Bacterial DNA Replication and Cell Division Help (page 2)

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By — McGraw-Hill Professional
Updated on Aug 24, 2011

Sigma Replication

Another type of bacterial replication is used to transfer a linear DNA molecule during bacterial conjugation or for the production of linear phage genomes. A nick occurs in one strand of a DNA double helix, creating free 3'-OH and 5'-P termini. Helicase and SSB proteins establish a replication fork. No primer is necessary because a strand with a free 3'-OH is available for elongation by DNA polymerase III as the leading strand. Simultaneously with replication of the leading strand, the template for the lagging strand is displaced. The displaced strand is discontinuously replicated to produce Okazaki fragments in the usual way (see Fig. 3-11).

The result of this replication model is a circle with a linear tail, resembling the Greek letter sigma (σ). Hence, this model is called sigma replication or rolling-circle replication (Fig. 10-4).

The circle may revolve several times, creating concatemers or covalently connected, linear repetitions of bacterial genomes. An endonuclease makes cuts at slightly different positions on each DNA strand of the concatemer to create genome-sized segments containing "sticky ends" (single-stranded complementary ends). The linear genomes circularize by base pairing of the sticky ends. DNA ligase seals each gap to create covalently closed (circular), double-stranded DNA molecules.

Binary Fission

A replicating bacterial chromosome is thought to be attached to invaginations of the cell membrane at each replication fork. After DNA replication, the cell elongates by growth of the sector between the two attachment points, causing the two chromosomal replicas to move apart. A septum of new cell membrane and wall is then synthesized between the two chromosomes, creating two progeny cells (Fig. 10-5). The "passing down" of DNA from parent to progeny cell is called vertical gene transfer and the overall process of bacterial cell division is called binary fission.

Bacterial DNA Replication and Cell Division

Fig. 10-5. A model for segregation of bacterial DNA ("chromosome") replicas. (1) A circular DNA molecule is attached to invaginations of the cell membrane at two points. The DNA is a theta structure (about half replicated). (2) Replication is complete. (3) Cell division is beginning via growth of the membrane region shown in medium gray. Both daughter chromosomes are already partially replicated. (4) Cell division is complete. (5) Daughter cells are midway through the next generation. The membrane attachment points have moved to the center of the cell as a result of growth of new membrane (dark gray) after cell division. (After G. S. Stent and R. Calendar, Molecular Genetics, 2nd ed., W. H. Freeman and Company, New York, 1978.)

When bacteria are growing exponentially, most cells contain two to four identical chromosomes in various stages of replication. If a mutation occurs during replication, the new copy of DNA will be slightly different from the parental template. If this mutation occurs in a coding region that results in a defective protein, the effect of the mutation (i.e., the phenotype) will only be observable once the new cell has depleted its level of wild-type protein. This is a phenomenon known as phenotypic lag.

EXAMPLE 10.4 Resistance to a specific bacteriophage can be acquired by mutation of a gene responsible for the phage receptor on the cell's surface. Resistance cannot be fully realized until the receptor sites (synthesized under the direction of the former phage-sensitive genotype) have been completely diluted out through successive cell divisions. If even one receptor remains on the mutant cell, it is still susceptible to phage infection. Thus, many cell generations may be required before a phage-resistant mutation can be fully expressed in a progeny cell.

Practice problems for these concepts can be found at:

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