# Question #3beaf

Dec 3, 2017

See below.

#### Explanation:

$\text{Total length" = "1,000 bp}$

The first restriction site is after 150 base pairs, thus

$\text{fragment 1"="150 bp}$

The second restriction site is 100 base pairs after the first site, so

$\text{fragment 2"=250-150="100 bp}$

The third restriction site is: 700 base pairs after the second site, so

$\text{fragment 3"=950-250="700 bp}$

The third restriction site wasn't at 1,000 base pairs so it actually split the last piece of DNA into 2 fragments. The last fragment is the last 50 base pairs of the DNA, so

$\text{fragment 4"=1,000-950="50 bp}$

The sum of the fragment lengths adds up to 1,00 base pairs, so we know they are the correct lengths.

I hope this helps!

Dec 3, 2017

I notice you actually mentioned the correct answer in your question?

#### Explanation:

Circular plasmid, 3 restriction sites, so after complete digestion: 3 fragments.

Length of the circular plasmid is 1000 BP, so the total, combined length of all three resulting fragments should be 1000.

Fragment 1: .
First restriction site is at pos(ition)=150 , second at pos=250 , so the difference = 100 BP.

Fragment 2:
Second restriction site is at pos250 , third at pos950 , so the difference = 700 BP.

Fragment3:
You have already cut at all three restriction sites for frags 1 & 2, so the remainder is the third fragment:
1000BP minus 700BP (frag2) minus 100BP (frag1) leaves 200 BP .

This third fragment is 200 BP long, because: 50BP starting from the third restriction site, and OVER pos0 (there is no restriction site there, so no cut ) to Pos150 ...

Which is basically what you mentioned in your question already...