# Homework Help: DNA molecules

1. Dec 6, 2005

### BBboy

If a DNA molecule containes 28% thymine, what percent of it will be cytosine?

will it be 72%?

2. Dec 6, 2005

### iansmith

Staff Emeritus
You'll have to show some work.

First, is there anymore information that comes with the question? There four different base pair that makes up DNA. Do you think knowing only one will help in solving the percentage of a certain base pair?

3. Dec 6, 2005

### BBboy

T - thymine bonds with A - adenine
C - cytosine bonds with G - guanine
how can you calculate the percentage of cytosine if it only gives you 28% thymine?

4. Dec 6, 2005

### mrjeffy321

You know that thymine will only bond with adenine, so the percentages of each one must be related to eachother somehow.
The same thing with cytosine and guanine.

5. Dec 7, 2005

### quasi426

Also assume that DNA is only composed of C,G,T and A. Therefore the total percentages of each individual base pair will equal 100 percent.

6. Dec 8, 2005

### Moonbear

Staff Emeritus
You can't if that's all the information you're given. Are you sure you weren't given more information than that?

7. Dec 10, 2005

### quasi426

Actually if you assume perfect Watson-Crick base pairing, then all you need is the percentage of one base, since the base it bonds to will be the same percentage.

So the percentage of A = percentage of T
And 100- percentage of A + percentage of T = percentage of C and T

percentage of C = percentage of T = 1/2 percentage of C and T

8. Dec 10, 2005

### cronxeh

This is a question from P. 232, #17:

http://www.emporia.edu/biosci/genetics/prob9.htm

P. 232, #17. A double stranded DNA molecule is 28% guanosine (G).
a. What is the complete base composition of this molecule?
In double stranded DNA the concentration of guanosine equals the concentration of cytosine, thus the molecule is also 28% cytosine. This leaves 44% for adenosine and thymine combined (1-28%-28% = 44%). As the concentration of adenosine equals the concentrations of thymine, the concentration of adenosine must be 22% and the concentration of thymine must also be 22%.

b. Answer the same question, but assume the molecule is double-stranded RNA.
For double stranded RNA the reasoning is the same except that uracil replaces thymine. Thus for double stranded RNA the base composition is 28% guanosine, 28% cytosine, 22% adenosine, and 22% uracil.