Replication bidirectional

Figure 36-17. The generation of "replication bubbles" during the process of DNA synthesis. The bidirectional replication and the proposed positions of unwinding proteins at the replication forks are depicted.

Each eukaryotic chromosome contains one linear molecule of DNA having multiple origins of replication. Bidirectional replication occurs by means of a pair of replication forks produced at each origin. Completion of the process results in the production of two identical linear molecules of DNA. DNA replication occurs in the nucleus during the S phase of the eukaryotic cell cycle. The two identical sister chromatids are separated om each other when the ceU divides during mitosis. 

Bidirectional replication is initiated at the origin of a 6,000 bp plasmid in vitro, in the absence of topoisomerases. The plasmid initially has a a of -0.06. How many base pairs will be unwound and replicated by each replication fork before the forks stall Assume that each fork travels at the same rate and that each includes all components necessary for elongation except topoisomerase. 

The use of autoradiographic methods confirmed bidirectional replication. Strains of amino acid aux-otrophs with small nucleoside triphosphate pools were used. The addition of amino acids after starvation led to initiation of replication with only a 6-min lag. The cells were labeled with [3H]thymidine, and after the replication forks had moved a short distance from the origin of replication the cells were given a pulse of "super-hot" [3H]thymidine. Using autoradiography it was possible to observe the clearly bidirectional replication forks378 (Fig. 27-17). Replication in other bacteria is also bidirectional. 

Figure 27-19 Hypothetical scheme for initiation of bidirectional replication in E. coli. The closed boxes R1 through R5 represent the 9-residue recognition sequences for the E. coli dnaA protein. The open boxes 1, 2, and 3 represent the three 13-residue repeats, possible sites for binding of the dnaB-dnaC protein complex. From McMacken et al.m Redrawn in simplified form.

Amplification of DNA of chromosomes. During formation of oocytes parts of the DNA are "amplified" by repeated replication. This provides a way for the ovum to accumulate ribosomal RNA and various proteins in large amounts. Similarly, genes for two abundant proteins of the egg shell or chorion of insects are amplified. Bidirectional replication initiated at discrete positions yields an "onion skin" structure containing many copies of an 90-kb sequence containing the two genes. The polyploidy observed in some highly specialized cells such as the Purkinje cells... 

Evidence for bidirectional replication is supported by additional experiments. Growing bacteria were briefly ex-... 

Schematic diagrams of two different modes of DNA synthesis at the growth fork(s). In unidirectional replication (a) one growth fork occurs in bidirectional replication (b) two occur. Red indicates regions containing newly synthesized DNA.

Bidirectional replication. Replication in both directions away from the origin, as opposed to replication in one direction only (unidirectional replication). 

Yes, both are actively involved in DNA replication and move at approximately equal rates in opposite directions around the circular molecule. This is known as bidirectional replication. The replicated portion of the molecule is referred to as a replication bubble or eye form (because of its appearance in diagrams). The size of the bubble varies from being extremely small up to nearly twice the size of the nonreplicating chromosome. Obviously, the site on the circular molecule of a very small bubble represents the region within which replication was initiated. 

DNA polymerase activity and 3 —> 5 direction exonuclease activity. Because there are 2 strands to be replicated the 2 strands of the dsDNA have to unwind and in a circular dsDNA bidirectional replication results in a replication bubble bounded by two Y-shaped replication forks that move around the circle. The continuous or leading strand is made unbroken around its ssDNA template. However the other (antiparallel) strand (the lagging strand), proceeding from the same starting point as the dsDNA opens up, is made with the same 5 3 ... 

In the third mechanism two replication forks are initiated at the origin and as synthesis proceeds the two forks migrate away from one another. This type of replication is called bi-directional. Most organisms, including mammals, use bidirectional replication (Fig 11.13). 

In both bacterial and plasmid DNA, replication is initiated at a unique origin and proceeds in both directions along the DNA molecule to a termination site that is located approximately 180 degrees from the origin. Thus, replication of DNA is, in most cases, bidirectional replication. The ability of DNA to replicate from many origins and in both directions means that cells can replicate all of their DNA in a fairly short period of time. The shortest generation time for the bacterium E. coli is approximately 30 minutes in rich medium in the laboratory whereas the shortest division time for a human cell is approximately 24 hours (Table 24-1). 

Location of A-T rich sequences and the four 9-base-pair repeats within OriC. (B). Model showing how DnaA protein binding to the 9-base-pair repeats leads to melting of the A-T rich sequence and formation of the two bidirectional replication forks. 

Topoisomerases - Bidirectional replication of the circular E. coli chromosome unwinds about 100,000 base pairs per minute. Relief of this torsional stress is essential for DNA replication to occur. Topoisomerases are enzymes with a "swivel" mechanism that can relieve this stress. There are two general classes of topoisomerases, type I (Figure 24.30) and type II (Figure 24.31). Type I enzymes change the DNA linking number (see here for reference) in units of 1, whereas type II enzymes change the linking number in units of 2. 

Fig. 13.2. Bidirectional replication of a circular chromosome. Replication begins at the point of origin (oriC) and proceeds in both directions at the same time.

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