Problem 10 - Cell and Molecular Biology: Concepts and Experiments

Question

According to Chargaff's determination of base composition, which of the following would characterize any sample of DNA? (1) \([\mathrm{A}]+[\mathrm{T}]=[\mathrm{G}]+[\mathrm{C}] ;(2)[\mathrm{A}] /[\mathrm{T}]=1 ;(3)[\mathrm{G}]=[\mathrm{C}]\) \((4)[\mathrm{A}]+[\mathrm{G}]=[\mathrm{T}]+[\mathrm{C}] .\) If the \(\mathrm{C}\) content of a preparation of double-stranded DNA is 15 percent, what is the A content?

Step-by-Step Solution

Answer

The adenine (A) content is 35%.

Step 1: Understand Chargaff's Rule

Chargaff's rule states that in a double-stranded DNA molecule, the amount of adenine (A) is equal to thymine (T), and the amount of guanine (G) is equal to cytosine (C). This is expressed as \([A] = [T]\) and \([G] = [C]\).

Step 2: Define the Total Base Content

The total sum of the contents of all the bases in DNA represents 100% of the DNA bases. Therefore, \([A] + [T] + [G] + [C] = 100\%\).

Step 3: Set Given Values

From the problem, we know the content of cytosine (C) is 15%. Since \([G] = [C]\), the guanine (G) content is also 15%.

Step 4: Calculate A + T Content

Since we have \([G] + [C] = 15\% + 15\% = 30\%\), the remaining percentage of DNA must be composed of adenine (A) and thymine (T). Thus, \([A] + [T] = 100\% - 30\% = 70\%\).

Step 5: Determine A Content

Using \([A] = [T]\), each makes up half of the \([A] + [T]\) content. Consequently, \([A] = \frac{70}{2}\% = 35\%\).

Key Concepts

DNA base composition

DNA's structural makeup is fascinating due to its simplicity and complexity. DNA is composed of four nucleotide bases: adenine (A), thymine (T), guanine (G), and cytosine (C). These bases pair up in specific ways to form the double helix structure of DNA. The base composition refers to the ratio or percentage of each of these bases in a DNA molecule.
One of the key rules governing the base composition of DNA is Chargaff's Rule, which states that in any DNA molecule, the amount of adenine equals thymine \([A] = [T]\) and the amount of guanine equals cytosine \([G] = [C]\).
Understanding the base composition is crucial because it helps us figure out the proportions of each base present in the DNA sample. This information is fundamental for DNA replication and gene expression studies, which are foundational to many biological processes and research.

adenine content

Adenine is one of the four nucleotide bases that are critical components of DNA. Chargaff's Rule is particularly useful when dealing with adenine because it establishes that adenine and thymine are present in equal quantities in double-stranded DNA.
If you know the amount of any of these bases, you can also determine the adenine content. For example, if we know the cytosine content, we also know the guanine content due to the rule \([G] = [C]\). With this information, we can deduce the extent of adenine and thymine, as \([A]+[T]\) would equal the amount left after accounting for \([G]+[C]\).
In the provided exercise example, once we determined that \([C]\) is 15%, we inferred that \([G]\) is also 15%. Thus, \([A]+[T] = 100\%-30\%=70\%\). Since \([A] = [T]\), adenine content \([A]\) is 35%.

Cytosine percentage

Cytosine is another crucial nucleotide base in DNA. According to Chargaff's rule, its content in a DNA molecule is equal to that of guanine. Given a specific cytosine percentage, you automatically know the guanine percentage because they pair together in DNA strands.
For instance, if the cytosine percentage in a DNA sample is given as 15%, the guanine percentage is equally 15%. This provides a symmetrical balance due to their complementary nature.
Such information is pivotal for determining the leftover percentage, which is occupied by adenine and thymine. This knowledge underpins the calculation of DNA's overall base composition and contributes to the understanding of various genetic and molecular biology phenomena.