Gene expression in prokaryotes and eukaryotes pdf

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gene expression in prokaryotes and eukaryotes pdf

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Regulation of gene expression , or gene regulation , [1] includes a wide range of mechanisms that are used by cells to increase or decrease the production of specific gene products protein or RNA. Sophisticated programs of gene expression are widely observed in biology, for example to trigger developmental pathways, respond to environmental stimuli, or adapt to new food sources. Virtually any step of gene expression can be modulated, from transcriptional initiation , to RNA processing , and to the post-translational modification of a protein.

Overview: Eukaryotic gene regulation

Regulation of gene expression , or gene regulation , [1] includes a wide range of mechanisms that are used by cells to increase or decrease the production of specific gene products protein or RNA. Sophisticated programs of gene expression are widely observed in biology, for example to trigger developmental pathways, respond to environmental stimuli, or adapt to new food sources.

Virtually any step of gene expression can be modulated, from transcriptional initiation , to RNA processing , and to the post-translational modification of a protein. Often, one gene regulator controls another, and so on, in a gene regulatory network. Gene regulation is essential for viruses , prokaryotes and eukaryotes as it increases the versatility and adaptability of an organism by allowing the cell to express protein when needed.

In multicellular organisms, gene regulation drives cellular differentiation and morphogenesis in the embryo, leading to the creation of different cell types that possess different gene expression profiles from the same genome sequence.

Although this does not explain how gene regulation originated, evolutionary biologists include it as a partial explanation of how evolution works at a molecular level , and it is central to the science of evolutionary developmental biology "evo-devo".

Any step of gene expression may be modulated, from the DNA-RNA transcription step to post-translational modification of a protein. The following is a list of stages where gene expression is regulated, the most extensively utilised point is Transcription Initiation:.

Hence these modifications may up or down regulate the expression of a gene. Some of these modifications that regulate gene expression are inheritable and are referred to as epigenetic regulation. Transcription of DNA is dictated by its structure. In general, the density of its packing is indicative of the frequency of transcription.

Octameric protein complexes called histones together with a segment of DNA wound around the eight histone proteins together referred to as a nucleosome are responsible for the amount of supercoiling of DNA, and these complexes can be temporarily modified by processes such as phosphorylation or more permanently modified by processes such as methylation.

Such modifications are considered to be responsible for more or less permanent changes in gene expression levels. Methylation of DNA is a common method of gene silencing. DNA is typically methylated by methyltransferase enzymes on cytosine nucleotides in a CpG dinucleotide sequence also called " CpG islands " when densely clustered. Analysis of the pattern of methylation in a given region of DNA which can be a promoter can be achieved through a method called bisulfite mapping. Methylated cytosine residues are unchanged by the treatment, whereas unmethylated ones are changed to uracil.

Abnormal methylation patterns are thought to be involved in oncogenesis. Histone acetylation is also an important process in transcription. Often, DNA methylation and histone deacetylation work together in gene silencing. The combination of the two seems to be a signal for DNA to be packed more densely, lowering gene expression.

Regulation of transcription thus controls when transcription occurs and how much RNA is created. Transcription of a gene by RNA polymerase can be regulated by several mechanisms. Specificity factors alter the specificity of RNA polymerase for a given promoter or set of promoters, making it more or less likely to bind to them i.

Repressors bind to the Operator , coding sequences on the DNA strand that are close to or overlapping the promoter region, impeding RNA polymerase's progress along the strand, thus impeding the expression of the gene. The image to the right demonstrates regulation by a repressor in the lac operon. General transcription factors position RNA polymerase at the start of a protein-coding sequence and then release the polymerase to transcribe the mRNA.

Activators enhance the interaction between RNA polymerase and a particular promoter , encouraging the expression of the gene. Activators do this by increasing the attraction of RNA polymerase for the promoter, through interactions with subunits of the RNA polymerase or indirectly by changing the structure of the DNA.

Enhancers are sites on the DNA helix that are bound by activators in order to loop the DNA bringing a specific promoter to the initiation complex. Enhancers are much more common in eukaryotes than prokaryotes, where only a few examples exist to date. In vertebrates, the majority of gene promoters contain a CpG island with numerous CpG sites.

