How many adenines are there when a 100 base pair DNA helix contains 45 cytosines?

Answer 1

I don't think the question, as it is currently phrased, is answerable. It's like saying, here's a room that contains 100 animals and those animals are dogs, cats, sheep and mice. 45 are dogs. How many sheep are there? Can't tell. Knowing the number of dogs doesn't help.

BUT, if you mean if a double stranded DNA sequence is 45% cytosine, how many percent is adenine? The answer is 5%. This is because cytosine always pairs with guanidine and adenine with thymidine. So, if 45% of the DNA is cytosine, then 45% must be guanidine. Since 45%+45%=90% that leaves 10%, which has to be shared evenly between adenine and thymidine. Half of 10% is 5%, therefore 5% is adenine.

Answer 2

This question does not admit of a definite answer because we don't know what the DNA is coding for. Each codon encodes for one particular amino acid. The pattern of base pairs in the codon determines which amino acid. The no of adenines in one codon can be anywhere from zero to three. Zero and three are both numbers of fairly low probability, but one and two are about equally likely. The statistical answer for a totally unknown protein being coded for is between 40 and 60.

Answer 3

75

100 nucleotide pairs are a total of 200 nucleotides. Because of base pairing, if there are 25 adenine there must also be 25 thymine. This leaves 200-50 = 150 nucleotides to be divided evenly between guanine and cytosine.

Answer 4

The DNA pairs are C-G and A-T. In your question you have 45 Cs, which means they are paired with 45 Gs. The rest of the base pair are A-T, so you have 100-45=55 A-T pairs. 55 As and 55 Ts.

Answer 5

55 adenines

Since there are 45 cytosine, guanine pairs, that means that all the remaining 55 base pairs would be adenine, thymine pairs.