Cellular genome

Fig. 9. The cell cycle. M mitosis G1+S+G2 interphase, i.e., the period between ceU division. During G1 (G for gap) the cellular genome is in the diploid state. This is followed by the S-phase (S for synthesis) during which the DNA is replicated, and finally G2. Insert Cell-cycle analysis of HM02 cells cultured with and without vinblastine. CeU-cycle distribution was determined by staining DNA with propidium iodide, and the number of cells in different phases of the cell cycle was measured with a FACStar flow cytometer. Vinblastine, which disrupts the formation of microtubules, causes cell-cycle arrest in the G2/M-phase...

Table 11.1. Some miniature cellular genomes (from Mas et al, 2004)...

Some well-characterized eukaryotic DNA transposons from sources as diverse as yeast and fruit flies have a structure very similar to that of retroviruses these are sometimes called retrotransposons (Fig. 26-33). Retro-transposons encode an enzyme homologous to the retroviral reverse transcriptase, and their coding regions are flanked by LTR sequences. They transpose from one position to another in the cellular genome by means of an RNA intermediate, using reverse transcriptase to make a DNA copy of the RNA, followed by integration of the DNA at a new site. Most transposons in eukaryotes use this mechanism for transposition, distinguishing them from bacterial transposons, which move as DNA directly from one chromosomal location to another (see Fig. 25-43). 

Of the very few variations in the genetic code that we know of, most occur in mitochondrial DNA (mtDNA), which encodes only 10 to 20 proteins. Mitochondria have their own tRNAs, so their code variations do not affect the much larger cellular genome. 

The most common changes in mitochondria (and the only code changes that have been observed in cellular genomes) involve termination codons. These changes affect termination in the products of only a subset of genes, and sometimes the effects are minor because the genes have multiple (redundant) termination codons. 

The leader sequence of the Serratia marcescens extracellular nuclease has been removed and the gene for the resulting nontransportable protein cloned behind the leftward promoter (PL) of lambda (Ahrenholtz, Lorenz Wackernagel, 1994). In the presence of a temperature-sensitive repressor (cl857), the cells can be induced to produce this altered enzyme by an increase in temperature. This produces the nuclease within the cell and, because it cannot be exported, its intracellular concentrations rise, rapidly degrading the cellular genome. Limitations to this system are that cell survival is reduced to only 2 x 10 5. 

Before one cell can divide into two cells, the cell must make a copy of the cellular DNA so that after cell division, each cell will contain a complete complement of the genetic material. Replication is the cellular process by which DNA or the cellular genome is duplicated with almost perfect (and sometimes perfect) fidelity. The replicative process in prokaryotic cells, such as Escherichia coli (E. coli) cells, is best understood and will be described in detail, and the aspects that differ in replicating eukaryotic cells will be noted. 

Without making a definite commitment, let us assume the hypothesis that expression of viral DNA is the causative factor in the cell transformation to a cancer state. There is certainly a significant body of experiments indicating that this is true in many mammals, but solid evidence in humans still eludes us. The viral genome, incorporated in the cellular genome, is completely repressed for long periods compared to most cells division times. As in the case of lysogenic bacteria, a wide variety of chemical and physi-... 

The level of radioactivity in a single-stranded DNA is taken as an indication of SSBs. Calculation of the approximate number of breaks requires reference to the molecular size of the cellular genome and comparison with the known number of breaks induced in genomes of specific sizes by X-ray and UV-irradiation of known dosage (see Collins, 1977). 

A DNA fragment, which may contain a specific gene, can be isolated from the cellular genome. Genes isolated from eukaryotic cells usually contain introns. 

DEF has been reported to cause extensive alterations in morphological features of erythrocyte and nuclear membranes and affected the permeability properties of rat liver mitochondrial membrane. A reduction in the activity of cytochrome-c-oxidase and NAD. H-oxidase has also been observed. Content of both DNA and RNA decreased in tissues studied within 1 month of DEF intoxication and was usually restored within 3 months. Histological study showed development of necrodystrophy in liver tissue and of fibroplastic glomerulonephritis in kidney. The deteriorating effect of DEF on cellular genome functions... 

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