Topoisomer

Topoisomer

Diagram of DNA topoisomers

A topoisomer is a type of isomer that differs in the topology of its molecular structure. Topoisomers are particularly important in the study of DNA, where they refer to different forms of DNA that have the same sequence but differ in their supercoiling or linking number.

DNA Topoisomers

DNA topoisomers are crucial in the context of DNA replication, transcription, and chromosome segregation. The double helix structure of DNA can become supercoiled, and the degree of supercoiling can affect the biological processes that involve DNA.

Types of DNA Topoisomers

DNA topoisomers can be classified based on their linking number, which is the number of times one strand of DNA winds around the other. The linking number is a topological property that remains constant unless the DNA is cut and rejoined.

• Relaxed DNA: This form of DNA has no supercoiling and is in its most stable state.
• Supercoiled DNA: This form of DNA is twisted upon itself, which can be either positive or negative supercoiling.

Enzymes Involved

Topoisomerases are enzymes that play a critical role in managing DNA topoisomers. They can cut one or both strands of DNA, allowing the DNA to be untangled or unwound, and then rejoin the strands.

• Type I Topoisomerases: These enzymes cut one strand of DNA and allow it to rotate around the other strand, changing the linking number by increments of one.
• Type II Topoisomerases: These enzymes cut both strands of DNA and pass another segment of the double helix through the break, changing the linking number by increments of two.

Biological Significance

The regulation of DNA topology is essential for maintaining the integrity of the genome during cell division and for the proper functioning of gene expression. Supercoiling can affect the accessibility of DNA to RNA polymerase and other transcription factors, influencing the rate of transcription.

Related pages

• DNA replication
• Chromosome
• Enzyme
• Isomer
• Supercoiling