Chromosomes semiconservative replication

The two major types of nucleic acids are DNA and RNA. Nucleic acids are polyphosphate esters containing the phosphate, sugar, and base moieties. Nucleic acids contain one of five purine or pyrimidine bases that are coupled within double-stranded helices. DNA, which is an essential part of the cell s chromosome, contains the information for the synthesis of protein molecules. For double-stranded nucleic acids, as the two strands separate, they act as a template for the construction of a complementary chain. The reproduction or duplication of the DNA chains is called replication. The DNA undergoes semiconservative replication where each of the two new strands contains one of the original strands. 

Initiation of a second round of replication leads to a replication eye at the initiation site of replication (fig. 26.3). As synthesis proceeds the size of the replication eye becomes larger at this stage the replicating chromosome is referred to as a theta structure because it has the appearance of the Greek letter d. Semiconservative replication is consistent with the density of the autoradiographic tracks made by different parts of the chromosome after one and two rounds of replication in [3H]thymidine (see fig. 26.3). 

Simulated autoradiographs of the E. coli chromosome after one or more replications in the presence of [3H]thymidine. After one round of replication, the autoradiograph shows a circular structure that is uniformly labeled. The second round of replication begins with the formation of a replication eye. One branch in the replication eye is twice as strongly labeled as the remainder of the chromosome, indicating that this branch contains two labeled strands. This structure is consistent with semiconservative replication for the E. coli chromosome. 

The semiconservative replication of DNA at the chromosomal level was shown by J. H. Taylor and coworkers. Using autoradiography and bean seedling root cells in tissue culture, they showed that, after a part of a cycle of duplication with [3H]thymidine (a selective label for DNA), the two chromosomes, descended from an original unlabeled chromosome, were both labeled. Following an additional duplication in the absence of labeled thymidine, the labeled chromosome yielded one labeled and one unlabeled descendant, as predicted by the semiconservative mechanism. 

The single-strand DNA penetrates the cell, where it has converted by enzymes to a duplex replicative form through Watson-Crick base pairing. The replicative form is then reproduced by a mechanism similar to that used for the semiconservative replication of the duplex chromosome of the host cell. Finally, after this stage of replication, the mechanism shifts to one in which the replicative form serves as a template to produce copies of the single-strand DNA found in the mature virus. 

There are a number of different reasons why non-DNA linkers have been considered necessary to hold DNA molecules in tandem arrangement in chromosomes. The most relevant of these to the regulation of DNA synthesis are as follows the presumption that replication units of DNA within a sin e chromosome cannot be directly connected to one another without interruption of the DNA double helix the need for a mechanism to relieve torsion of the DNA molecule, torsion developed in connection with semiconservative replication. The latter proposal that non-DNA linkers mi t serve to allow rotation of the DNA double helix during semiconservative replication is no longer considered valid since it has been repeatedly demonstrated that a single-stranded break in a DNA double helix allows the intact chain to serve as a swivel and release torsion within the molecule (Vlnograd and Lebowitz, 1966). 

That DNA synthesis is prokaryotes is associated with the cell membrane is supported by two findings. The first is the electron microscopic evidence (Jacob et al., 1963) that shows the bacterial chromosome is attached to the membrane surface. The second is the isolation of DNA-membrane fractions from E, coli capable of supporting semiconservative replication (Knippers and Stratling, 1970 Smith et al., 1970). These systems mark the nearest approach so far in vitro to the form of replication which prevails in vivo. 

Fig. 138. Taylor s autoradiographic investigations on Vida faba. The semiconservative replication of only one chromosome is depicted. Radioactively labeled strands are red. Blackening of the photographic plate by tritium radiation = red dots (modified from Taylor et al. 1957).

Subsequent studies initiated by others have shown that the DNA in all chromosomes regardless of their size replicate in this semiconservative fashion. Thus, the semiconservative mode of DNA replication is well established and essentially universal. The genetic information implanted in... 

DNA replication proceeds by the synthesis of one new strand on each of the parental strands. This mode of replication is called semiconservative, and it appears to be universal. DNA synthesis initiates from a primer at a unique point on a prokaryotic template such as the E. coli chromosome. From the initiation point, DNA synthesis proceeds bidirectionally on the circular bacterial chromosome. The bidirectional mode of synthesis is not followed by all chromosomes. For some chromosomes, usually small in size, replication is unidirectional. 

From the complementary duplex structure of DNA described in chapter 25, it is a short intuitive hop to a model for replication that satisfies the requirement for one round of DNA duplication for every cell division. In chapter 26, DNA Replication, Repair, and Recombination, key experiments demonstrating the semiconservative mode of replication in vivo are presented. This is followed by a detailed examination of the enzymology of replication, first for how it occurs in bacteria and then for how it occurs in animal cells. Also included in this chapter are select aspects of the metabolism of DNA repair and recombination. The novel process of DNA synthesis using RNA-directed DNA polymerases is also considered. First discovered as part of the mechanisms for the replication of nucleic acids in certain RNA viruses, this mode of DNA synthesis is now recognized as occurring in the cell for certain movable genetic segments and as the means whereby the ends of linear chromosomes in eukaryotes are synthesized. 

Watson and Crick (see Volume 1) immediately saw the relationship of the double helix to genetic replication. They proposed that each strand of the chromosome serves as a template to specify a new, complementary DNA strand. A template is a pattern for making something DNA acts as a template because each strand specifies the new daughter strand by base-pairing. This template feature makes DNA replication semiconservative after replication, each daughter chromosome has one strand of newly synthesized DNA and one strand of DNA from the parental chromosome. See Figure 8-6. 

The answer is d. (Murray, pp 412—434. Scriver, pp 3—45. Sack, pp 3-29. Wilson, pp 99-121.) Like bacterial DNA, eukaryotic DNA is replicated in a semiconservative manner. However, in contrast to most bacterial DNA, which is circular in structure, nuclear chromosomal DNA is a single, uninterrupted molecule that is linear and unbranched. A eukaryotic chromosome contains a strand of DNA at least 100 times as large as the DNA molecules found in prokaryotes. Eukaryotic, but not prokaryotic, DNA molecules are bound to small basic proteins called histones. The histone-DNA complex formed is referred to as chromatin. 

Each daughter chromosome contains one of the parental DNA strands and one newly synthesized, complementary strand. Therefore, replication is said to be semiconservative i.e., the parental strands are conserved but are no longer together. Each one is paired with a newly synthesized strand (see Figs. 13.1 and 13.2). 

Fig. 13.8. Replication of a eukaryotic chromosome. Synthesis is bidirectional from each point of origin (O) and semiconservative, each daughter DNA helix contains one intact parental strand (solid line) and one newly synthesized strand (dashed line).

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