# Homework Help: Double-stranded DNA molecule is 22% cytosine

1. Nov 20, 2008

### wwwaazup

i am stuck on this problem and was hoping someone could help..

If a double-stranded DNA molecule is 22% cytosine, what are the percentages of guanine, adenine, and thymine?

Thanks

2. Nov 20, 2008

### Staff: Mentor

Re: chem

Hint: there will be the same amount of one of the other three.

3. Nov 20, 2008

### wwwaazup

Re: chem

thanks, so 26% each

4. Nov 20, 2008

### Staff: Mentor

Re: chem

Which each, and why 26%?

5. Nov 20, 2008

### wwwaazup

Re: chem

if 22% is is cytosine, i subtract 22 from 100 and get 78, then i divided that by 3 which is 26
so there is 26%, guanine, 26% adenine, and 26% thymine..

Right??

6. Nov 21, 2008

### Staff: Mentor

Re: chem

No, there must be other base that is present in exactly 22% quantity. Do you know how they pair?

7. Nov 21, 2008

### wwwaazup

Re: chem

T & A pair and C & G pair..
so what are the steps i need to take?

8. Nov 21, 2008

### Staff: Mentor

Re: chem

Thinking will be enough. If there is 22% of C, how much G do you expect?

9. Nov 21, 2008

### wwwaazup

Re: chem

So the hint u gave me was that there will be the same amount of one of the other three.
Then u said there must be other base that is present in exactly 22% quantity...

10. Nov 21, 2008

### GCT

Re: chem

If C pairs with G then there must be equal amounts of each assuming this is good DNA. Draw pictures if it helps.

11. Nov 22, 2008

### Staff: Mentor

Re: chem

Perhaps my English fails me, but from what I understand you have stated twice exactly the same.