Prokaryotic cells transcription

Functional mRNA is single-stranded. In prokaryotic cells, transcription and translation are usually coupled mRNA becomes bound to ribosomes and translation begins before transcription is complete. The messenger is usually translated by several ribosomes at once, and thus several to many protein molecules are made from it. In prokaryotes, however, mRNA lifetimes are short, with half-times of several minutes Eukaryotic mRNA is normally stable for hours or days... 

Busby S, Ebright RH Promoter structure, promoter recognition, and transcription activation in prokaryotes. Cell 1994 79 ... 

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. 

There are three major types of RNA that participate in the process of protein synthesis ribosomal RNA (rRNA), transfer RNA (tRNA), and messenger RNA (mRNA). They are unbranched polymers of nucleotides, but differ from DNA by containing ribose instead of deoxyribose and uracil instead of thymine. rRNA is a component of the ribosomes. tRNA serves as an adaptor molecule that carries a spe dfic amino acid to the site of protein synthesis. mRNA carries genetic information from the nuclear DNA to the cytosol, where it is used as the template for protein synthesis. The process of RNA synthesis is called transcription, and its substrates are ribonucleoside triphosphates. The enzyme that synthesizes RNA is RNA polymerase, which is a multisub-irit enzyme. In prokaryotic cells, the core enzyme has four subunits—... 

We have seen how interactions between DNA-binding proteins such as CAP and RNA polymerase can activate transcription in prokaryotic cells (Section 31.1.6). Such protein-protein interactions play a dominant role in eukaryotic gene regulation. In contrast with those of prokaryotic transcription, few eukaryotic transcription factors have any effect on transcription on their own. Instead, each factor recruits other proteins to build up large complexes that interact with the transcriptional machinery to activate or repress Panscription. 

Gene expression in human cells is regulated primarily at the level of transcription as it is in prokaryotic cells. However, because transcription is more complex in eukaryotic cells, gene expression can be regulated at many different... 

In prokaryotic cells, which have no nuclei, translation of an mRNA into protein can begin from the 5 end of the mRNA even while the 3 end is still being synthesized by RNA polymerase. In other words, transcription and translation can occur concurrently in prokaryotes. In eukaryotic cells, however, not only is the nucleus separated from the cytoplasm where translation occurs, but also the primary transcripts of protein-coding genes are precursor mRNAs (pre-mRNAs) that must undergo several modifications, collectively termed RNA processing, to yield a functional mRNA (see Figure... 

Figure 3.2 Summary illustration of prokaryotic cell such as Escherichia coli. Prokaryotic cells do not exhibit discrete compartmentalization so transcription and translation can take place concomitantly (Reproduced from Voet Voet, 1995 [Wiley] Fig. 1-2).

Although many RNAs and proteins are produced in even a simple prokaryotic cell, not all of them are produced at the same time or in the same quantities. In prokaryotes, the control of transcription is largely responsible for controlling the level of protein production. In fact, many equate transcription control with gene expression. 

Protein synthesis in prokaryotes takes place as a coupled process with simultaneous transcription of mRNA and translation of the message in protein synthesis. This is possible because of the lack of compartmentaliza-tion in prokaryotic cells. In eukaryotes, mRNA is transcribed and processed in the nucleus and only then exported to the cytoplasm to direct protein synthesis. 

Describe the differences in the transcription and translation processes as they occnr in prokaryotic cells and eukaryotic cells. 

Derepression the release of an operon from repression of transcription. In prokaryotic cells it occurs by inactivation of a repressor, either by removal of a corepressor (see Enzyme repression) or by binding of an inducer (see Enzyme induction). D. in eukaryotic cells involves regulatory proteins and effectors, such as hormones (see Gene activation). 

The most important difference between RNA synthesis in eukaryotic and prokaryotic cells is, however, at the level of the primary transcript, particularly in the case of hnRNA which must undergo three types of post-transcriptional modification (Figure 21.2) before it is transported to the cytoplasm as a fully mature mRNA molecule. These processes are as follows, (i) Modification of the 5 end of the RNA to form a cap structure. This entails the enzyme addition of a guanosine residue via a 5 -5 triphosphate linkage to the 5 end, as well as varying degrees of methylation of the first two or three nucleotides, (ii) In most eukaryotic mRNAs the addition of... 

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