If the molar percentage of guanine in a human DNA is 30%, what is the molar percentage of adenine in that molecule?
**Core Concept:** DNA composition and molar percentages of nucleotides. DNA is a double-stranded molecule composed of four nitrogenous bases: adenine (A), guanine (G), cytosine (C), and thymine (T). The molar percentage of each base in a typical human DNA molecule can be calculated using the formula:
1. Guanine has a molar percentage of 30%.
2. Thymine is absent in the human genome, so we will not calculate its molar percentage.
**Why the Correct Answer is Right:**
First, we need to understand that DNA is composed of equal amounts of A and T base pairs due to base pairing (A with T and G with C). Therefore, the molar percentage of adenine (A) and thymine (T) will be half of the molar percentage of guanine (G) and cytosine (C). In this case, since G has a molar percentage of 30%, half of that is 15%. Similarly, T has a molar percentage of 0 (since it is absent in the human genome), and half of that is also 0.
**Why Each Wrong Option is Incorrect:**
A. 15% (this is incorrect as it is the molar percentage of cytosine, not adenine)
B. 0% (this is incorrect as it is the molar percentage of thymine, not adenine)
C. 15% (similarly incorrect, this is the molar percentage of cytosine)
**Clinical Pearl:**
The molar percentages of DNA bases help in understanding the genetic material's composition and can provide insights into genetic disorders and mutations. For example, if a mutation results in a higher or lower percentage of a specific nucleotide, it might indicate a genetic abnormality, such as a deletion or duplication of DNA segments.
**Correct Answer:** D. 0% (molar percentage of thymine)
Explanation: Since thymine is absent in the human genome, its molar percentage will be 0%. The remaining bases are guanine (30%) and cytosine (70%). Half of 70% is 35%, which is the molar percentage of cytosine. The remaining base, adenine, has half of that percentage, i.e., 17.5%.