Problem 4 - Biology

Question

If DNA of a particular species was analyzed and it was found that it contains 27 percent \(\mathrm{A},\) what would be the percentage of \(\mathrm{C} ?\) a. 27 percent b. 30 percent c. 23 percent d. 54 percent

Step-by-Step Solution

Answer

23 percent (c)

Step 1 - Understand Base Pair Rules

In DNA, adenine (A) pairs with thymine (T), and cytosine (C) pairs with guanine (G). Hence, the amount of adenine equals the amount of thymine, and the amount of cytosine equals the amount of guanine.

Step 2 - Calculate Percentage of Thymine

Since A pairs with T, if there is 27 percent of adenine (A), there must also be 27 percent of thymine (T).

Step 3 - Calculate Total Percentage of A and T

Add the percentages of adenine (A) and thymine (T): Percentage of A + Percentage of T = 27% + 27% = 54%

Step 4 - Calculate Remaining Percentage for C and G

Since the total percentage of all bases must be 100%, the remaining percentage for both cytosine (C) and guanine (G) is: 100% - 54% = 46%

Step 5 - Determine the Percentage of One Base

Cytosine (C) and guanine (G) are always present in equal amounts, so the percentage of cytosine (C) is half of the remaining 46%. Percentage of C = 46% / 2 = 23%

Key Concepts

adenine thymine pairing

DNA is made up of four types of nucleotides: adenine (A), thymine (T), cytosine (C), and guanine (G). One of the key principles of DNA structure is how these bases pair with each other.

Adenine always pairs with thymine, forming what are known as 'adenine–thymine pairs.' This means that the amount of adenine in a DNA molecule will always equal the amount of thymine.

For example, if we know that a DNA sample contains 27% adenine, we can automatically conclude it also contains 27% thymine, because of this specific pairing rule. This balance helps to maintain the structure of the DNA double helix.

• Adenine (A) pairs with thymine (T)
• Amount of adenine = amount of thymine

Understanding this pairing rule is crucial for calculating the percentage of other bases in DNA.

cytosine guanine pairing

Just like adenine pairs with thymine, cytosine (C) pairs with guanine (G). These are known as 'cytosine–guanine pairs.'

The amount of cytosine in a DNA sample will always equal the amount of guanine. This rule works the same way as the adenine-thymine rule and is essential for maintaining the structure of DNA.

In our example exercise, if we know that 54% of the DNA is made up of adenine and thymine together, the remaining percentage must be for cytosine and guanine. Since cytosine pairs with guanine, we can split the remaining percentage equally between them.

• Cytosine (C) pairs with guanine (G)
• Amount of cytosine = amount of guanine

Following these rules helps us correctly determine the composition of a DNA molecule.

percentage calculations

To solve the problem, we need to apply our knowledge of base pairings to calculate precise percentages. Here's a step-by-step guide on how to do it:

1. Determine the percentage of bases that pair directly with each other. In the example, if adenine is 27%, thymine is also 27%. That makes 54% of adenine and thymine combined.

• Given: 27% adenine (A)
• Therefore: 27% thymine (T)
• Total A + T = 27% + 27% = 54%

2. Calculate the remaining percentage for the other base pairs. Since DNA is 100%, subtract the total percentage of A and T from 100%.

• Total DNA = 100%
• Remaining percentage = 100% - 54% = 46%

3. Determine the percentage of cytosine (C) and guanine (G) by splitting the remaining percentage equally. Because C pairs with G, they split the remaining percentage equally.

• Total remaining percentage for C and G = 46%
• Percentage of cytosine (C) = 46% / 2 = 23%

This simple method ensures you can always determine the percentage each base will occupy in a DNA molecule based on given data.