1
$\begingroup$

In my Biology class we were asked this question:

Simple DNA Strand

This DNA strand consists of eight pairs of nitrogenous bases. How many different sequences of eight bases can you make? Explain how you found your answer.

I guessed either 28 or 8!. Apparently, the answer is 8! ÷ 24. I asked my teacher, but she did not know the answer. Does anyone know why this would be?

||improve this question
$\endgroup$
  • $\begingroup$ The answer your teacher gave you might be the answer to the question of how many sequences of 8 bases can be formed using only the bases shown in the diagram, each one can be used once. The factorial comes from the fact that once you pick a base there are n-1 options left and so on. 2^4 is 2*2*2*2 which accounts for there being four duplicate bases so that count only unique sequences. $\endgroup$ – Cell Mar 15 '19 at 0:47
  • $\begingroup$ Don't guess! You need to learn to approach this sort of simple statistics logically as laid out by @Remi.b. The relevence of this sort of problem to biology is more in relation to the frequency of restriction sites, which are smaller and a better place to start. You can find lots of practice questions and an explanation of how to answer them on a self-teaching resource I put up for students at my own university. Give it a try. $\endgroup$ – David Mar 15 '19 at 18:32
2
$\begingroup$

At each base, you can have 4 different bases (A,T,C or G). Therefore for the first base there are 4 possibilities, namely

  • A
  • T
  • C
  • G

For the first two base pairs there are $4^2 = 16$ possible combinations

  • AA
  • AT
  • AC
  • AG
  • TA
  • TT
  • TC
  • TG
  • CA
  • CT
  • CC
  • CG
  • GA
  • GT
  • GC
  • GG

For the first three bases, there are $4^3$ possible combinations. For 8 base pairs, there are $4^8 = 65536$ possible combinations. $2^8$, $8!$ and $8! + 2^4$ are all wrong.

||improve this answer
$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.