Eukaryotic cells transcription

In eukaryotic cells transcription and translation occur in two distinct temporal and spacial events, whereas in prokaryotic cells they occur in one step. As humans have eukaryotic cells, we will look at this process. Transcription occurs on DNA in the nucleus and translation occurs on ribosomes in the cytoplasm. 

Although fundamentally similar in bacteria and eukaryotic cells, transcription and translation must differ in details because of the difference in structural organization and of the surrounding environment. The environment of bacterial cells may change suddenly and considerably, but that of mammalian cells is stable because of homeostatic control. Therefore, metabolic patterns are much more stable in mammalian than in bacterial cells, and ordinarily there is no need for the sudden appearance of enzyme clusters like those that appear in bacteria challenged by inducers. Thus, it may not be necessary to produce polycistronic messenger in mammalian cells. Moreover, the mammalian cell may find it advantageous to maintain stable templates. 

Plasmid-. Small circular DNA molecule that can replicate independently of the genomic DNA. Used extensively as a vector for DNA cloning. Is usually amplified in E. coli from where it can be isolated and used for many applications, e.g., gene transfer into eukaryotic cells. Transcription-. Copying of one strand of DNA into a complementary RNA sequence by the enzyme RNA polymerase. 

FIGURE 11.24 The properties of mRNA molecules in prokaryotic versus eukaryotic cells during transcription and translation. 

The eukaryotic expression cassette is the part of an expression vector that enables production of a protein in a eukaryotic cell. The cassette consists of a eukaryotic promoter for mRNA transcription, the gene and an mRNA termination and processing signal (Poly-A signal). 

Figure 37-1. This figure illustrates that genes can be transcribed off both strands of DNA. The arrowheads indicate the direction of transcription (polarity). Note that the template strand is always read in the 3 to 5 direction. The opposite strand is called the coding strand because it is identical (except for T for L) changes) to the mRNA transcript (the primary transcript in eukaryotic cells) that encodes the protein product of the gene.

It is clear that the signals in DNA which control transcription in eukaryotic cells are of several types. Two types of sequence elements are promoter-proximal. One of these defines where transcription is to commence... 

The mechanisms whereby introns are removed from the primary transcript in the nucleus, exons are ligated to form the mRNA molecule, and the mRNA molecule is transported to the cytoplasm are being elucidated. Four different splicing reaction mechanisms have been described. The one most frequently used in eukaryotic cells is described below. Although the sequences of nu-... 

The relationship between hnRNA and the corresponding mature mRNA in eukaryotic cells is now apparent. The hnRNA molecules are the primary transcripts plus their early processed products, which, after the addition of caps and poly(A) tails and removal of the portion corresponding to the introns, are transported to the cytoplasm as mature mRNA molecules. 

Table 39-4. Gene expression is regulated by transcription and in numerous other ways in eukaryotic cells.

In addition to affecting the efficiency of promoter utilization, eukaryotic cells employ alternative RNA processing to control gene expression. This can result when alternative promoters, intron-exon splice sites, or polyadenylation sites are used. Occasionally, heterogeneity within a cell results, but more commonly the same primary transcript is processed differendy in different tissues. A few examples of each of these types of regulation are presented below. 

Schreck, R., Albermann, K. and Baeuerle, P.A. (1992). Nuclear fector kB an oxidative stress-responsive transcription fector of eukaryotic cells (a review). Free Rad. Res. Commun. 17, 221-237. 

The protein synthesis machinery reads the RNA template starting from the 5 end (the end made first) and makes proteins beginning with the amino terminus. These directionalities are set up so that in prokaryotes, protein synthesis can begin even before the RNA synthesis is complete. Simultaneous transcription-translation can t happen in eukaryotic cells because the nuclear membrane separates the ribosome from the nucleus. 

What are the mechanisms by which trichothecenes exert their transcriptional and post-transcriptional effects The 60S ribosomal subunit is a well-known molecular target of trichothecenes in leukocytes and other actively proliferating eukaryotic cells,3 whereas attempts to demonstrate alternative receptors have not been successful.37 38 Translational inhibitors that bind to ribosomes rapidly activate mitogen-activated protein kinases (MAPKs) and apoptosis via a mechanism termed the ribotoxic stress response. 39-40... 

Rutherford, J.C. and Bird, A.J. (2004) Metal-responsive transcription factors that regulate iron, zinc, and copper homeostasis in eukaryotic cells, Eukaryot. Cell, 3, 1-13. 

Technically, in vitro transcription is achieved from standard expression plasmids typically carrying SP6 or T7 promoters using marketed kits. Translation into the polypeptide may be either coupled directly to the transcription (in vitro TnT) or require isolation of the RNA. Again, a large number of suitable prokaryotic and eukaryotic cell extracts as well as complementation factors are freely available. 

Heterogeneous nuclear RNA (huRNA or pre-mRNA), which is found only in the nucleus of eukaryotic cells. It represents precursors of mRNA, formed during its post-transcriptional processing. 

---