Escherichia coli cells are grown for many generations in a...

Escherichia coli cells are grown for many generations in a medium in which the only available nitrogen is the heavy isotope $^{15} \mathrm{N}$. They are then transferred to a medium containing $^{14} \mathrm{N}$ as the only source of nitrogen. (a) What distribution of $^{15} \mathrm{N}$ and $^{14} \mathrm{N}$ would be expected in the DNA molecules of cells that had grown for one generation in the $^{14} \mathrm{N}$ -containing medium assuming that DNA replication was (i) conservative, (ii) semiconservative, or (iii) dispersive? (b) What distribution would be expected after two generations of growth in the $^{14} \mathrm{N}$ -containing medium assuming (i) conservative, (ii) semiconservative, or (iii) dispersive replication?

Escherichia coli cells are grown for many generations in a medium in which the only available nitrogen is the heavy isotope $^{15} \mathrm{N}$. They are then transferred to a medium containing $^{14} \mathrm{N}$ as the only source of nitrogen.
(a) What distribution of $^{15} \mathrm{N}$ and $^{14} \mathrm{N}$ would be expected in the DNA molecules of cells that had grown for one generation in the $^{14} \mathrm{N}$ -containing medium assuming that DNA replication was (i) conservative,
(ii) semiconservative, or
(iii) dispersive?
(b) What distribution would be expected after two generations of growth in the $^{14} \mathrm{N}$ -containing medium assuming
(i) conservative,
(ii) semiconservative, or
(iii) dispersive replication?

Key Concepts

Isotope Labeling in DNA

Isotope labeling involves incorporating atoms with distinct isotopic signatures, such as heavy or light nitrogen, into biological molecules like DNA. This technique allows researchers to track the distribution and mixing of molecules over generations, providing insight into the mechanism of DNA replication by differentiating between old and new nucleotides based on isotope content.

Dispersive Replication

Dispersive replication is a theoretical model where the parental DNA and newly synthesized DNA are broken into segments, and each daughter molecule is a patchwork of old and new segments. Unlike semiconservative replication, the original strands are not preserved in their entirety, but rather, are intermingled with newly synthesized DNA.

Conservative Replication

Conservative replication is a hypothetical model in which the entire parental DNA molecule remains intact and is copied to produce an entirely new molecule. As a result, one daughter molecule would contain all old DNA, and the other would contain entirely new DNA. This model is not supported by experimental data in biological systems.

Semiconservative Replication

In semiconservative replication, each daughter DNA molecule consists of one original (parental) strand and one newly synthesized strand. This model is supported by experimental evidence and is the standard mechanism of DNA replication in living organisms, ensuring that half of the original genetic material is conserved in each new molecule.

DNA Replication Models

DNA replication models describe the theoretical ways in which a DNA molecule can be duplicated and its strands distributed into the two daughter molecules. The three main models are conservative, semiconservative, and dispersive replication, each proposing a different mechanism for the separation and mixing of the old and new DNA strands during cell division.

Transcript

00:01 Let's make a table to answer this question.

00:03 So down the side, let's put our three modes of hypothesized replication for dna, so conservative.

00:19 We have semi -conservative, which is the actual mode of dna replication.

00:28 And then we have dispersive.

00:34 We can quickly just remind ourselves what each of these mean.

00:37 Conservative means that the parental strands of dna stay together.

00:41 So the daughter strands form a new dna molecule.

00:44 And the parental strands form the other.

00:48 Semiconservative means that every resulting dna molecule has one parent and one daughter strand, and dispersive is crazy.

00:58 And it's a model of dna replication where the parental strands and the daughter strands are basically cut up and mixed together to form a single wholly functional strand of dna.

01:11 And so let's finish making our, oops.

01:15 That's a crazy line.

01:16 Let's make our table here.

01:18 And we are interested in what happens when we label the genetic material that's being produced with different kinds of nitrogen isotopes.

01:29 So we're interested in what happens after one generation and the second generation.

01:41 So let's start by comparing conservative between the two generations.

01:45 If you grow nitrogen in heavy nitrogen, if you grow dna in heavy nitrogen and then switch it to light nitrogen, what's going to happen is you are going to end up after one generation with a one -to -one ratio of heavy chains, which is in 15, to light chains.

02:07 And so we can look at it like this.

02:10 That means that every parental strand will be like this.

02:14 And then your daughter strands will be together like this.

02:18 And they will be in a one -to -one ratio.

02:22 Now, that changes a little bit after two generations, because after two generations, you actually end up with a one -to -three ratio of heavy nitrogen to light nitrogen.

02:36 And so we can draw us a little picture here.

02:39 And so what this means, and we'll keep with our color scheme, we'll just have one heavy chain.

02:45 So we're going to put that in black...

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