Completed answer key for a DNA structure worksheet, featuring handwritten responses and labeled diagrams of the double helix, nucleotides, and base pairs.

Let’s go step by step through each question to make sure we get the right answers.

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

- Nucleotide: Circle one full unit that includes a phosphate (P), sugar (S), and base (A, T, G, or C). In the diagram, there’s already a box around one nucleotide — that’s correct.
- Sugar and phosphate molecules: Label “S” for deoxyribose sugar and “P” for phosphate group — they alternate in the backbone.
- Bases not already labeled: The bases are A, T, G, C. If any are missing labels, add them. In the diagram, most are labeled, but check if all are marked.
- Base pair: Draw a line or box around two complementary bases connected by hydrogen bonds — like A-T or G-C. There’s already a label “BASE PAIR” pointing to A=T and G≡C.
- Sugar-phosphate backbones: These are the outer rails of the ladder — label both sides as “sugar-phosphate backbone”.
- Hydrogen bonds: These are the dotted lines between base pairs — label them “hydrogen bonds”. Also note: A-T has 2 H-bonds, G-C has 3.

✔ All these are correctly shown or described in the answer key.

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Question 2:
“A nucleotide is made of three parts: a ______ group, a five carbon ______, and a ______ base.”

→ Phosphate group
→ Sugar (deoxyribose)
→ Nitrogenous base

✔️ Correctly filled in.

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Question 3:
“In a single strand of DNA, the phosphate group binds to the ______ of the next group.”

→ It binds to the sugar of the next nucleotide. This forms the sugar-phosphate backbone.

✔️ Answer: sugar

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Question 4:
“Chargaff’s rule states that the DNA of any species contains equal amounts of ______ & ______ and also equal amounts of ______ & ______.”

→ Adenine (A) = Thymine (T)
→ Guanine (G) = Cytosine (C)

✔️ Answers: adenine (A) & thymine (T); guanine (G) & cytosine (C)

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Question 5:
“In DNA, thymine is complementary to (or pairs with) ______; cytosine is complementary to ______.”

→ Thymine pairs with adenine (A)
→ Cytosine pairs with guanine (G)

✔️ Correct.

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Question 6:
“If the percentage of thymine is 30%, what would the percentage of cytosine in the same DNA strand be?”

Step-by-step:

- In double-stranded DNA, A = T and G = C (Chargaff’s rules).
- So if T = 30%, then A = 30% → total A + T = 60%
- That leaves 40% for G + C
- Since G = C, each must be half of 40% → 20%

But wait — the question says: “in the same DNA strand” — not the whole molecule.

⚠️ Important: Chargaff’s rules apply to double-stranded DNA, not necessarily to a single strand.

However, in many school-level problems, when they say “a strand of DNA” and give % of one base, they often mean the whole double helix — unless specified otherwise.

Looking at the handwritten answer: it says “20%”, and adds “30% A, 20% G, 20% C” — which implies they’re treating it as double-stranded.

Also, the problem likely intends for us to use Chargaff’s rules assuming double-stranded context.

So:

T = 30% → A = 30% → A+T = 60% → G+C = 40% → C = 20%

✔️ Final answer: 20%

*(Note: If this were truly a single strand, we couldn’t determine C from just T — but given the context and expected answer, 20% is correct for standard curriculum.)*

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Question 7:
“James Watson and Francis Crick... determined that the shape of the DNA molecule was a ______.”

→ Double helix

✔️ Correct.

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Question 8:
“Why do purines pair with pyrimidines?”

Purines (A and G) are larger, two-ring structures. Pyrimidines (T and C) are smaller, one-ring structures.

They pair this way so that the width of the DNA helix stays constant — A always pairs with T, G with C — because their shapes fit together perfectly via hydrogen bonding.

Handwritten answer: “Because of their shape, adenine can only bond with thymine and cytosine only with guanine”

That’s acceptable, though slightly incomplete. Better phrasing: Purines pair with pyrimidines to maintain uniform width of the DNA ladder and allow proper hydrogen bonding.

But since the student wrote a reasonable answer referencing shape and specific pairing, and it matches textbook logic — we’ll accept it.

✔️ Acceptable answer.

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Question 9:
“What type of bonds connect the deoxyribose sugars to the phosphate groups?”

→ These are covalent bonds, specifically called phosphodiester bonds (which are a type of covalent ester bond).

Student wrote: “Covalent ester bond” — close enough for high school level.

More precise term: phosphodiester bond — but “covalent bond” is fundamentally correct.

✔️ We’ll go with covalent bonds (or phosphodiester bonds if being precise). Student’s answer is acceptable.

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Question 10:
“What type of bonds connect the bases to each other?”

→ Hydrogen bonds

And student added:
“2 hydrogen bonds connect A and T
3 hydrogen bonds connect G and C”

✔️ Perfect — accurate and complete.

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Now, compiling final answers clearly:

Final Answer:
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 of their shape, adenine can only bond with thymine and cytosine only with guanine (to maintain consistent width and proper bonding)
9. covalent bonds (specifically phosphodiester bonds)
10. hydrogen bonds (2 between A-T, 3 between G-C)