Which enzyme elongates the dna strand

In subsequent cell divisions, an increasing amount of DNA contains 14 N only. These data support the semi-conservative replication model. During the density gradient ultracentrifugation, the DNA was loaded into a gradient Meselson and Stahl used a gradient of cesium chloride salt, although other materials such as sucrose can also be used to create a gradient and spun at high speeds of 50, to 60, rpm.

In the ultracentrifuge tube, the cesium chloride salt created a density gradient, with the cesium chloride solution being more dense the farther down the tube you went. At the point, the molecules stopped sedimenting and formed a stable band. By looking at the relative positions of bands of molecules run in the same gradients, you can determine the relative densities of different molecules. The molecules that form the lowest bands have the highest densities.

So DNA grown in 15 N had a higher density, as would be expected of a molecule with a heavier isotope of nitrogen incorporated into its nitrogenous bases.

Meselson and Stahl noted that after one generation of growth in 14 N after cells had been shifted from 15 N , the DNA molecules produced only single band intermediate in position in between DNA of cells grown exclusively in 15 N and DNA of cells grown exclusively in 14 N. This suggested either a semi-conservative or dispersive mode of replication. Conservative replication would have resulted in two bands; one representing the parental DNA still with exclusively 15 N in its nitrogenous bases and the other representing the daughter DNA with exclusively 14 N in its nitrogenous bases.

The single band actually seen indicated that all the DNA molecules contained equal amounts of both 15 N and 14 N. These results could only be explained if DNA replicates in a semi-conservative manner. Dispersive replication would have resulted in exclusively a single band in each new generation, with the band slowly moving up closer to the height of the 14 N DNA band.

Therefore, dispersive replication could also be ruled out. When two daughter DNA copies are formed, they have the identical sequences to one another and identical sequences to the original parental DNA, and the two daughter DNAs are divided equally into the two daughter cells, producing daughter cells that are genetically identical to one another and genetically identical to the parent cell.

DNA replication employs a large number of proteins and enzymes, each of which plays a critical role during the process. One of the key players is the enzyme DNA polymerase, which adds nucleotides one by one to the growing DNA chain that are complementary to the template strand. The addition of nucleotides requires energy; this energy is obtained from the nucleotides that have three phosphates attached to them, similar to ATP which has three phosphate groups attached.

When the bond between the phosphates is broken, the energy released is used to form the phosphodiester bond between the incoming nucleotide and the growing chain. There are specific nucleotide sequences called origins of replication where replication begins. The origin of replication is recognized by certain proteins that bind to this site. An enzyme called helicase unwinds the DNA by breaking the hydrogen bonds between the nitrogenous base pairs.

ATP hydrolysis is required for this process. As the DNA opens up, Y-shaped structures called replication forks are formed. Two replication forks at the origin of replication are extended bi-directionally as replication proceeds. Single-strand binding proteins coat the strands of DNA near the replication fork to prevent the single-stranded DNA from winding back into a double helix.

DNA polymerase then extends this RNA primer, adding nucleotides one by one that are complementary to the template strand. The DNA tends to become more highly coiled ahead of the replication fork. Single-strand binding proteins bind to the single-stranded DNA to prevent the helix from re-forming.

Primase synthesizes an RNA primer. On the leading strand, DNA is synthesized continuously, whereas on the lagging strand, DNA is synthesized in short stretches called Okazaki fragments. The replication fork moves at the rate of nucleotides per second. Okazaki fragments are named after the Japanese scientist who first discovered them.

The leading strand can be extended by one primer alone, whereas the lagging strand needs a new primer for each of the short Okazaki fragments. The sliding clamp a ring-shaped protein that binds to the DNA holds the DNA polymerase in place as it continues to add nucleotides. Topoisomerase prevents the over-winding of the DNA double helix ahead of the replication fork as the DNA is opening up; it does so by causing temporary nicks in the DNA helix and then resealing it.

The primers are removed by the exonuclease activity of DNA pol I, while the gaps are filled in by deoxyribonucleotides. The table summarizes the enzymes involved in prokaryotic DNA replication and the functions of each. DNA replication in eukaryotes occurs in three stages: initiation, elongation, and termination, which are aided by several enzymes.

Because eukaryotic genomes are quite complex, DNA replication is a very complicated process that involves several enzymes and other proteins. It occurs in three main stages: initiation, elongation, and termination. Eukaryotic DNA is bound to proteins known as histones to form structures called nucleosomes. During initiation, the DNA is made accessible to the proteins and enzymes involved in the replication process.

