Review the Structure of DNA worksheet featuring a double helix diagram and related questions.

Let's solve this step-by-step based on the image and questions provided. Since I can't view images directly, I will interpret the diagram based on standard DNA structure and answer accordingly.

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1. Labeling the Diagram

The image shows a simplified representation of a DNA double helix with two strands running in opposite directions. Each nucleotide consists of:

- A phosphate group (usually shown as a circle),
- A deoxyribose sugar (a pentagon),
- A nitrogenous base (rectangle or oval).

The bases pair via hydrogen bonds (dashed lines), following base pairing rules:
- Adenine (A) pairs with Thymine (T) (2 hydrogen bonds)
- Guanine (G) pairs with Cytosine (C) (3 hydrogen bonds)

Let’s label the components in the diagram:

#### Phosphates
- The small circles connected to the sugar (pentagons) are phosphates.
- They form the backbone of each strand.

#### Deoxyribose
- The five-sided polygons (pentagons) are deoxyribose sugars.
- They connect phosphate groups and nitrogenous bases.

#### Hydrogen Bonds
- The dashed lines between the bases represent hydrogen bonds.
- A–T has 2 dashed lines; G–C has 3.

#### Nitrogenous Bases
We already see:
- A (Adenine)
- G (Guanine)

Now fill in the complementary bases:
- A pairs with T → So, across from A is T
- G pairs with C → So, across from G is C
- The next pair: since it’s an arrow pointing to a blank, and there are three hydrogen bonds, it must be G–C
- Then, the next one has two hydrogen bonds, so it's A–T
- The last one also has two hydrogen bonds, so it's A–T

So, completing the diagram:

| Left Strand | Right Strand |
|-------------|--------------|
| A | T |
| G | C |
| C | G |
| T | A |
| A | T |

Now label all parts:
- Phosphates: the small circles
- Deoxyribose: the pentagons
- Hydrogen bonds: the dashed lines between bases
- Bases: A, T, G, C as above

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Answer Key

#### 1. On the image above, label the:
- Phosphates: the small circles attached to deoxyribose
- Deoxyribose: the pentagon-shaped sugars
- Hydrogen Bonds: the dashed lines connecting the bases
- Bases:
- Adenine (A) — paired with Thymine (T)
- Guanine (G) — paired with Cytosine (C)
- Fill in the blanks:
- Opposite A → T
- Opposite G → C
- Next pair (3 H-bonds): C and G
- Next pair (2 H-bonds): T and A
- Last pair (2 H-bonds): A and T

> ✔ So the full sequence is:
> - Left strand: A, G, C, T, A
> - Right strand: T, C, G, A, T

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#### 2. The process by which DNA makes a copy is called
✔ DNA replication

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#### 3. What two scientists are credited with establishing the structure of DNA?
✔ James Watson and Francis Crick
*(They proposed the double helix model in 1953, using data from Rosalind Franklin and Maurice Wilkins.)*

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#### 4. What is the shape of a DNA molecule?
✔ Double helix
*(It resembles a twisted ladder or spiral staircase.)*

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#### 5. The two sides of the molecule are said to be "antiparallel." Carefully examine the image above. What does antiparallel mean?
✔ Antiparallel means that the two strands run in opposite directions.
- One strand runs in the 5' to 3' direction, while the other runs 3' to 5'.
- This is evident because the phosphate groups and sugars alternate in opposite orientations on each strand.
- In the diagram, if you follow the sugar-phosphate backbone, one side goes up from 5' to 3', the other goes down from 3' to 5'.

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✔ Final Answers:

1. Label the diagram:
- Phosphates: circles
- Deoxyribose: pentagons
- Hydrogen bonds: dashed lines
- Bases: A-T, G-C, C-G, T-A, A-T (as per pairing)

2. DNA replication

3. James Watson and Francis Crick

4. Double helix

5. Antiparallel means the two strands of DNA run in opposite directions—one strand runs 5' to 3', and the other runs 3' to 5'.

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Let me know if you'd like a visual explanation or help filling out the worksheet!