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The genetic regions
©2001 Timothy Paustian, University of Wisconsin-Madison
Bacterial cells lack a membrane defined nucleus. However a descrete region in the bacterial cytoplasm seems to contain the genetic material and this nucleiod region can often be distinguished on EMs of cells. Most cells have only one main chromosome although a few species do have 2 or more. For the rest of this section we will focus on the majority of bacteria; assuming one chromosome per cell.
The Bacterial Chromosome
The bacterial chromosome consists of a single, circle of deoxyribonucleic acid. To learn more about basic nucleic acid stucture follow the link.
DNA is double stranded- two strands line up antiparrallel to each other and the bases are linked together with hydrogen bonds.
Due to the nature of these bonds the strands form a left handed double helix
The entire molecule is a linear sequence of bases. The typical E. coli cell has 4.6 x 106 bases. This would make a strand of DNA 1400 µm long, but E. coli is only 2-3 µm long. How does it fit? The size of bacterial DNA is halved by the circular structure of the DNA, but much more is needed of course. A series of strategies are employed by microbes to compact the DNA, yet leave it accessible for its necessary functions.
The relaxed circular chromosome, with a diameter of appoximately 430 µm, is first segregated into about 50 chromosomal domains by DNA-protein interactions with nucleotide binding proteins such as HU, IHF and H-NS. There is sufficient HU protein in the cell to bind the DNA every 300 to 400 bp and these protiens are thought to act in a fashion similar to the histones in eucaryotic organisms. An intact chromosome with its full complement of DNA binding proteins will result in a particle with a maximum diameter of 17 µm.
The DNA is further compacted by twisting the DNA in each domain around itself, called supercoiling.
Figure 1 - Supercoiling of DNA.
A collection of enzymes (one of the more notable being DNA gyrase) is responsible for winding the DNA. Imagine the DNA as a rubber band held at one end. DNA gyrase twists the DNA about itself causing it to fold over. By repeating this process many times the DNA is organized into a series of supercoiled regions. A fully supercoiled chrmosome will be about 1 µm in diameter and is small enough to fit inside a bacterium.
The single most important purpose of the genetic material of any cell is that it holds all the information necessary for a cell to carry out its many functions. The sequence of bases in the DNA contain this information or genetic code. Generally, this is translated into messanger RNA and then into protein that then carry out the many necessary functions of the cell. This "central dogma" of biology is explained in much greater detail in chapter 7
Figure 2 - DNA is transcribed into RNA that is then translated into protein by the ribosome.
DNA must be able to replicate itself to pass on this genetic information. It must do this precisely. The overall process is surprisingly simple to understand, but involves the action of numerous proteins in a very complex molecular dance. Since DNA is double stranded, the two strands separate and each one serves as a template to make another complementary strand.
This process is known as semiconservative replication. At each cell division, each cell gets one old strand and one new strand. Replication is accomplished by the coordinated efforts of many cellular enzymes (about 20). One of the best understood enzymes is polymerase, which forms the phospodiester bond between the phosphate residue on the sugar of the incoming nucleotide and OH residue on the sugar of the growing DNA chain. Synthesis is always 5' ->3'. This enzyme can also proof read and correct any mistakes made along the way.
Because bacterial chromosomal DNA is supercoiled it makes replication a little trickier. For most bacterial chromosomes, replication of the circular DNA is bi-directional. This leads to a characteristic structure known as a theta structure (looks like the Greek letter theta when viewed in the electron microscope
Figure 3 - A Cartoon of Replicating DNA
Although bacterial cells have only one main chromosome, they may have other pieces of genetic material. These smaller pieces of DNA are known as plasmids and are defined as extrachromosomal pieces of DNA which are capable of autonomous (or self-regulated) replication.
Similar to most bacterial chromosomes, plasmids are covalently closed circular DNA, but considerably smaller. In a few species linear plasmids have been found.
Often we refer to the size of a piece of DNA by the number of 1000s of base pairs (kilobases (kb)) Chromosomal DNA is typically about 4000 kb, plasmid DNA ranges from 1-200 kb. There may also be anywhere from 1-700 copies of a plasmid in a cell.
The function of plasmids is not always known, but they are not normally essential to the host, although their presence generally gives the host some advantage.Examples of advantages plasmids bestow on the host
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