Extra-chromosomal genetics

When genetic information (nucleic acid) is transferred between different cells, species or genera it is often carried by a specialized DNA molecule called a vector. Viruses are natural vectors, as are some kinds of small independently replicating circular extra-chromosomal DNA molecules (plasmids). A few of the basic features of plasmids used in molecular biology are reviewed in Figure 2.6. 

Maynard Smith I don t think that need contradict what has been said here. The difference between someone with Down s syndrome and their parents is that the child has an extra chromosome. The child will still resemble the parents at other gene loci. There would still be genetic similarity between parent and offspring. 

The transport of the therapeutic transgenes toward the nuclei of the target cells can be carried out both by viral [19] and nonviral vectors such as plasmid DNA [20]. Plasmid DNA molecules are extra-chromosomal carriers of genetic information that have the ability to replicate autonomously. These vectors constitute an attractive gene transfer system because they are safer and easier to produce when compared with viral vectors [20-22]. However, plasmid DNA vectors are less effective in transfecting cells when compared with viral vectors, which have a natural ability to deliver and express their genes in a wide variety of cell types and tissues [11,23]. 

This may be seen in genes related to potassium. For plants, potassium is the element of summer and of stress. When summer heat and droughts stress the plant, it needs extra potassium to balance the shifts in osmotic pressure. Plants that make extra chromosomes have more genetic space to evolve genes that move the potassium around. Extra DNA gives the plant a better ability to control its potassium and respond to its environment. 

Some chromosomal conditions are caused by changes in the number of chromosomes. These changes are not inherited, but occur as random events during the formation of reproductive cells (eggs and sperm). An error in cell division called nondisjunction results in reproductive cells with an abnormal number of chromosomes. For example, a reproductive cell may accidentally gain or lose one copy of a chromosome. If one of these atypical reproductive cells contributes to the genetic makeup of a child, the child will have an extra or missing chromosome in each of the body s cells. 

Aneuploidy, which indicates a deviation from the euploid number of chromosomes, is the term used to describe the loss or gain of specific chromosomes. Two major types of aneuploidy are observed trisomy (three copies of a specific chromosome) and monosomy (one copy of a specific chromosome). Monosomies and trisomies are usually caused by nondisjunction (the failure of the two members of the chromosome pair to disjoin or separate) during meiosis (Fig II-3-2). All autosomal monosomies are lethal, but trisomies of three different autosomes (13,18, and 21) are compatible with survival to term in at least some cases. This difference illustrates the fact that the body tolerates extra genetic material more successfiiUy than a loss of genetic material. 

H. J. Muller s discovery in 1927 that radiation could induce mutations in living organisms. Whether congenital diseases such as Down s syndrome (in which an extra copy of a particular chromosome, or aneu-ploidy, is present) are much influenced by chemical mutagens in the environment remains an active area of study by genetic toxicologists. 

A favored method for constructing merodiploids is to infect the bacterial cell with a virus or a plasmid DNA that carries the extra genes of interest. The F plasmid is commonly used for this purpose. In strict usage, the genetic representation for a cell carrying the lac operon on the chromosome and the F plasmid would be z+y+a+l/Fz+y+a+,... 

Cytogenetics is the oldest of the medical genetic sciences, but it did not become important clinically until 1959, when the presence of an extra copy of chromosome 21 (Trisomy 21) was first implicated in Down syndrome. Since then, many other chromosomal abnormalities have been directly associated with different diseases. By viewing metaphase cells with the light microscope, trained personnel can detect a broad range of numerical and structural chromosome anomalies that can then be correlated to specific diagnoses ranging from Down syndrome and Thrner syndrome to various types of cancer. 

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