Semiconservative replication

The copying mechanism to which Watson and Crick referred is called semiconservative and is diagramed in Figure 7-11. The sugar-phosphate backbones are represented by thick ribbons, and the sequence of base pairs is random. Let's imagine that the double helix is like a zipper that unzips, starting at one end (at the bottom in Figure 7-11). We can see that, if this zipper analogy is valid, the unwinding of the two strands will expose single bases on each strand. Each exposed base has the potential to pair with free nucleotides in solution. Because the DNA structure imposes strict pairing requirements, each exposed base will pair only with its complementary base, A with T and G with C. Thus, each of the two single strands will act as a template, or mold, to direct the assembly of complementary bases to reform a double helix identical with the original. The newly added nucleotides are assumed to come from a pool of free nucleotides that must be present in the cell.

If this model is correct, then each daughter molecule should contain one parental nucleotide chain and one newly synthesized nucleotide chain. However, a little thought shows that there are at least three different ways in which a parental DNA molecule might be related to the daughter molecules. These hypothetical modes of replication are called semiconservative (the Watson-Crick model), conservative, and dispersive (Figure 7-12). In semiconservative replication, the double helix of each daughter DNA molecule contains one strand from the original DNA molecule and one newly synthesized strand. However, in conservative replication, the parent DNA molecule is conserved, and a single daughter double helix is produced consisting of two newly synthesized strands. In dispersive replication, daughter molecules consist of strands each containing segments of both parental DNA and newly synthesized DNA.

Meselson-Stahl experiment

The first problem in understanding DNA replication was to figure out whether the mechanism of replication was semiconservative, conservative, or dispersive.

In 1958, two young scientists, Matthew Meselson and Franklin Stahl, set out to discover which of these possibilities correctly described DNA replication. Their idea was to allow parental DNA molecules containing nucleotides of one density to replicate in medium containing nucleotides of different density. If DNA replicated semiconservatively, the daughter molecules should be half old and half new and therefore of intermediate density. To carry out their experiment, they grew E. coli cells in a medium containing the heavy isotope of nitrogen (15N) rather than the normal light (14N) form.

Watson And Crick Copying Mechanism

Figure 7-11 Semiconservative DNA replication. The model of DNA replication proposed by Watson and Crick is based on the hydrogen-bonded specificity of the base pairs. Parental strands, shown in blue, serve as templates for polymerization. The newly polymerized strands, shown in orange, have base sequences that are complementary to their respective templates.

Figure 7-11 Semiconservative DNA replication. The model of DNA replication proposed by Watson and Crick is based on the hydrogen-bonded specificity of the base pairs. Parental strands, shown in blue, serve as templates for polymerization. The newly polymerized strands, shown in orange, have base sequences that are complementary to their respective templates.

12 Chapter 7 • DNA: Structure and Replication

Semiconservative replication

Conservative replication

Dispersive replication

Figure 7-12 Three alternative patterns for DNA replication.

The Watson-Crick model would produce the first (semiconservative) pattern. Orange lines represent the newly synthesized strands.

This isotope was inserted into the nitrogen bases, which then were incorporated into newly synthesized DNA strands. After many cell divisions in 15N, the DNA of the cells were well labeled with the heavy isotope. The cells were then removed from the 15N medium and put into a 14N medium; after one and two cell divisions, samples were taken and the DNA was isolated from each sample.

Meselson and Stahl were able to distinguish DNA of different densities because the molecules can be separated from each other by a procedure called cesium chloride gradient centrifugation. If cesium chloride is spun in a centrifuge at tremendously high speeds (50,000 rpm) for many hours, the cesium and chloride ions tend to be pushed by centrifugal force toward the bottom of the tube. Ultimately, a gradient of ions is establshed in the tube, with the highest ion concentration, or density, at the bottom. DNA centrifuged with the cesium chloride forms a band at a position identical with its density in the gradient. DNA of different densities will form bands at different places. Cells initially grown in the heavy isotope 15N showed DNA of high density. This DNA is shown in red at the left-hand side of Figure 7-13a. After growing these cells in the light isotope 14N for one generation, the researchers found that the DNA was of intermediate density, shown half red (15N) and half blue (14N) in the central part. After two generations, both intermediate- and low-density DNA was observed (right-

(a) Predictions of semiconservative model

Parental 1st generation

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  • Lino
    Where are the parental strands on dna strands dna replication?
    8 years ago

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