Question

Human DNA contains approximately \(30.4 \%\) A. Estimate the percentages of \(\mathrm{G}, \mathrm{C}\), and \(\mathrm{T}\) and compare them with the values presented in Table 28.1.

Short answer

Answer: The estimated percentages for human DNA are: A - 30.4%, T - 30.4%, G - 19.6%, and C - 19.6%.

Step-by-step solution

Find the percentage of Thymine (T)

Since the percentage of Adenine (A) in human DNA is given as \(30.4\%\), and A pairs with T, the percentage of T will also be \(30.4\%\).

Calculate the combined percentage of A and T

The combined percentage of A and T will be the sum of their individual percentages. So, A and T together make up \(30.4\% + 30.4\% = 60.8\%\) of the DNA.

Determine the combined percentage of Guanine (G) and Cytosine (C)

Since A and T make up \(60.8\%\) of the DNA, the remaining percentage will be occupied by G and C. Therefore, the combined percentage of G and C will be \(100\% - 60.8\% = 39.2\%\).

Find the individual percentages of G and C

As G pairs with C, their individual percentages should be equal. Therefore, the percentage of G will be half of the combined percentage of G and C, which is \(\frac{39.2\%}{2} = 19.6\%\). The same applies to C, so the percentage of C will also be \(19.6\%\). In conclusion, the estimated percentages of nucleotides in human DNA are: - A: \(30.4\%\) - T: \(30.4\%\) - G: \(19.6\%\) - C: \(19.6\%\) We can now compare these values with those presented in Table 28.1.

Key concepts

Base Pairing Rule

The concept of base pairing is foundational to understanding DNA structure and function. Base pairing refers to the principle that in DNA, the nitrogenous bases adenine (A) always pairs with thymine (T), and guanine (G) always pairs with cytosine (C).

This pairing occurs due to the specific hydrogen bonding patterns between these bases. A pairs with T through two hydrogen bonds, while G pairs with C through three hydrogen bonds. This results in a uniform width of the DNA double helix, ensuring stability and proper replication. Whenever you know the percentage of one of these bases in a strand of DNA, you can directly infer the percentage of its complement due to this strict base pairing rule.

Adenine (A) Percentage

Adenine's percentage in human DNA can be indicative of the overall base composition. Given the information that adenine accounts for approximately 30.4% of bases in human DNA, this directly dictates the percentage of thymine due to the base pairing rule. Thus, A and T together account for a substantial portion of the DNA's composition. This knowledge is crucial for various applications, including genetics, forensics, and molecular biology, as alterations in these percentages can lead to mutations and genetic disorders.

Guanine (G) and Cytosine (C) Percentage

Guanine and cytosine pair together in DNA, which means their percentages must also be equal. Once you know the combined percentage of these two nucleotides completes the remaining portion of the DNA not accounted for by adenine and thymine, it's simple to find the individual percentages.

They contribute significantly to DNA's overall structure and their chemical properties influence aspects like the DNA's melting point and its overall stability. Equally crucial to any genetic analysis, the GC content can affect the function and structure of genes, so knowing that guanine and cytosine both make up 19.6% of DNA plays a role in the broader understanding of genomic organization.

Thymine (T) Percentage

The thymine percentage in human DNA is directly correlated with the amount of adenine present based on the base-pairing rule. If adenine represents 30.4% of the DNA, thymine will have the same percentage, meaning thymine also constitutes 30.4%. Understanding the percentage of thymine is vital because it helps in determining DNA's susceptibility to damage from UV light, as thymine dimers can form with excessive exposure, leading to potential mutations.