Book problem:

How many different DNA molecules 10 nucleotide pairs long are possible?

 

My solution:

I was thinking that the base pair has to be either A-T or C-G. Thus, two pairs are possible.

 

(1 of 2 possible) x (1 of 2 possible) ... 8 more times... = 2^10 = (1 of 2 possible: A-T or C-G)^10

 

Book solution:

But I'm guessing the book has it as 4^10.

 

My question:

What am I not seeing?

Because

 

AT

CG

 

is different from

 

AT

GC

 

In other words, there are not 2, but 4 possible base pairs in that retrospect.

You are forgetting that the base pairing has no impact on the question. At any given on one strand the choices are A,T,G or C. The pairing only affects the choice of bases for the the opposite strand.

But isn't a DNA molecule two strands, thus my answer is right?

And since there is symmetry, only two possibilities exist for pairs (A-T and C-G).

 

If it were only one side of the molecule, then I would see your point.

However, it said "pairs."

If it didn't say "pairs," then it would have said, "...10 nucleotides long."

 

TA

CG

 

is...

 

AT

GC

 

&

 

TA

GC

 

is

 

AT

CG

 

Because of the mirroring, there are only two possibilities.

Edited January 17, 201015 yr by Genecks

No, while the question refers to the DNA molecule, however the sequence is only depending on one of the strands.

You have to take into account that the DNA is read only in one direction, 5' ---> 3' and each strand has a different direction.

Even not considering the direction, it should be able to figure it out. In DNA one chain determines the sequence of another chain by base pairing, so just ignore one of them. Now ten nucleotide pairs are reduced to ten nucleotides in a sequence, each slot gets 4 possible entries, so possible combination equals 4^10.