The Chromosomes

One of the most surprising observations made with the aid of this new technique involved a change in the accepted number of chromosomes in humans. For years, the number of chromosomes in human cells had been assumed to be 48 now it is established that they contain only 46 chromosomes. The human diploid cell contains 22 pairs of autosomal chromosomes and one pair of sex chromosomes. In females, the sex chromosomes are identical in males, there is one sex chromosome of the X type and a sex chromosome of the Y type. Thus, the male is the heterogametic sex in humans as Wilson [47] postulated previously. 

Cross-of red-eyed female with white-eyed male 

The mode of inheritance of numerous human diseases is similar to that just described for the sex-linked trait in Drosophila. Two diseases are most typical in this respect—color blindness and hemophilia. Color blindness is a rather common disease, but the distribution among male and female varies considerably. The 

2nd generation half the males have white eyes 

1st generation all females red-eyedj all males white-eyed 

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In the style of the Darwinian Theory, the quality of a chromosome is called its fitness. The quality or fitness of a ehromosome is usually caleulated with the help of an objeetive function, which is a mathematical function indicating how good the solution, and thus the chromosome, is for the optimization problem. This computation of the fitness is done for each chromosome in each population,... 

Roulette wheel selection In this selection variant the probability for selecting a chromosome is proportional to its fitness. The idea can be illustrated when we imagine a roulette wheel, where a slot is allocated to each chromosome and the size of the slot is chosen with respect to the quality of the chromosome. When the wheel is spinning the chromosomes with a better quality are more likely to be chosen than those of a minor quality. Figure 9-28 illustrates this procedure,... 

Truncation selection Prior to the application of this selection variant the chromosomes are ranked according to their fitness. The top 1 jx of the population are then selected and each of these chromosomes gets x copies for the population, c.g., the top onc-third of the chromosomes are each given three copies in the population. All other chromosomes are neglected. 

For the so-called one-point crossover a position within the chromosomes is randomly picked at the same position in both parents and the chromosomes are both cut at this position. Then the first part of chromosome 1 is concatenated with the second part of chromosome 2, and vice versa. This procedure is shown in Figure 9-29. 

Thus, if the chromosome is represented as a bit string, the mutation operator will change one bit in the string as illustrated in Figure 9-30. Usually, the mutation operator is applied with only a small probability. 

Figure 9-31. Descriptor selection using a CA. The upper part of the figure shows the original set of descriptors, below which Is the chromosome, Those genes containing the value 1 are selected for the final set of descriptors, which is given at the bottom.

As explained in Chapter 8, descriptors are used to represent a chemical structure and, thus, to provide a coding which allows electronic processing of chemical data. The example given here shows how a GA is used to Rnd an optimal set of descriptors for the task of classification using a Kohoncii neural network. The chromosomes of the GA are to be used as a means for selecting the descriptors they indicate which descriptors are used and which are rejected ... 

For the selection of descriptors, GA simulated evolution of a population. Each individual of the population represents a subset of descriptors and is defined by a chromosome of binary values. The chromosome has as many genes as there are possible descriptors (92 for the aromatic group, 119 for non-rigid aliphatic,... 

The chromosome in a genetic algorithm codes for the torsion angles of the rotatable bonds. 

Most plasmids are topologically closed circles of DNA. They can be separated from the bulk of the chromosomal DNA by virtue of their resistance to alkaline solution. The double-stranded stmcture of DNA is denatured at high pH, but because the two strands of the plasmid are topologically joined they are more readily renatured. This property is exploited in rapid procedures for the isolation of plasmid DNA from recombinant microorganisms (5,6). 

Plasmid Vectors for Facile Introduction of Passenger DNA and Selection of Recombinants. The map of a commonly used plasmid vector, pUC19 (7), is shown in Figure 2. Three parts of the vector are key to its utility. The origin sequence, oh, allows the repHcation of plasmid DNA in high copy number relative to the chromosome. A gene, amp, encoding the enzyme beta-lactamase, which hydrolyzes penicillin compounds, allows... 

Bacteria produce chromosomady and R-plasmid (resistance factor) mediated P-lactamases. The plasmid-mediated enzymes can cross interspecific and intergeneric boundaries. This transfer of resistance via plasmid transfer between strains and even species has enhanced the problems of P-lactam antibiotic resistance. Many species previously controded by P-lactam antibiotics are now resistant. The chromosomal P-lactamases are species specific, but can be broadly classified by substrate profile, sensitivity to inhibitors, analytical isoelectric focusing, immunological studies, and molecular weight deterrnination. Individual enzymes may inactivate primarily penicillins, cephalosporins, or both, and the substrate specificity predeterrnines the antibiotic resistance of the producing strain. Some P-lactamases are produced only in the presence of the P-lactam antibiotic (inducible) and others are produced continuously (constitutive). 

One method used to achieve genetic stabiUty is to insert the plasmid or the recombinant DNA direcdy into the chromosome. Since no cell can afford to lose a chromosome, this assures that the recombinant DNA is not lost as long as it remains an integral part of the chromosome. 

Genetic damage can take place at the level of the chromosome or at the gene level. In addition, chemicals can also induce alterations in the number of chromosomes in the cells. Aneuploidy is an excess or a shortage of a single chromosome. Polyploidia is an excess of a whole set of chromosomes tn the ceil. 

Eukaryotic ceils possess a discrete, membrane-bounded nucleus, the repository of the cell s genetic material, which is distributed among a few or many chromosomes. During ceil division, equivalent copies of this genetic material must be passed to both daughter ceils through duplication and orderly partitioning of the chromosomes by the process known as mitosis. Like prokaryotic... 

Nucleus The nucleus is separated from the cytosol by a double membrane, the nuclear envelope. The DNA is complexed with basic proteins (histones) to form chromatin fibers, the material from which chromosomes are made. A distinct RNA-rich region, the nucleolus, is the site of ribosome assembly. The nucleus is the repository of genetic information encoded in DNA and organized into chromosomes. During mitosis, the chromosomes are replicated and transmitted to the daughter cells. The genetic information of DNA is transcribed into RNA in the nucleus and passes into the cytosol where it is translated into protein by ribosomes. 

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