Jul 1, 2014
112
16
upload_2015-9-4_19-7-18.png
upload_2015-9-4_19-7-51.png

How are you supposed to know that the negative values in the table indicate the chance of incorrect insertions?..
 
May 20, 2015
170
116
Status
Medical Student (Accepted)
This never got answered but I now am confused on reading this table. Can anyone explain?
 

NextStepTutor_3

MCAT Tutor
Vendor
2+ Year Member
Nov 6, 2014
263
162
Status
Pre-Medical
Here's my take on this question, which is a good one:

The key is understanding which base should be incorporated and going from there. The table headings mention a "G template" and an "oxo-G template." If a template strand of DNA includes guanine (or a base that is very structurally similar), then the base that should be incorporated into the new strand is cytosine. We can then conclude that C represents the correct insertions, while all other nucleotides (A, T, G) could only be incorporated in error.

From there, we need to understand how they chose to represent the numbers. The table mentions "log10 insertion frequency," so they've simply taken the actual insertion frequency and converted it to logarithmic form. In other words, if this table were NOT logarithmic, the "oxo-G" column could have read A: 10^-2, C: 10, T: 10^-2.1, G: 10^-4. That would mean that, for every 10 times that the template includes an oxo-G and C is correctly inserted, A will be INcorrectly inserted 10^-2 times. We can adjust these numbers however we want - for every 100 times that C is correctly included, A will be incorrectly included 10^-1 times; for every 1000 correct Cs, we'll observe 10^0 (or one) incorrect As, etc.

(Note: this is the kind of table that, due to its unusual nature and use of logarithms, 99% of students won't understand right away, and that's fine. When you aren't sure how to read a table, look for any unusual results - any values that are much larger than the others, etc. From the table, we see that the values for C are the only positive ones, so we must make sense of that somehow. From there, when we try to understand why C is positive, we're drawn to the "G template" and "oxo-G template" headings, and we can conclude that C is unique in that it is supposed to pair with G.)
 
  • Like
Reactions: rlaboss

aldol16

2+ Year Member
Nov 1, 2015
4,912
3,436
Status
Medical Student
How are you supposed to know that the negative values in the table indicate the chance of incorrect insertions?..
The negative values don't indicate incorrect insertions. The table is just telling you the frequency of insertion of a specific base versus the others. So for example, A is inserted 10^-2 times for every 10^1 times that C is inserted. In this case, C is the correct base because it pairs with G. So for every 10 times C is inserted, A is inserted 0.01 times. If you have 1000 correct pairings of C, then you should have 1 incorrect pairings of A by that ratio.
 

lostnconfused

5+ Year Member
Sep 18, 2011
253
51
Status
I also did not get this question as well... If we are using 10^-2, wouldn't it be 1/0.01 = 1000/x, and solving for X you would get 10, not 1? Or would it be 1/0.1 = 1000/x?
 

aldol16

2+ Year Member
Nov 1, 2015
4,912
3,436
Status
Medical Student
I also did not get this question as well... If we are using 10^-2, wouldn't it be 1/0.01 = 1000/x, and solving for X you would get 10, not 1? Or would it be 1/0.1 = 1000/x?
10/0.01 = 1000/x
10*x = 1000*0.01
x = 1000*0.01/10
x = 1

You're forgetting that it's 10:0.01, not 1:0.01.