In DNA replication, why is a G on the original strand partnered by a C on the complementary strand, and not by an \(\mathrm{A},\) a \(\mathrm{T},\) or a \(\mathrm{G} ?\)

The base pairing rule is fundamental to the structure and function of DNA. This rule states that in DNA, adenine (A) pairs with thymine (T), and guanine (G) pairs with cytosine (C). This specificity is due to the molecular structures of the bases that allow them to form hydrogen bonds with their complements: A and T form two hydrogen bonds, whereas G and C form three. This base pairing is critical during DNA replication because it ensures that each new DNA molecule is an exact copy of the original, maintaining genetic information.

Understanding this rule helps us grasp why a G on the original DNA strand is always paired with a C on the complementary strand. If G were to pair with any other base, such as A, T, or another G, it would disrupt the DNA's double helical structure and the genetic code could be altered, possibly leading to mutations.

A nucleotide is the basic building block of DNA and consists of three parts: a sugar (deoxyribose), a phosphate group, and a nitrogenous base. There are four different nitrogenous bases in DNA: adenine (A), thymine (T), guanine (G), and cytosine (C). These bases are attached to the sugar molecule, which in turn is linked to a phosphate group. The phosphate group of one nucleotide binds to the sugar of the next, forming a sugar-phosphate backbone that is the same along the entire length of the DNA molecule.

The structure of nucleotides allows them to connect in a specific order, forming a strand of DNA. The nitrogenous bases protrude from the backbone and are vital for base pairing during DNA replication. The integrity of the nucleotide structure ensures that genetic information is correctly stored and transmitted from one generation to the next.

Chargaff's rule, discovered by biochemist Erwin Chargaff, is a central principle in the study of DNA. The rule states that in a double-stranded DNA molecule, the amount of adenine (A) is equal to that of thymine (T), and the amount of guanine (G) is equal to that of cytosine (C). This observation laid the foundation for understanding the base pairing rule, as Chargaff's rule hints at the pairs A-T and G-C forming because of their chemical compatibility and equal ratios.

This rule reinforces the base pairing concept because it is rooted in the pairing mechanism where A will only bind with T, and G with C. The base pairing rule is a direct reflection of Chargaff's observation and is crucial for the fidelity of DNA replication. When replicating, an enzyme called DNA polymerase ensures that A pairs with T and G pairs with C, maintaining the A to T and G to C ratio according to Chargaff's rule.

Hydrogen bonding in DNA is responsible for the specific pairing of nitrogenous bases and the stability of the double helix structure. Hydrogen bonds are weak attractions between a hydrogen atom on one molecule and a nitrogen or oxygen atom on another molecule. In the context of DNA, hydrogen bonds occur between the complementary nitrogenous bases.

Adenine and thymine form two hydrogen bonds, while guanine and cytosine form three, which is why G-C pairs are slightly stronger than A-T pairs. The number and arrangement of these bonds ensure a stable yet flexible DNA structure. These hydrogen bonds can be easily broken and reformed during processes such as DNA replication, allowing the strands to separate and serve as templates for the creation of new complementary strands, and then re-anneal to form the familiar double helix. The specificity and number of hydrogen bonds formed between bases are critical for accurate base pairing and the integrity of genetic information.