Question

If DNA replication was conservative, after 1 round of replication in \({ }^{14} \mathrm{~N}\), the density of the double stranded DNA would be: A. all intermediate B. all heavy C. half intermediate, half heavy D. half heavy, half light

Short answer

D. half heavy, half light

Step-by-step solution

Understanding Conservative Replication

Conservative replication is a hypothetical model of DNA replication where the original DNA molecule is conserved, and the new DNA consists of all new nucleotides. After one round of replication, this would result in one molecule completely composed of the original material (heavy strand) and one molecule completely composed of new material (light strand).

Analyzing the Outcome After One Round

Since the exercise specifies replication in ^{14}N after one round, conservative replication would yield one double-stranded DNA molecule with the original heavy isotopes (^{15}N) and one double-stranded DNA molecule with the new light isotopes (^{14}N).

Determining the Correct Answer

Based on the conservative replication hypothesis, we expect one molecule to be heavy (^{15}N-^{15}N) and the other to be light (^{14}N-^{14}N). There would be no intermediate density DNA as no strands would be a mix of ^{15}N and ^{14}N.

Key concepts

Conservative Replication

Understanding the basics of conservative replication is essential in grasping the fundamentals of DNA replication. In this hypothetical model, the parental DNA molecule remains intact, and a completely new complementary strand is synthesized to pair with it. This means that after one cycle of DNA replication, there would be two distinct DNA molecules – one with the original 'heavy' strands of DNA and one with new 'light' strands. The 'heavy' DNA refers to DNA composed of a denser isotope of nitrogen, such as ucleotide{^{15}N}, while 'light' DNA is composed of the more common isotope, ucleotide{^{14}N}.
Imagine taking a photograph and its negative. The photograph is the original, the negative is a brand-new copy; they are distinctly separate entities. Similarly, in conservative replication, the parent DNA and the newly synthesized DNA are separate after the replication event. If DNA replication were purely conservative, after one round of replication in the presence of ucleotide{^{14}N}, the result would be one molecule fully 'heavy' and one fully 'light', without any intermediate densities.

Isotopic Labeling in DNA Replication

Isotopic labeling is a powerful tool used to track molecular changes, especially in DNA replication. By incorporating isotopes such as ucleotide{^{15}N} or ucleotide{^{14}N} into the nucleotides that make up the DNA strands, scientists can distinguish between 'old' and 'new' DNA segments based on their density through a process called density gradient centrifugation.
Picture isotopic labeling like color-coding elements in a complex structure to see what changes over time. When the DNA replicates, the isotopes reveal whether both strands of a DNA molecule are old, new, or a mix. This helps determine which model of DNA replication is accurate. In the case of conservative replication, if DNA has gone through one round of replication after being tagged with the heavy isotope of nitrogen (ucleotide{^{15}N}), and then allowed to replicate in the presence of the lighter isotope (ucleotide{^{14}N}), the outcome would be a clear distinction between the original heavy DNA and the new light DNA, with no intermediate densities.

DNA Replication Hypothesis

The DNA replication hypothesis encompasses several models, including conservative, semi-conservative, and dispersive. Each model predicts different outcomes for the post-replication DNA in terms of the distribution of old and new DNA strands. The semi-conservative hypothesis, which was confirmed through the famous Meselson-Stahl experiment, posits that each new DNA molecule consists of one strand from the original molecule and a new complementary strand.
In contrast, the conservative hypothesis assumes no mixing of old and new strands across the DNA molecules, and the dispersive model suggests a more random integration, with old and new DNA segments interspersed within a single strand. To verify the validity of these hypotheses, isotopic labeling was a critical tool, demonstrating that DNA replication is, in fact, semi-conservative. This experiment provided crucial insight into the mechanics of genetic inheritance and the molecular continuity of life.