Problem 4 What are the topological propert... [FREE SOLUTION]

Chapter 3: Problem 4

What are the topological properties of DNA, and how are they related to each other? Which of these can be changed without cleaving the DNA backbone?

Short Answer

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Answer: The three main topological properties of DNA are linking number (Lk), twist (Tw), and writhe (Wr). They are related through the equation Lk = Tw + Wr. To change these properties without cleaving the DNA backbone, enzymes such as topoisomerases can introduce or remove supercoils, altering the linking number. The twist and writhe can be changed by proteins like histones that bind to DNA and cause it to bend or unwind.

Step by step solution

1. Introduction to DNA structure

DNA (Deoxyribonucleic acid) is a double helical structure made of two complementary strands. The strands are connected by hydrogen bonds between nucleotide pairs (Adenine-Thymine and Guanine-Cytosine). The DNA molecule is negatively charged due to the phosphate groups on its backbone.

2. Topological properties of DNA

There are three main topological properties of DNA: linking number (Lk), twist (Tw), and writhe (Wr). These three properties are related through the equation: Lk = Tw + Wr.

3. Linking number (Lk)

Linking number is the number of times one DNA strand wraps around the other strand in the double helix. It is an integer value that represents the number of turns of the helix and provides information about the global structure of the DNA molecule.

4. Twist (Tw)

Twist is the number of helical turns in the DNA double helix. It measures the tightness of the winding of the two DNA strands around each other. Generally, there are about 10.5 base pairs per helical turn in B-DNA, which is the most common form of DNA found in cells.

5. Writhe (Wr)

Writhe is the number of times the double helix itself crosses over. This property measures the degree of supercoiling present in the DNA molecule. A DNA molecule can have positive or negative supercoiling, depending on the direction of the turns.

6. Changing topological properties without cleaving the DNA backbone

The linking number (Lk) can be changed without cleaving the DNA backbone by introducing or removing supercoils via enzymes such as topoisomerases. These enzymes can either cut one or both DNA strands, allow the passage of another strand through the break, and then reseal the break, thus altering the linking number. The twist (Tw) and the writhe (Wr) can be changed by proteins that bind to DNA and cause it to bend or unwind, such as histones that help package DNA into chromatin.

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91影视

What are the topological properties of DNA, and how are they related to each other? Which of these can be changed without cleaving the DNA backbone?

Short Answer

Step by step solution

1. Introduction to DNA structure

2. Topological properties of DNA

3. Linking number (Lk)

4. Twist (Tw)

5. Writhe (Wr)

6. Changing topological properties without cleaving the DNA backbone

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