DNA Structure Worksheet showing the double helix and related questions.

Problem Description:

The image provided is a worksheet about DNA structure. The task involves identifying and explaining the components of DNA, as well as answering questions related to its structure. Below is a detailed explanation and solution for each part of the worksheet.

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Worksheet Analysis and Solution:

#### 1. Label the parts of the DNA molecule shown below.
The diagram shows a double helix structure of DNA. Here are the labels for the parts:

- A: Phosphate group
- B: Sugar (deoxyribose)
- C: Nitrogenous base
- D: Hydrogen bond
- E: Backbone (phosphate-sugar backbone)
- F: Base pair

#### Explanation:
- Phosphate group (A): Located at the top of each "rung" in the ladder-like structure. It forms part of the backbone along with the sugar.
- Sugar (B): Specifically deoxyribose, which connects to the phosphate group and the nitrogenous base.
- Nitrogenous base (C): Forms the "rungs" of the DNA ladder. There are four types: adenine (A), thymine (T), cytosine (C), and guanine (G).
- Hydrogen bond (D): Bonds between complementary nitrogenous bases (A-T and C-G) that hold the two strands together.
- Backbone (E): Composed of alternating phosphate groups and sugars, running along the outside of the double helix.
- Base pair (F): A pair of nitrogenous bases held together by hydrogen bonds (e.g., A-T or C-G).

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#### 2. How many different kinds of nitrogenous bases are present in DNA? Name them using their one-letter abbreviations.
- Number of nitrogenous bases: 4
- One-letter abbreviations: A, T, C, G

#### Explanation:
DNA contains four types of nitrogenous bases:
1. Adenine (A): Purine base
2. Thymine (T): Pyrimidine base
3. Cytosine (C): Pyrimidine base
4. Guanine (G): Purine base

These bases are essential for encoding genetic information.

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#### 3. What is the complementary base pairing rule for DNA?
- Complementary base pairing rule:
- Adenine (A) pairs with Thymine (T) via two hydrogen bonds.
- Cytosine (C) pairs with Guanine (G) via three hydrogen bonds.

#### Explanation:
The complementary base pairing ensures that the two strands of DNA are held together stably. The specific pairing is as follows:
- A-T: Two hydrogen bonds (weaker but stable enough).
- C-G: Three hydrogen bonds (stronger, providing additional stability).

This rule is crucial for DNA replication and transcription processes.

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#### 4. Write out the complementary sequences for the following DNA strands:
- Strand 1: ATCGTAGCTAG
- Strand 2: CGATCGATCGA

#### Solution:
To find the complementary strand, replace each base with its complement:
- A → T, T → A, C → G, G → C

- For Strand 1 (ATCGTAGCTAG):
- Replace each base:
- A → T
- T → A
- C → G
- G → C
- T → A
- A → T
- G → C
- C → G
- T → A
- A → T
- G → C
- Complementary strand: TAGCATCGATC

- For Strand 2 (CGATCGATCGA):
- Replace each base:
- C → G
- G → C
- A → T
- T → A
- C → G
- G → C
- A → T
- T → A
- C → G
- G → C
- A → T
- Complementary strand: GCTAGCTAGCT

#### Final Answer for Question 4:
- Complementary strand for Strand 1: TAGCATCGATC
- Complementary strand for Strand 2: GCTAGCTAGCT

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Summary of Answers:

1. Labels:
- A: Phosphate group
- B: Sugar (deoxyribose)
- C: Nitrogenous base
- D: Hydrogen bond
- E: Backbone (phosphate-sugar backbone)
- F: Base pair

2. Number of nitrogenous bases: 4
- One-letter abbreviations: A, T, C, G

3. Complementary base pairing rule:
- A-T (2 hydrogen bonds)
- C-G (3 hydrogen bonds)

4. Complementary sequences:
- Strand 1: TAGCATCGATC
- Strand 2: GCTAGCTAGCT

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Final Boxed Answers:

1. Labels:
- A: Phosphate group
- B: Sugar (deoxyribose)
- C: Nitrogenous base
- D: Hydrogen bond
- E: Backbone (phosphate-sugar backbone)
- F: Base pair

2. Number of nitrogenous bases: 4
- One-letter abbreviations: A, T, C, G

3. Complementary base pairing rule:
- A-T (2 hydrogen bonds)
- C-G (3 hydrogen bonds)

4. Complementary sequences:
- Strand 1: \boxed{\text{TAGCATCGATC}}
- Strand 2: \boxed{\text{GCTAGCTAGCT}}