Replication And Expression Of Genetic Information

Replication

Bacteria multiply by cell division that results in the production of two daughter cells from one parent cell. As part of this process, the genome must be replicated so that each daughter cell receives the same complement of functional DNA. Replication is a complex process that is mediated by various enzymes, such as DNA polymerase and cofactors; replication must occur quickly and accurately. For descriptive purposes, replication may be considered in four stages that are depicted together in Figure 2-4:

  1. Unwinding or relaxation of the chromosome's supercoiled DNA
  2. Unzipping, or disconnecting, the complementary strands of the parental DNA so that each may serve as a template (i.e., pattern) for synthesis of new DNA strands
  3. Synthesis of the new DNA strands
  4. Termination of replication with release of two identical chromosomes, one for each daughter cell

Relaxation of supercoiled chromosomal DNA is required so that enzymes and cofactors involved in replication can access the DNA molecule at the site where the replication process will originate (i.e„ origin of replication). On exposure of the replication site (a specific sequence of approximately 300 base pairs that is recognized by several initiation proteins), unzipping of the complementary strands of parental DNA begins. Each parental strand serves as a template for the synthesis of a new complementary daughter strand. The site of active replication is referred to as the replication fork; there are two during the replication process. Each replication fork moves through the parent DNA molecule in opposite directions so that replication is a bidirectional process. Activity at each replication fork involves different cofactors and enzymes, with DNA polymerases playing a central role. Using each parental strand as a template, DNA polymerases add nucleotide bases to each growing daughter strand in a sequence that is complementary to the base sequence of the template (parent) strand. The complementary bases of each strand are then crosslinked. The new nucleotides can only be added to the 3' hydroxyl end of the growing strand so that synthesis for each daughter strand only occurs in a 5' to 3' direction.

Termination of replication occurs when the two replication forks meet, resulting in two complete

Figure 2-4 Bacterial DNA replication depicting bidirectional movement of two replication forks from origin of replication. Each parent strand serves as a template for production of a complementary daughter strand and, eventually, two identical chromosomes.

Replication Factory Bacteria

Figure 2-4 Bacterial DNA replication depicting bidirectional movement of two replication forks from origin of replication. Each parent strand serves as a template for production of a complementary daughter strand and, eventually, two identical chromosomes.

chromosomes, each containing two complementary strands, one of parental origin and one newly synthesized daughter strand. Although the time required for replication can vary among bacteria, the process generally takes approximately 40 minutes in rapidly growing bacteria such as E. coli. However, the replication time for a particular bacterial strain can vary depending on environmental conditions such as the availability of nutrients or the presence of toxic substances (e.g., antimicrobial agents).

Expression of Genetic Information

Gene expression is the processing of information encoded in genetic elements (i.e., chromosomes, plasmids, and transposons) that results in the production of biochemical products. The overall process is composed of two complex steps, transcription and translation, and requires various components, including a DNA template representing a single gene or cluster of genes, various enzymes and cofactors, and RNA molecules of specific structure and function.

Transcription. Gene expression begins with transcription, which converts the DNA base sequence of the gene (i.e., the genetic code) into an .mRNA (messenger RNA) molecule that is complementary to the gene's DNA sequence (Figure 2-5). Usually only one of the two DNA strands (the sense strand) encodes for a functional gene product, and this same strand is the template for mRNA synthesis.

RNA polymerase is the enzyme central to the transcription process. The enzyme is composed of four protein subunits (a [two copies], p, P') and a sigma (o) factor. Sigma factor is loosely affiliated with the enzyme structure and identifies the appropriate site on the DNA template where transcription of mRNA is initiated. This initiation site is also known as the promoter sequence. The remainder of the enzyme

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