Question

question 20
negative dna supercoiling ______ while positive dna supercoiling ______.

can be found in most cells; prevents dna annealing at low temperatures
is not found in most cells; prevents dna melting at high temperatures
results in double stranded breaks; involves a singular protein
can be found in most cells; helps prevent dna melting at high temperatures

Explanation:

Define DNA supercoiling types

DNA supercoiling refers to the over- or under-winding of a DNA strand. Negative supercoiling is the under-winding of the double helix, which introduces torsional strain that facilitates the unwinding of the double helix. Positive supercoiling is the over-winding of the double helix, which packs the base pairs more tightly together.

Analyze negative supercoiling distribution

Negative supercoiling is the predominant state of DNA found in almost all living cells (both prokaryotes and eukaryotes). Because it under-winds the DNA, it makes strand separation easier, which is essential for replication and transcription processes.

Analyze positive supercoiling function

Positive supercoiling is rare and is primarily found in thermophilic and hyperthermophilic archaea and bacteria. The tight over-winding of the DNA double helix increases its thermal stability, helping to prevent DNA denaturation (melting) at extremely high environmental temperatures.

Evaluate the given options

• Option 1: "can be found in most cells; prevents DNA annealing at low temperatures" — Incorrect, positive supercoiling is not related to preventing low-temperature annealing.
• Option 2: "is not found in most cells; prevents DNA melting at high temperatures" — Incorrect, negative supercoiling is found in most cells.
• Option 3: "results in double stranded breaks; involves a singular protein" — Incorrect, supercoiling itself does not inherently result in double-stranded breaks.
• Option 4: "can be found in most cells; helps prevent DNA melting at high temperatures" — Correct, as negative supercoiling is universal in most cells, and positive supercoiling stabilizes DNA against thermal denaturation in thermophiles.

Answer:

• can be found in most cells; prevents DNA annealing at low temperatures
• is not found in most cells; prevents DNA melting at high temperatures
• results in double stranded breaks; involves a singular protein
• can be found in most cells; helps prevent DNA melting at high temperatures (Correct answer)