Question

Base Composition of DNAs Made from Single-Stranded Templates Predict the base composition of the total DNA synthesized by DNA polymerase on templates provided by an equimolar mixture of the two complementary strands of bacteriophage \(\phi \mathrm{X} 174 \mathrm{DNA}\) (a circular DNA molecule). The base composition of one strand is A, \(24.7 \% ; \mathrm{G}, 24.1 \% ; \mathrm{C}\), \(18.5 \%\); and \(\mathrm{T}, 32.7 \%\). What assumption is necessary to answer this problem?

Short answer

Assume DNA polymerase perfectly pairs bases. Result: A/T 28.7%, G/C 21.3%.

Step-by-step solution

Understand the Problem

We have an equimolar mixture of two complementary strands of bacteriophage \(\phi \mathrm{X} 174 \mathrm{DNA}\). We need to predict the base composition of the DNA synthesized by DNA polymerase. Given the base composition of one strand, we must understand the relationship between complementary bases (A-T and C-G) to find the base composition of the synthesized DNA.

Determine Complementary Base Composition

The strand provided includes bases \(A: 24.7\%\), \(G: 24.1\%\), \(C: 18.5\%\), and \(T: 32.7\%\). The complementary strand will have \(T\) instead of \(A\), \(C\) instead of \(G\), \(G\) instead of \(C\), and \(A\) instead of \(T\). Therefore, the complementary strand would have:- T: 24.7%- C: 24.1%- G: 18.5%- A: 32.7%

Calculate Total DNA Base Composition

To find the overall base composition, we average the percentage of each base between the two strands:- \(\text{A} = \frac{24.7\% + 32.7\%}{2} = 28.7\%\)- \(\text{G} = \frac{24.1\% + 18.5\%}{2} = 21.3\%\)- \(\text{C} = \frac{18.5\% + 24.1\%}{2} = 21.3\%\)- \(\text{T} = \frac{32.7\% + 24.7\%}{2} = 28.7\%\)

Make Necessary Assumption

The assumption necessary here is that DNA polymerase synthesizes the DNA by matching each base with its complementary base, creating a double-stranded DNA from the provided single strands. This assumes complete fidelity of base pairing during synthesis and no alteration of base composition.

Key concepts

Base Composition

Base composition refers to the proportion of the different nucleotide bases that make up a strand of DNA. In this exercise, we are dealing with the bases adenine (A), guanine (G), cytosine (C), and thymine (T). These bases pair up with their complementary partners to form the structure of DNA.
For any given DNA strand, the base composition is expressed as the percentage of each base present. Here, for one strand of the bacteriophage \(\phi \mathrm{X} 174\) DNA, the base composition is:

• A: 24.7\%
• G: 24.1\%
• C: 18.5\%
• T: 32.7\%

Understanding the base composition of a DNA strand is crucial when predicting the composition of the DNA synthesized from such a strand, especially when using tools like DNA polymerase.

Complementary Bases

The concept of complementary bases is foundational to understanding DNA structure and replication. In DNA, adenine (A) always pairs with thymine (T), and guanine (G) pairs with cytosine (C). These pairs are held together by hydrogen bonds and are complementary to each other.
When given a single strand of DNA, one can predict the sequence of its complementary strand by substituting each base with its complement. For instance, if the original strand contains A at 24.7%, the complementary strand will have T at the same percentage.
In our example, the complementary strand of the bacteriophage \(\phi \mathrm{X} 174\) DNA translates as follows:

• A becomes T
• T becomes A
• G becomes C
• C becomes G

Complementary base pairing is essential in DNA synthesis as it ensures that the genetic information is accurately copied during cell division.

DNA Polymerase

DNA polymerase is an enzyme that plays a vital role in DNA replication. It functions by adding nucleotides to a growing DNA strand, using an existing strand as a template. This process results in the formation of a new double-stranded DNA molecule.
During DNA synthesis, DNA polymerase matches each base on the template strand with its complementary counterpart on the new strand. This ensures that the resulting DNA is almost identical to the original sequence.
In order to predict the base composition of synthesized DNA, an assumption is made that DNA polymerase works with complete fidelity, meaning it perfectly matches each base with its complement, and the base composition is maintained without alteration. This assumption was crucial for solving the given exercise by averaging the bases of the two complementary strands to find the total DNA base composition.