at the start of the strand
12. 3' end – hydroxyl (-OH) group at the end of the strand

Let’s go number by number:

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✔ Final Answers:

#### Base Pairing Rules:

a. A pairs with T with 2 hydrogen bonds
b. G pairs with C with 3 hydrogen bonds
c. A and G are purines made of a double ring structure
d. T and C are pyrimidines made of a single ring structure

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#### Label Identification:

1. Phosphate group
2. Deoxyribose sugar
3. Nitrogenous base (likely cytosine)
4. Hydrogen bond
5. Nitrogenous base (likely guanine)
6. Phosphate group
7. Nitrogenous base (likely adenine)
8. Deoxyribose sugar
9. Nitrogenous base (likely thymine)
10. Deoxyribose sugar
11. 5' end
12. 3' end

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🔍 Explanation:

- Purines (A & G) have two rings (imidazole + pyrimidine).
- Pyrimidines (T & C) have one ring.
- Hydrogen bonds hold complementary bases together:
- A-T: 2 H-bonds
- G-C: 3 H-bonds
- The sugar-phosphate backbone forms the sides of the helix.
- The 5' end has a free phosphate group; the 3' end has a free hydroxyl (-OH) group.
- The strands are antiparallel: one goes 5' → 3', the other 3' → 5'.

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✔ This completes the identification of DNA structure and base pairing rules. Let me know if you'd like a visual breakdown!