Supercoiling of DNA can occur (choose all that apply):
A. when double stranded DNA is overwound
B. when double stranded DNA is underwound
C. when double stranded DNA is heated above it's Tm
D. when DNA is treated with nucleases
and
Generally, cellular DNA is:
A. Slightly underwound
B. Slightly overwound
C. Chemically unstable in basic solutions
D.Single stranded
For the first question:
The process of supercoiling occurs when the double-stranded DNA molecule undergoes changes in its structure due to changes in the number of turns or twists. Double-stranded DNA can be either overwound or underwound to achieve supercoiling.
A. When double-stranded DNA is overwound: This means that additional turns or twists are introduced into the DNA molecule, resulting in positive supercoiling.
B. When double-stranded DNA is underwound: This indicates that the DNA molecule has fewer turns or twists than its relaxed state, resulting in negative supercoiling.
C. When double-stranded DNA is heated above its Tm: This statement is incorrect. Supercoiling does not occur when DNA is heated above its melting temperature (Tm). At high temperatures, DNA denatures, separating into single strands.
D. When DNA is treated with nucleases: This statement is incorrect. Nucleases are enzymes that degrade DNA, but they do not directly cause supercoiling.
Therefore, the correct answers to the first question are A. when double stranded DNA is overwound and B. when double stranded DNA is underwound.
For the second question:
A. Slightly underwound: The statement that cellular DNA is slightly underwound is correct. In most cells, the DNA in the nucleus is slightly underwound, exhibiting negative supercoiling. This underwinding helps to facilitate various DNA processes, such as replication and transcription.
B. Slightly overwound: This statement is incorrect. Cellular DNA is usually slightly underwound, not overwound.
C. Chemically unstable in basic solutions: This statement is incorrect. DNA is generally chemically stable in basic solutions unless exposed to extreme pH conditions.
D. Single stranded: This statement is also incorrect. Cellular DNA exists as a double-stranded helix, not as single-stranded.
Therefore, the correct answer to the second question is A. Slightly underwound.