Biology worksheet on DNA and RNA review, featuring questions and a diagram illustrating DNA replication with base pairing and strand labeling.

Let's go through the Biology: DNA and RNA Review KEY worksheet step by step, solving each question and explaining the answers clearly.

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DNA Basic Structure

#### 1. How many nucleotides are shown in the DNA segment pictured?
- Answer: 4
- Explanation: Each nucleotide consists of a sugar (deoxyribose), a phosphate group, and a nitrogenous base. In the diagram, there are four distinct units connected together — one for each base: A, T, C, G. So, there are four nucleotides.

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#### 2. Circle an entire nucleotide on the DNA segment.
- Answer: You should circle one complete unit that includes:
- One phosphate group (P)
- One sugar (S) (deoxyribose)
- One nitrogenous base (A, T, C, or G)
- For example, you could circle the top nucleotide (with base A), including the P, S, and A.
- Note: This is a visual task; in your answer key, it should be circled appropriately.

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#### 3. Name the three parts of a DNA nucleotide.
a. Sugar
b. Phosphate
c. Nitrogen base

✔ These are the three components of a DNA nucleotide.

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#### 4. Use the letters “P” (phosphate) and “S” (sugar) to label the sugar and phosphates of the DNA molecule to the right.
- The diagram shows a strand with bases A-T-C-G from top to bottom.
- The phosphates (P) connect the sugars in the backbone.
- The sugars (S) are attached to the bases and link to phosphates.
- Labeling:
- Place P at the phosphate groups (on the left side of each sugar).
- Place S at the sugar molecules (between the phosphate and base).
- Example labeling:
```
P S P S P S P S
| | | | | | | |
A T C G A T C G
```
- But since only one strand is shown, label accordingly: P → S → P → S → etc., along the backbone.

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#### 5. Which part does the phosphate molecule connect with?
- Answer: Sugar
- Explanation: The phosphate group connects to the sugar of the adjacent nucleotide, forming the phosphodiester bond in the sugar-phosphate backbone of DNA.

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DNA Replication

> *The diagram below shows DNA replication.*

We have two strands labeled Area A and Area B, with sequences:

```
Area A: Area B:
A T A T
C G T A
G C C G
T A G C
A T A T
G C T A
C G C G
T A G C
```

An arrow shows DNA replication moving from left to right.

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#### 6. In area A, fill in the missing DNA bases from the parent helix.
- We need to complete the complementary strand in Area A.
- Remember: A pairs with T, C pairs with G.

Given:
```
Original strand (top): A C G T A G C T
Complementary strand (bottom): ? ? ? ? ? ? ? ?
```

So, the bottom strand must be:
- A → T
- C → G
- G → C
- T → A
- A → T
- G → C
- C → G
- T → A

Thus, the missing bases in Area A (bottom strand) are:
```
T G C A T C G A
```

✔ So fill in:
- First blank: T
- Second: G
- Third: C
- Fourth: A
- Fifth: T
- Sixth: C
- Seventh: G
- Eighth: A

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#### 7. Name the enzyme that will separate the DNA strands in Area A.
- Answer: Helicase
- Explanation: Helicase unwinds the double helix by breaking hydrogen bonds between complementary bases, creating the replication fork.

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#### 8. In area B, perform DNA replication and fill in the new strand of DNA.
- Area B shows the original template strand:
```
Original (top): A T G C A T C G
New strand (bottom): ? ? ? ? ? ? ? ?
```

We need to add the complementary strand:
- A → T
- T → A
- G → C
- C → G
- A → T
- T → A
- C → G
- G → C

So the new strand is:
```
T A C G T A G C
```

✔ Fill in the blanks in Area B:
- First: T
- Second: A
- Third: C
- Fourth: G
- Fifth: T
- Sixth: A
- Seventh: G
- Eighth: C

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#### 9. After DNA replication, name the two strands of DNA identical?
- Answer: No
- Explanation: After DNA replication, each DNA molecule has one old (parental) strand and one new strand. This is called semi-conservative replication. Therefore, the two DNA molecules are not identical because they contain different combinations of old and new strands.

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✔ Final Answers Summary:

| Question | Answer |
|--------|--------|
| 1 | 4 nucleotides |
| 2 | Circle one full nucleotide (P-S-base) |
| 3 | Sugar, Phosphate, Nitrogen base |
| 4 | Label P on phosphates, S on sugars |
| 5 | Sugar |
| 6 | Bottom strand in Area A: T G C A T C G A |
| 7 | Helicase |
| 8 | New strand in Area B: T A C G T A G C |
| 9 | No |

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