For example, in colorectal cancers about to genes are transcriptionally silenced by CpG island methylation see regulation of transcription in cancer. Transcriptional repression in cancer can also occur by other epigenetic mechanisms, such as altered expression of microRNAs.

One of the cardinal features of addiction is its persistence. The persistent behavioral changes appear to be due to long-lasting changes, resulting from epigenetic alterations affecting gene expression, within particular regions of the brain. These are 1 histone acetylations and histone methylations , 2 DNA methylation at CpG sites , and 3 epigenetic downregulation or upregulation of microRNAs. Chronic nicotine intake in mice alters brain cell epigenetic control of gene expression through acetylation of histones.

This increases expression in the brain of the protein FosB, important in addiction. These CpG sites occurred in over 7, genes, or roughly a third of known human genes.

The majority of the differentially methylated CpG sites returned to the level of never-smokers within five years of smoking cessation. However, 2, CpGs among genes remained differentially methylated in former versus never smokers. In rodent models, drugs of abuse, including cocaine, [13] methampheamine, [14] [15] alcohol [16] and tobacco smoke products, [17] all cause DNA damage in the brain. In mammals, methylation of cytosine see Figure in DNA is a major regulatory mediator.

Methylated cytosines primarily occur in dinucleotide sequences where cytosine is followed by a guanine, a CpG site. The total number of CpG sites in the human genome is approximately 28 million.

In a rat, a painful learning experience, contextual fear conditioning , can result in a life-long fearful memory after a single training event. Methylation of CpGs in a promoter region of a gene represses transcription [23] while methylation of CpGs in the body of a gene increases expression. When contextual fear conditioning is applied to a rat, more than 5, differentially methylated regions DMRs of nucleotides each occur in the rat hippocampus neural genome both one hour and 24 hours after the conditioning in the hippocampus.

The pattern of induced and repressed genes within neurons appears to provide a molecular basis for forming the first transient memory of this training event in the hippocampus of the rat brain. Cells do this by modulating the capping, splicing, addition of a Poly A Tail, the sequence-specific nuclear export rates, and, in several contexts, sequestration of the RNA transcript. These processes occur in eukaryotes but not in prokaryotes. This modulation is a result of a protein or transcript that, in turn, is regulated and may have an affinity for certain sequences.

By binding to specific sites within the 3'-UTR, miRNAs can decrease gene expression of various mRNAs by either inhibiting translation or directly causing degradation of the transcript. These are prevalent motifs within 3'-UTRs. Among all regulatory motifs within the 3'-UTRs e. As of , the miRBase web site, [27] an archive of miRNA sequences and annotations, listed 28, entries in biologic species. The effects of miRNA dysregulation of gene expression seem to be important in cancer.

The effects of miRNA dysregulation of gene expression also seem to be important in neuropsychiatric disorders, such as schizophrenia , bipolar disorder , major depressive disorder , Parkinson's disease , Alzheimer's disease and autism spectrum disorders. The translation of mRNA can also be controlled by a number of mechanisms, mostly at the level of initiation.

Recruitment of the small ribosomal subunit can indeed be modulated by mRNA secondary structure, antisense RNA binding, or protein binding. In both prokaryotes and eukaryotes, a large number of RNA binding proteins exist, which often are directed to their target sequence by the secondary structure of the transcript, which may change depending on certain conditions, such as temperature or presence of a ligand aptamer.

Some transcripts act as ribozymes and self-regulate their expression. A large number of studied regulatory systems come from developmental biology. Examples include:. Up-regulation is a process that occurs within a cell triggered by a signal originating internal or external to the cell , which results in increased expression of one or more genes and as a result the protein s encoded by those genes.

Conversely, down-regulation is a process resulting in decreased gene and corresponding protein expression. In general, most experiments investigating differential expression used whole cell extracts of RNA, called steady-state levels, to determine which genes changed and by how much.

