Replication lagging strand

Lagging strand (Section 28.10) In DNA replication, the strand that grows away from the replication fork. 

Registiy of Mass Spectral Data, 412 Replication (DNA). 1106-1107 direction of, 1107 error rate during. 1107 lagging strand in, 1107 leading strand in, 1107 Okazaki fragments in, 1107 replication fork in, 1107 Replication fork (DNA), 1107 Reserpine, structure of, 65 Residue (protein), 1027 Resist, photolithography and, 505-506... 

Each strand of the double helix is replicated simultaneously but by somewhat different mechanisms. A complex of proteins, including DNA polymerase, replicates the leading strand continuously in the 5 to 3 direction. The lagging strand is replicated discon-tinuously, in short pieces of 150-250 nucleotides, in the 3 to 5 direction. 

DNA replication differs between the leading strand and the lagging strand of the DNA double helix. In cells, replication Af the lagging strand involves the formation of short RNA primers by action of an enzyme called RNA primase (or primase for short). Such RNA primers are made at intervals on the lagging strand and are then removed and replaced with DNA by DNA polymerase. 

At each replication fork there are two DNA polymerase complexes. As the double-stranded DNA is unwound, two template strands are exposed. One of the templates can be replicated in a continuous fashion by DNA polymerase since a continuous synthesis of new strands can occur in the 5 to 3 direction as the template strand is exposed. Since all growing chains must be synthesized in file 5 to 3 direction, the lagging chain must be continuously reinitiated as new template is exposed. The lagging strand is then synthesized discontinuously, in pieces that must be joined together later. 

There s not just one DNA polymerase there s a whole army. DNA replication actually occurs in large complexes containing many proteins and sometimes many polymerases. In eukaryotic cells we have to replicate both mitochondrial and nuclear DNA, and there are specific DNA polymerases for each. In addition to DNA replication, you have to make new DNA when you repair. Consequently, the function may be specialized for repair or replication. There can also be specialization for making the leading or lagging strand. Some of the activities of DNA polymerases from eukaryotes and prokaryotes are shown in the table on the next page. 

There is a leading and a lagging strand for each of the two replication forks on the... 

DNA polymerase III is the main DNA replicating enzyme and consequently is involved in duplication of both the leading and the lagging strands. Besides the deoxyribonu-cleoside triphosphates as substrates, it requires a single-stranded DNA template and an RNA primer. The RNA primer is synthesised according to the DNA sequence at each replication fork. It is DNA polymerase I that is... 

Figure 11-2. The prokaryotic DNA replication fork. A schematic representation of semi-conservative replication of DNA by different mechanisms on the leading and lagging strands by DNA polymerase III (DNA pol III) is shown. Other enzymes and accessory proteins that participate in initiation, elongation, and ligation phases of the process are indicated, with DNA pol I depicted as having just dissociated from a completed Okasaki fragment. SSBs, single-stranded DNA binding proteins.

The chromosomes become shorter at each round of DNA replication after removal of the RNA primer from the lagging strand. 

Elongation The elongation phase of replication includes two distinct but related operations leading strand synthesis and lagging strand synthesis. Several enzymes at the replication fork are important to the synthesis of both strands. Parent DNA is first unwound by DNA helicases, and the resulting topological stress is relieved by topo-isomerases. Each separated strand is then stabilized by... 

Like bacteria, eukaryotes have several types of DNA polymerases. Some have been linked to particular functions, such as the replication of mitochondrial DNA. The replication of nuclear chromosomes involves DNA polymerase a, in association with DNA polymerase S. DNA polymerase a is typically a multisubunit enzyme with similar structure and properties in all eukaryotic cells. One subunit has a primase activity, and the largest subunit (Afr -180,000) contains the polymerization activity. However, this polymerase has no proofreading 3 —>5 exonuclease activity, making it unsuitable for high-fidelity DNA replication. DNA polymerase a is believed to function only in the synthesis of short primers (containing either RNA or DNA) for Okazaki fragments on the lagging strand. These primers... 

Yet another polymerase, DNA polymerase e, replaces DNA polymerase S in some situations, such as in DNA repair. DNA polymerase e may also function at the replication fork, perhaps playing a role analogous to that of the bacterial DNA polymerase I, removing the primers of Okazaki fragments on the lagging strand. 

DNA is synthesized in the 5 —>3 direction by DNA polymerases. At the replication fork, the leading strand is synthesized continuously in the same direction as replication fork movement the lagging strand is synthesized discontinuously as Okazaki fragments, which are subsequently ligated. 

Terms in bold are defined template 950 semiconservative replication 950 replication fork 951 origin 952 Okazaki fragments 952 leading strand 952 lagging strand 952 nucleases 952 exonuclease 952 endonuclease 952 DNA polymerase I 952 primer 954 primer terminus 954... 

Leading and Lagging Strands Prepare a table that lists the names and compares the functions of the precursors, enzymes, and other proteins needed to make the leading versus lagging strands during DNA replication in E. coli. 

Fidelity of Replication of DNA What factors promote the fidelity of replication during the synthesis of the leading strand of DNA Would you expect the lagging strand to be made with the same fidelity Give reasons for your answers. 

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