DNA Structure: The Molecule of Heredity - Diagram showing the double helix with labeled parts such as nucleotides, sugar-phosphate backbone, base pairs, and hydrogen bonds.

Let’s go through each question one by one, step by step. I’ll explain clearly so you understand why each answer is correct.

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

A nucleotide is the building block of DNA. Think of it like a Lego piece with 3 parts:
- One part is called a phosphate group (it helps link nucleotides together).
- The middle part is a sugar — in DNA, it’s always a five-carbon sugar called deoxyribose.
- The third part is a nitrogenous base — that’s the letter part (A, T, C, or G).

✔ So the answers are:
phosphate, sugar, nitrogenous

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

In a DNA strand, nucleotides are linked like beads on a string. The phosphate of one nucleotide connects to the sugar of the next nucleotide. Specifically, it binds to the 3’ carbon of the sugar in the next nucleotide — but for this level, we just say “sugar”.

✔ 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 ______________ & ______________.

Chargaff discovered that in DNA:
- Amount of Adenine (A) = Amount of Thymine (T)
- Amount of Guanine (G) = Amount of Cytosine (C)

This is because A pairs with T, and G pairs with C.

✔ Answers:
Adenine & Thymine, Guanine & Cytosine

*(You can write them in either order as long as the pairs match.)*

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

Base pairing rules:
- Thymine (T) always pairs with Adenine (A)
- Cytosine (C) always pairs with Guanine (G)

✔ Answers:
adenine, guanine

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Question 6: In a strand of DNA, if the percentage of thymine is 30%, what would the percentage of cytosine in the same DNA strand be?

Use Chargaff’s Rule again!

If T = 30%, then A must also = 30% (because A=T).

So A + T = 30% + 30% = 60%

That leaves 100% - 60% = 40% for G + C.

Since G = C, divide 40% by 2 → 20% each.

✔ So cytosine = 20%

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Question 7: James Watson and Francis Crick with the help of Rosalind Franklin and others, determined that the shape of the DNA molecule was a ______________.

They figured out DNA has a twisted ladder shape — called a double helix.

✔ Answer: double helix

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Question 8: Why do purines pair with pyrimidines?

Purines (A and G) are bigger molecules. Pyrimidines (T and C) are smaller. If two purines paired, they’d be too big to fit inside the DNA ladder. If two pyrimidines paired, they’d be too small. Pairing one big with one small keeps the width of the DNA constant — like having matching rungs on a ladder.

✔ Simple answer: To keep the DNA strand the same width all along.

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Question 9: A ______________ is a segment of DNA that determines all traits.

This is asking for the basic unit of heredity — a gene. Genes are sections of DNA that code for specific traits (like eye color, height, etc.).

✔ Answer: gene

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

The bases (A-T, G-C) are held together by weak bonds called hydrogen bonds. These allow the DNA strands to separate easily when needed (like during copying).

✔ Answer: hydrogen bonds

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Final Answer:

2. phosphate, sugar, nitrogenous
3. sugar
4. adenine & thymine, guanine & cytosine
5. adenine, guanine
6. 20%
7. double helix
8. To keep the DNA strand the same width all along.
9. gene
10. hydrogen bonds