Completed answer key for a DNA structure worksheet, detailing nucleotide components, base pairing rules, and hydrogen bonding in DNA.

Let’s go through each question one by one and solve them carefully.

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Question 1:
This is a diagram labeling task. Since we can’t draw here, we’ll describe what should be labeled based on standard DNA structure:

- Nucleotide: A single unit made of phosphate + sugar + base. In the diagram, it’s shown as a box around one P (phosphate), one S (sugar), and one base (like A, T, G, or C).
- Sugar and phosphate molecules: Sugar is usually drawn as a pentagon (labeled “S”), phosphate as a circle or square (labeled “P”).
- Bases not already labeled: The bases are A, T, G, C. If any are missing labels, fill them in using complementary pairing: A with T, G with C.
- Base pair: Two bases connected by dotted lines (hydrogen bonds) — e.g., A-T or G-C.
- Sugar-phosphate backbones: The two outer rails of the ladder — alternating P and S going up each side.
- Hydrogen bonds: Dotted lines between base pairs (2 for A-T, 3 for G-C).

*(Since this is visual, student should refer to their diagram and label accordingly.)*

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Question 2:
A nucleotide is made of three parts:
→ a phosphate group,
→ a five-carbon sugar (deoxyribose in DNA),
→ and a nitrogenous base (A, T, G, or C).

✔ Final Answer for Q2: phosphate, sugar, nitrogenous

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Question 3:
In a single strand of DNA, the phosphate group binds to the sugar of the next nucleotide. This forms the backbone.

✔ Final Answer for Q3: sugar

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Question 4:
Chargaff’s rule says:
Amount of adenine (A) = amount of thymine (T)
Amount of guanine (G) = amount of cytosine (C)

✔ Final Answer for Q4: adenine (A) & thymine (T); guanine (G) & cytosine (C)

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Question 5:
In DNA:
Thymine (T) pairs with → adenine (A)
Cytosine (C) pairs with → guanine (G)

✔ Final Answer for Q5: adenine (A); guanine (G)

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Question 6:
If thymine = 30%, then adenine = 30% (because A=T).
So A + T = 60%.
That leaves 40% for G + C.
Since G = C, each is half of 40% → 20%

✔ Final Answer for Q6: 20%

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Question 7:
Watson, Crick, Franklin, etc. discovered DNA’s shape is a double helix.

✔ Final Answer for Q7: double helix

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Question 8:
Why do purines pair with pyrimidines?
Because purines (A and G) are bigger (two rings), pyrimidines (T and C) are smaller (one ring). Pairing one big with one small keeps the DNA width constant. Also, hydrogen bonding only works properly between specific pairs: A-T and G-C.

✔ Final Answer for Q8: Because purines are larger and pyrimidines are smaller; pairing them keeps the DNA strand uniform width and allows proper hydrogen bonding.

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Question 9:
The bond connecting deoxyribose sugar to phosphate group is a covalent bond, specifically called a phosphodiester bond.

✔ Final Answer for Q9: covalent ester bond (or phosphodiester bond)

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Question 10:
Bases connect to each other via hydrogen bonds.
- A and T form 2 hydrogen bonds
- G and C form 3 hydrogen bonds

✔ Final Answer for Q10: hydrogen bonds; 2 for A-T, 3 for G-C

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Final Answer:
Here are all the answers clearly listed:

1. (Diagram labeling – see description above)
2. phosphate, sugar, nitrogenous
3. sugar
4. adenine (A) & thymine (T); guanine (G) & cytosine (C)
5. adenine (A); guanine (G)
6. 20%
7. double helix
8. Because purines are larger and pyrimidines are smaller; pairing them keeps the DNA strand uniform width and allows proper hydrogen bonding.
9. covalent ester bond (or phosphodiester bond)
10. hydrogen bonds; 2 for A-T, 3 for G-C