II. THE PROKARYOTIC CELL: BACTERIA
B. PROKARYOTIC CELL STRUCTURE
3. Structures Located Within the Cytoplasm
b. The Nucleoid
The overall purpose of this Learning Object is:
1) to learn the chemical makeup and the functions associated with the nucleoid of bacteria;
2) to introduce the relationship between components of the bacterial nucleoid and the initiation of body defenses; and
3) to introduce how certain antimicrobial agents intefere with bacterial DNA replication.
LEARNING OBJECTIVES FOR THIS SECTION
In this section on Prokaryotic Cell
Structure we are looking at the various organelles or structures that make up
a bacterium. As mentioned in the introduction to this section, a typical bacterium
usually consists of:
Structures located within the cytoplasm of bacteria include the nucleoid, ribosomes, and in some bacteria, plasmids, endospores, inclusion bodies, and organelles used for photosynthesis. We will now look at the bacterial nucleoid.
The Nucleoid (def)
A. Structure and Composition
The term genome (def) refers to the sum of an organism's genetic material. The bacterial genome is composed of chromosomal deoxyribonucleic acid or DNA and represents the bacterium's nucleoid. Unlike the eukaryotic nucleus, the bacterial nucleoid has no nuclear membrane or nucleoli (see Fig. 1).
In general it is thought that during DNA replication (discussed in Unit 6), each strand of the replicating bacterial DNA attaches to proteins at what will become the cell division plane. For example, Par proteins function to separate bacterial chromosomes to opposite poles of the cell during cell division. They bind to the origin of replication of the DNA and physically pull or push the chromosomes apart, similar to the mitotic apparatus of eukaryotic cells. (see Fig. 3).
In the center of the bacterium, a group of proteins called Fts (filamentous temperature sensitive) proteins interact to form a ring at the cell division plane. These proteins form the cell division apparatus known as the divisome and are directly involved in bacterial cell division by binary fission. The divisome is responsible for directing the synthesis of new cytoplasmic membrane and new peptidoglycan to form the division septum.
Since bacteria are haploid (def), that is they have only one chromosome and only reproduce asexually, there is also no meiosis (def) in bacteria.
The nucleoid is one long, single molecule of double stranded, helical, supercoiled DNA (def). In most bacteria, the two ends of the double-stranded DNA covalently bond together to form both a physical and genetic circle (def). The chromosome is generally around 1000 µm long and frequently contains as many as 3500 genes (see Fig. 2). E. coli, a bacterium that is 2-3 µm in length, has a chromosome approximately 1400 µm long.
To enable a macromolecule this large to fit within the bacterium, histone-like proteins bind to the DNA, segregating the DNA molecule into around 50 chromosomal domains and making it more compact. A group of enzymes called DNA topoisomerases then supercoil each domain around itself, forming a compacted mass of DNA approximately 0.2 µm in diameter. In actively growing bacteria, projections of the nucleoid extend into the cytoplasm. Presumably, these projections contain DNA that is being transcibed into mRNA.Supercoils are both inserted and removed by topoisomerases.
DNA topoisomerases (def) are, therefore, essential in the unwinding, replication, and rewinding of the circular, supercoiled bacterial DNA. In order for the long molecule of DNA to fit within the bacterium, the DNA must be supercoiled. However, this supercoiled DNA must be uncoiled and relaxed in order for DNA polymerase to bind for DNA replication and RNA polymerase to bind for transcription of the DNA. For example, a topoisomerase called DNA gyrase catalyzes the negative supercoiling of the circular DNA found in bacteria. Topoisomerase IV, on the other hand, is involved in the relaxation of the supercoiled circular DNA, enabling the separation of the interlinked daughter chromosomes at the end of bacterial DNA replication.
The nucleoid is the genetic material of the bacterium. Genes (def) located along the DNA are transcribed into RNA that, in the case of mRNA, is then translated into protein at the ribosomes. In other words, DNA determines what proteins and enzymes an organism can synthesize and, therefore, what chemical reactions it is able to carry out.
From Drew Berry, wehi.edu.au. This animation takes some time to load.
C. Significance of the Nucleoid to the Initiation of Body Defense
In order to protect against infection, one of the things the body must initially do is detect the presence of microorganisms. The body does this by recognizing molecules unique to microorganisms that are not associated with human cells. These unique molecules are called pathogen-associated molecular patterns or PAMPS. (Because all microbes, not just pathogenic microbes, possess PAMPs, pathogen-associated molecular patterns are sometimes referred to as microbe-associated molecular patterns or MAMPs.)
Bacterial and viral genomes contain a high frequency of unmethylated cytosine-guanine (CpG) dinucleotide sequences (a cytosine lacking a methyl or CH3 group and located adjacent to a guanine). Mammalian DNA has a low frequency of cytosine-guanine dinucleotides and most are methylated. These unmethylated cytosine-guanine dinucleotide sequences in bacterial DNA are PAMPS that bind to pattern-recognition receptors on a variety of defense cells of the body and triggers innate immune defenses such as inflammation, fever, and phagocytosis.
D. Antimicrobial Agents that Inhibiting Normal Nucleic Acid Replication in Bacteria
Some antibacterial chemotherapeutic affect bacteria by inhibiting normal nucleic acid replication.
- The fluoroquinolones (norfloxacin, lomefloxacin, fleroxacin, ciprofloxacin, enoxacin, trovafloxacin, etc.) work by inhibiting one or more of the topoisomerases (def), the enzymes needed for bacterial nucleic acid synthesis.
- Co-trimoxazole, a combination of sulfamethoxazole and trimethoprim, block enzymes in the bacteria pathway required for the synthesis of tetrahydrofolic acid, a cofactor needed for bacteria to make the nucleotide bases thymine, guanine, uracil, and adenine. Without the tetrahydrofolic acid, the bacteria cannot synthesize DNA or RNA.
Antimicrobial chemotherapy will be discussed in greater detail later in Unit 2 under Control of Bacteria by Using Antibiotics and Disinfectants.
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Updated: Aug., 2012
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