Question

When samples of double-stranded DNA are analyzed, the quantity of adenine present equals that of thymine. Similarly, the quantity of guanine equals that of cytosine. Explain the significance of these observations.

Short answer

The equal quantities of adenine (A) and thymine (T), and guanine (G) and cytosine (C) in double-stranded DNA samples are significant because they reinforce the base pairing rules (A-T and G-C) and confirm the double-helical structure of DNA. This complementary base pairing is vital for accurate replication and transcription of genetic information during cell division, ensuring genetic material stability. Moreover, these observations, known as Chargaff's rules, were foundational in understanding DNA structure and function in living organisms.

Step-by-step solution

Introduce the structure of DNA

Double-stranded DNA consists of two long chains (strands) made up of nucleotides. Each nucleotide contains a sugar molecule, a phosphate group, and a nitrogenous base. There are four types of nitrogenous bases in DNA: adenine (A), thymine (T), guanine (G), and cytosine (C). The two strands of DNA run in opposite directions and form a double helix structure, held together by hydrogen bonds between the nitrogenous bases.

Base Pairing Rules

The nitrogenous bases pair up according to specific rules: adenine (A) only pairs with thymine (T), and guanine (G) only pairs with cytosine (C). This is known as complementary base pairing and is due to the specific hydrogen bond formations between the bases. A and T form two hydrogen bonds while G and C form three hydrogen bonds.

Explain the significance of equal quantities of A & T and G & C

Since adenine (A) always pairs with thymine (T) and guanine (G) with cytosine (C), it means that in a double-stranded DNA molecule, the number of A's will always be equal to the number of T's and the number of G's will always be equal to the number of C's. This observation is significant because it reinforces the base pairing rules and provides evidence for the double-helical structure of DNA. Furthermore, this complementary base pairing is essential for the accurate replication and transcription of genetic information during cell division, ensuring the stability of an organism's genetic material.

Mention Chargaff's Rules

This observation of equal quantities of A & T and G & C in double-stranded DNA is also known as Chargaff's rules, named after the biochemist Erwin Chargaff who first discovered this pattern. These rules laid the foundation for the understanding of the DNA structure and its function in living organisms.