The Unusual Nuclear And Mitochondrial Genomes Of T Cruzi

As with the other trypanosomatids, the chromosomes of T. cruzi do not condense during mitosis, and karyotype studies have had to rely on pulse field gel electrophoresis and related techniques. These indicate that the nuclear genome is organized into about 40 chromosomes ranging from 0.4 to 4.5 megabases. The genome size (haploid content) is 45 megabases and the total number of genes has been estimated to fall within the range of 8-10,000. There is significant variation in the size of chromosomes between different parasite strains and, although the genome is generally diploid, the sizes of homologous chromosomes within the same cell can often differ considerably. The genome contains an extensive range of repetitive DNA elements and several large surface glycoprotein gene families, some with over 500 members. T. cruzi has an extremely complex cell surface, and this large and diverse repertoire of genes, many of which are expressed in a stage-specific manner, is critical to several aspects of the parasite life cycle (Fig. 1). These include host cell invasion, protection from complement and disruption of the host immune response.'12- In the parasite genome there are no recognizable RNA polymerase II-specific promoters for protein-coding genes; transcription is polycistronic and each mRNA is modified posttranscriptionally by addition of spliced leader RNA to its 5' end. In the case of surface antigen genes, evidence is now accumulating that their stage-specific expression is regulated predominantly at the level of RNA stability.'13-T. cruzi contains a single mitochondrion that forms a complex structure that extends throughout much of the cell (Fig. 2). The mitochondrial genome, which contains 20-25% of the total cellular DNA, is sublocalized at a specific site organized as a tight compact disc called the kinetoplast[14] (Fig. 2). It contains two classes of circular DNA, maxicircles and minicircles, which form a concatenated network. The maxicircle DNAs are between 20 and 30 kb and are present in 25-50 copies per cell. The minicircle DNAs, of which there are 5-10,000 copies per cell, have a size range from 0.5 to 2.5 kb. Maxicircle DNA is analogous to the mitochondrial genomes of other organisms and contains genes encoding ribosomal RNA and components of the mitochondrial respiratory system. Most unusually, many of the primary transcripts of the protein coding genes do not contain a complete open reading frame and need to undergo RNA editing to produce functional mRNA.[15] This requires posttran-scriptional insertion or deletion of uridine residues, a process mediated by small transcripts called guide RNAs that are encoded by the minicircle DNAs. Minicircles have a conserved region, which encompasses the postulated origin of replication and a region of high variability that reflects the repertoire of expressed guide RNAs.

Getting Started With Dumbbells

Getting Started With Dumbbells

The use of dumbbells gives you a much more comprehensive strengthening effect because the workout engages your stabilizer muscles, in addition to the muscle you may be pin-pointing. Without all of the belts and artificial stabilizers of a machine, you also engage your core muscles, which are your body's natural stabilizers.

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