## NS FL4 B/B 53

erichsuh94
Posts: 13
Joined: Wed May 23, 2018 4:16 am

### NS FL4 B/B 53

I am having very difficult time visualizing the explanation of the solution.

As for the question, I understand that

Histone diameter: 11nm
Length of one nucleotide: 0.3nm

and that 200 basepairs coil around one histone.

So the length of unwound DNA (200 basepairs) which would be wrapped around a single histone is 0.3 nm X 200 = 60 nm.

But I am not quite clear where we are going with

" Once coiled around the protein, we can assume (without getting into unnecessary surface area calculations) that a single layer of DNA strand will be wrapped around the protein (strand thickness = 2 nm), meaning we now have the strand condensed to a ~ 4-nm layer (2 nm on each side) around the 11-nm-diameter protein. Thus, about 60 nm of DNA is replaced by 15 nm, meaning there is approximately a 45-nm decrease in length. 45/60 = 0.75 = a 75% decrease in length."

as in the explanation.

Please help!
NS_Tutor_Katelyn
Posts: 136
Joined: Wed May 30, 2018 11:51 am
Location: Minnesota
Contact:

### Re: NS FL4 B/B 53

erichsuh94 wrote:I am having very difficult time visualizing the explanation of the solution.

As for the question, I understand that

Histone diameter: 11nm
Length of one nucleotide: 0.3nm

and that 200 basepairs coil around one histone.

So the length of unwound DNA (200 basepairs) which would be wrapped around a single histone is 0.3 nm X 200 = 60 nm.

But I am not quite clear where we are going with

" Once coiled around the protein, we can assume (without getting into unnecessary surface area calculations) that a single layer of DNA strand will be wrapped around the protein (strand thickness = 2 nm), meaning we now have the strand condensed to a ~ 4-nm layer (2 nm on each side) around the 11-nm-diameter protein. Thus, about 60 nm of DNA is replaced by 15 nm, meaning there is approximately a 45-nm decrease in length. 45/60 = 0.75 = a 75% decrease in length."

as in the explanation.

Please help!

The original length of the DNA is 60 nm (0.3 x 200, as you said). After determining that, we have to figure out what the new length will be after it is wrapped around the histone. The histone has a diameter of 11 nm, as the passage tells us. The DNA has a diameter of 2 nm, and it wraps full around the histone once, so it adds 2 nm of diameter to each side of the histone. This makes the new diameter of the histone 15 nm (11 + 2 + 2). The question asked by what fraction is was shortened. 15/60 is 0.25, so it was shortened by 0.75, i.e. it lost 75% of it's length.

Let me know if that makes more sense. If it doesn't, you may need to bring this up during office hours, as it's difficult to truly visualize without drawing it out.

-Kate
Katelyn Sawyer
Senior Tutor
katelyn@nextsteptestprep.com

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