Consider a short double-stranded linear DNA molecule of 100 complete t...
For a relaxed circular plasmid the linking number is always equal to the twist (total no. of complete helix turns). So the linking number for the circular DNA is 100.
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Consider a short double-stranded linear DNA molecule of 100 complete t...
Explanation:
The linking number is defined as the number of times one strand of a closed DNA molecule winds around the other. It is an important topological property of DNA and is conserved during most DNA metabolic processes.
In a short double-stranded linear DNA molecule of 100 complete turns with 10.5bp/turn, the total number of base pairs will be:
Total number of base pairs = Number of turns x Base pairs per turn
= 100 x 10.5
= 1050 bp
When the ends of the DNA molecule are sealed together to make a relaxed circle, the linking number of the circle can be calculated using the following formula:
Linking number = Twist + Writhe
where Twist is the number of times the DNA strands cross each other in a right-handed or left-handed manner, and Writhe is the number of times the DNA strands cross each other in a circular or sigmoidal manner.
In a relaxed circle, the Twist is equal to the number of complete turns in the DNA molecule, which is 100 in this case. The Writhe is zero because the DNA strands do not cross each other in a circular or sigmoidal manner.
Therefore, the linking number of the relaxed circle will be:
Linking number = Twist + Writhe
= 100 + 0
= 100
Hence, the correct answer is '100'.