Understanding G + C Content in Mycobacterium tuberculosis DNA

When the base composition of DNA from Mycobacterium tuberculosis was determined, if 18% of the bases were adenine, what is the G + C content?

• A. 36%
• B. 64%
• C. 50%
• D. 18%

Answer: (B)

In DNA, the base pairing rules dictate that adenine (A) pairs with thymine (T) and cytosine (C) pairs with guanine (G). This pairing means that the amount of adenine should equal the amount of thymine, while the amount of cytosine should equal the amount of guanine. If 18% of the bases are adenine, then 18% of the bases will also be thymine. This accounts for 36% of the total base composition (18% A + 18% T). The remaining percentage of the bases must be made up of cytosine (C) and guanine (G). Since the total percentage must equal 100%, the sum of the percentages of cytosine and guanine would be: 100% - 36% = 64%. Since cytosine and guanine are equal in quantity, the G + C content will be 64%. This confirms that the correct answer is indeed 64%, indicating a significant proportion of the DNA that makes up these two bases, which are often associated with needs for DNA stability and function in organisms like Mycobacterium tuberculosis.

Have you ever wondered why the structure of DNA is so crucial for understanding biology? Let's take a closer look at the G + C content in Mycobacterium tuberculosis and explore how it connects to base pairing. If you’re gearing up for the USA Biology Olympiad (USABO) or simply curious, let's dive right in.

First off, what’s G + C content? Essentially, it refers to the combined percentage of guanine (G) and cytosine (C) bases in a DNA strand. This is important because these bases play a pivotal role in the stability and function of the DNA molecule. The more G and C bases present, the stronger the DNA structure tends to be due to three hydrogen bonds forming between them, compared to just two between adenine (A) and thymine (T).

Now, let’s tackle the problem at hand. If we know that 18% of the DNA bases are adenine, we can follow a straightforward approach to determine the G + C content. Here’s the thing: In DNA, A pairs with T and C pairs with G, as per the base pairing rules established by Watson and Crick. So, if you have 18% adenine, guess what? You also have 18% thymine! That alone totals 36% of your DNA’s base composition already accounted for—pretty neat, right?

But wait—what about the remaining bases? Since the total must always equal 100%, we can calculate how much is left for cytosine and guanine. Simply put, you take your total, 100%, and subtract the 36% already accounted for:

100% - 36% = 64%.

Now, since cytosine and guanine must be equal (given their pairing), we find that the G + C content equals 64%. So, if you were asked whether the G + C content is 36%, 64%, 50%, or 18%, you’d confidently circle 64%, knowing that this significant portion indicates a robust and stable structure for the DNA of Mycobacterium tuberculosis.

You might be saying, “Okay, but why does it matter for organisms like Mycobacterium tuberculosis?” Great question! This bacterium is notorious for causing tuberculosis, a severe respiratory infection. Understanding its DNA structure helps researchers find effective treatments, understand the disease's mechanisms, and develop public health strategies.

The base composition isn’t just a trivia question; it represents a real-world application of genetics that impacts health and disease. So, as you prep for the USABO or any biology test on genetic materials, keep these concepts in mind. The more you understand the foundational elements—like G + C content—the better equipped you’ll be to tackle complex questions.

If you found this breakdown useful, remember that every piece of information builds on what you learned before, creating a strong foundation for your biology understanding. And who knows? You might just find it fascinating how interconnected it all is, from the microscopic world of DNA to the macroscopic effects on health. Keep studying, keep asking questions, and you'll do great on your biology journey!