These are, however, not informative of where the regulation has occurred and may mask conflicting regulatory processes see post-transcriptional regulation , but it is still the most commonly analysed quantitative PCR and DNA microarray.

When studying gene expression, there are several methods to look at the various stages. In eukaryotes these include:. From Wikipedia, the free encyclopedia. For information on therapeutic regulation of gene expression, see therapeutic gene modulation. For vocabulary, see Glossary of gene expression terms. Main article: Transcriptional regulation. Main article: Regulation of transcription in cancer.

Main article: Post-transcriptional regulation. Main article: Three prime untranslated region. Main article: MicroRNA. Main article: Translational regulation.

Main article: Evolutionary developmental biology. Main article: Gene regulatory network. For protein methods, see protein methods. Genetics Home Reference.

Genome Biology. Cancer Research. Bibcode : PNAS.. Bibcode : Sci International Journal of Genomics. Nature Reviews. November Science Translational Medicine. October Circulation: Cardiovascular Genetics. The Journal of Medical Investigation.

Alcoholism, Clinical and Experimental Research. Clinical Science.

Regulation of gene expression

This is achieved via a conformational constraint which is relieved as ribosomes translate the upstream cistron. They do this inorder to save up energy and increase efficiency. Regulation of gene expression, or gene regulation, includes a wide range of mechanisms that are used by cells to increase or decrease the production of specific gene products protein or RNA. Sophisticated programs of gene expression are widely observed in biology, for example to trigger developmental pathways, respond to environmental stimuli, or adapt to new food sources. Eukaryotic genes typically have more regulatory elements to control gene expression compared to prokaryotes. For example, the arrival of a hormone may turn on or off certain genes in that cell.

To understand how gene expression is regulated, we must first understand how a gene becomes a functional protein in a cell. The process occurs in both prokaryotic and eukaryotic cells, just in slightly different fashions. Because prokaryotic organisms lack a cell nucleus, the processes of transcription and translation occur almost simultaneously. When the protein is no longer needed, transcription stops. As a result, the primary method to control what type and how much protein is expressed in a prokaryotic cell is through the regulation of DNA transcription into RNA. All the subsequent steps happen automatically.

References

Regulation of gene expression is achieved by the presence of cis regulatory elements; these signatures are interspersed in the noncoding region and also situated in the coding region of the genome. These elements orchestrate the gene expression process by regulating the different steps involved in the flow of genetic information. Current chapter describes the structural and functional elements present in the coding and noncoding region of the genome. Further we discuss role of regulatory elements in regulation of gene expression in prokaryotes and eukaryotes. Finally, we also discuss DNA structural properties of regulatory regions and their role in gene expression.

To understand how gene expression is regulated, we must first understand how a gene codes for a functional protein in a cell. The process occurs in both prokaryotic and eukaryotic cells, just in slightly different manners. Prokaryotic organisms are single-celled organisms that lack a cell nucleus, and their DNA therefore floats freely in the cell cytoplasm. To synthesize a protein, the processes of transcription and translation occur almost simultaneously. When the resulting protein is no longer needed, transcription stops.

Gene expression is the process by which information from a gene is used in the synthesis of a functional gene product that enables it to produce protein as the end product. Gene expression is summarized in the central dogma of molecular biology first formulated by Francis Crick in , [1] further developed in his article, [2] and expanded by the subsequent discoveries of reverse transcription [3] [4] [5] and RNA replication. The process of gene expression is used by all known life— eukaryotes including multicellular organisms , prokaryotes bacteria and archaea , and utilized by viruses —to generate the macromolecular machinery for life.

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Prokaryotes regulate gene expression by controlling the amount of transcription, whereas eukaryotic control is much more complex. To understand how gene expression is regulated, we must first understand how a gene codes for a functional protein in a cell. The process occurs in both prokaryotic and eukaryotic cells, just in slightly different manners. Prokaryotic organisms are single-celled organisms that lack a defined nucleus; therefore, their DNA floats freely within the cell cytoplasm. When the resulting protein is no longer needed, transcription stops.

Modulation of Gene Expression by Gene Architecture and Promoter Structure

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