Q.If Meselson and Stahl's experiment is continued for four generations in bacteria, the ratio of N15/N15 : N15/N14 : N14/N14 containing DNA in the fourth generation would be:

DNA replication is a semiconservative process, meaning each new DNA molecule consists of one original strand and one newly synthesized strand. This is possible due to the principle of complementary base pairing, where adenine always pairs with thymine, and guanine always pairs with cytosine. During replication, the two strands of the parent DNA separate, and each acts as a template for the synthesis of a new complementary strand.

In Meselson and Stahl's experiment, bacteria initially grown in a heavy nitrogen ($N^{15}$) medium have DNA containing $N^{15}/N^{15}$. When these bacteria are transferred to a light nitrogen ($N^{14}$) medium and allowed to replicate:

• Generation 0: All DNA is $N^{15}/N^{15}$.
• Generation 1: After one replication, all DNA molecules are hybrids ($N^{15}/N^{14}$) because each new molecule has one $N^{15}$ template strand and one newly synthesized $N^{14}$ strand. There are 2 such molecules.
• Generation 2: The two hybrid molecules replicate. Each $N^{15}$ strand forms a new $N^{14}$ strand, resulting in two $N^{15}/N^{14}$ molecules. Each $N^{14}$ strand forms a new $N^{14}$ strand, resulting in two $N^{14}/N^{14}$ molecules. So, there are 2 $N^{15}/N^{14}$ and 2 $N^{14}/N^{14}$ molecules.
• Generation 3: The two $N^{15}/N^{14}$ molecules produce two $N^{15}/N^{14}$ and two $N^{14}/N^{14}$ molecules. The two $N^{14}/N^{14}$ molecules produce four $N^{14}/N^{14}$ molecules. In total, there are 2 $N^{15}/N^{14}$ and 6 $N^{14}/N^{14}$ molecules. …

The very essence of life's continuity lies in the ability of genetic material to replicate itself accurately. For decades, scientists pondered how DNA, the blueprint of life, managed to copy itself. The structure of DNA, famously elucidated by Watson and Crick, provided a crucial clue: the double helix, with its two complementary strands. This structure immediately suggested a mechanism for replication, known as complementary base pairing.

Complementary base pairing is a fundamental principle where adenine (A) always pairs with thymine (T), and guanine (G) always pairs with cytosine (C). These specific pairings are maintained by hydrogen bonds between the bases. When the two strands of the DNA double helix separate, each individual strand can then serve as a template for the synthesis of a new complementary strand. For instance, if an old strand has an 'A', the new strand will incorporate a 'T' opposite it; if it has a 'G', a 'C' will be added. This precise pairing ensures that the genetic information is faithfully copied.

This mechanism led to the hypothesis of semi-conservative DNA replication. The term "semi-conservative" means that each new DNA molecule produced after replication consists of one original (parental) strand and one newly synthesized strand. This was a revolutionary idea, as it implied that the genetic information was passed down with remarkable fidelity, yet also allowed for the creation of entirely new molecules. Before this, other models, such as conservative (where the original DNA molecule remained intact and a completely new one was formed) or dispersive (where new and old DNA were mixed throughout both strands), were also considered.

In 1958, Matthew Meselson and Franklin Stahl conducted an elegant experiment using Escherichia coli bacteria to definitively prove that DNA replication is indeed semi-conservative. Their experiment relied on the difference in density between DNA containing a heavy isotope of nitrogen (N15) and DNA containing the common, lighter isotope (N14).

Here's how they conducted their groundbreaking experiment:

• Growing E. coli in N15 medium: They first grew E. coli for many generations in a medium containing N15 as the sole nitrogen source. This ensured that all the nitrogenous bases in the bacteria's DNA were incorporated with N15, making the DNA molecules "heavy."
• Transfer to N14 medium: They then transferred these N15-laden bacteria to a fresh medium containing only N14. From this point onwards, any new DNA strands synthesized would incorporate N14.
• DNA extraction and centrifugation: At various time intervals (corresponding to different generations of bacterial division), they extracted DNA from the bacterial cultures. This extracted DNA was then subjected to cesium chloride (CsCl) density gradient centrifugation. In this technique, DNA molecules separate into distinct bands based on their density. Heavy DNA (N15/N15) would settle at the bottom, light DNA (N14/N14) at the top, and hybrid DNA (N15/N14) in between.

Let's trace the DNA composition through four generations:

Generation 0 (Parental DNA):

Before transferring to the N14 medium, all the DNA in the E. coli cells was heavy, meaning both strands of every DNA molecule contained N15.

• Composition: All N15/N15 (heavy) DNA.

Generation 1 (After one replication in N14 medium):

When the N15/N15 DNA replicates in the N14 medium, each N15 strand acts as a template. A new complementary strand is synthesized using N14.

• Result: All DNA molecules are N15/N14 (hybrid). There are no N15/N15 or N14/N14 molecules. This result immediately ruled out the conservative model, which would have predicted N15/N15 and N14/N14 DNA.

Generation 2 (After two replications in N14 medium):

The two N15/N14 hybrid molecules from Generation 1 now replicate.

• Each N15 strand serves as a template for a new N14 strand, forming an N15/N14 hybrid molecule.
• Each N14 strand serves as a template for a new N14 strand, forming an N14/N14 (light) molecule. …

Use a general formula instead of re-deriving each generation: after n rounds of replication in the light medium, there are always exactly 2 hybrid (N15/N14) molecules (the two original heavy strands can never vanish, and once each is paired with a new light strand it stays hybrid forever), while the total molecule count doubles to 2^n. So hybrid : total = 2 : 2^n, and lig …