Whereas many bacterial plasmids see Unique Characteristics of Prokaryotic Cells replicate by a process similar to that used to copy the bacterial chromosome, other plasmids, several bacteriophages , and some viruses of eukaryotes use rolling circle replication Figure 7. The circular nature of plasmids and the circularization of some viral genomes on infection make this possible.

Rolling circle replication begins with the enzymatic nicking of one strand of the double-stranded circular molecule at the double-stranded origin dso site. Completion of DNA replication at the site of the original nick results in full displacement of the nicked strand, which may then recircularize into a single-stranded DNA molecule. Ligase is not involved in the initiation of replication. Telomerase is unique to eukaryotes.

More primers are used in lagging strand synthesis than in leading strand synthesis. Skip to content Mechanisms of Microbial Genetics. Learning Objectives Explain the meaning of semiconservative DNA replication Explain why DNA replication is bidirectional and includes both a leading and lagging strand Explain why Okazaki fragments are formed Describe the process of DNA replication and the functions of the enzymes involved Identify the differences between DNA replication in bacteria and eukaryotes Explain the process of rolling circle replication.

Think about It Which enzyme breaks the hydrogen bonds holding the two strands of DNA together so that replication can occur? Is it the lagging strand or the leading strand that is synthesized in the direction toward the opening of the replication fork? This animation illustrates the process of DNA replication. Think about It How does the origin of replication differ between eukaryotes and prokaryotes?

What polymerase enzymes are responsible for DNA synthesis during eukaryotic replication? What is found at the ends of the chromosomes in eukaryotes and why? Think about It Is there a lagging strand in rolling circle replication? Why or why not? In bacteria, the initiation of replication occurs at the origin of replication , where supercoiled DNA is unwound by DNA gyrase , made single-stranded by helicase , and bound by single-stranded binding protein to maintain its single-stranded state.

During elongation , the leading strand of DNA is synthesized continuously from a single primer. The lagging strand is synthesized discontinuously in short Okazaki fragments , each requiring its own primer.

Termination of replication in bacteria involves the resolution of circular DNA concatemers by topoisomerase IV to release the two copies of the circular chromosome. Eukaryotes typically have multiple linear chromosomes, each with multiple origins of replication. Overall, replication in eukaryotes is similar to that in prokaryotes. The linear nature of eukaryotic chromosomes necessitates telomeres to protect genes near the end of the chromosomes.

Telomerase extends telomeres, preventing their degradation, in some cell types. Rolling circle replication is a type of rapid unidirectional DNA synthesis of a circular DNA molecule used for the replication of some plasmids. What is the role of single-stranded binding protein in DNA replication? Below is a DNA sequence. Envision that this is a section of a DNA molecule that has separated in preparation for replication, so you are only seeing one DNA strand.

Why was it important that Meselson and Stahl continue their experiment to at least two rounds of replication after isotopic labeling of the starting DNA with 15 N, instead of stopping the experiment after only one round of replication? Previous: The Functions of Genetic Material. Next: RNA Transcription. Share This Book Share on Twitter. Opens the DNA helix by breaking hydrogen bonds between the nitrogenous bases.

Seals the gaps between the Okazaki fragments on the lagging strand to create one continuous DNA strand. Relaxes supercoiled chromosome to make DNA more accessible for the initiation of replication; helps relieve the stress on DNA when unwinding, by causing breaks and then resealing the DNA.

Introduces single-stranded break into concatenated chromosomes to release them from each other, and then reseals the DNA. The two new DNA molecules each consist of one parental strand and one newly made strand.

This is known as semiconservative DNA replication. Topoisomerase : alleviates positive supercoiling twisting of DNA ahead of the replication fork. The replication of DNA begins at special sites called origins of replication. Bacteria, which have relatively small circular chromosomes, contain just a single origin of replication. Eukaryotic chromosomes, which are long, linear strands of DNA contain many origins of replication along the DNA strands see below.

Replication then proceeds in both directions until the entire molecule is copied. At the end of each replication bubble is a replication fork , a Y shaped region where the parental strands of DNA are unwound so that the replication machinery can copy the DNA.

Helicases are enzymes that are responsible for untwisting the double helix at the replication forks, separating the two strands and making them available to serve as templates for DNA replication. The untwisting of the double helix by helicase causes additional twisting of the DNA molecule ahead of the replication fork.