RNA polymerase binds to the transcription initiation complex, allowing transcription to occur. Once this transcription initiation complex is assembled, RNA polymerase can bind to its upstream sequence. When bound along with the transcription factors, RNA polymerase is phosphorylated. This releases part of the protein from the DNA to activate the transcription initiation complex and places RNA polymerase in the correct orientation to begin transcription; DNA-bending protein brings the enhancer, which can be quite a distance from the gene, in contact with transcription factors and mediator proteins.
Transcription factors recognize the promoter. RNA polymerase II then binds and forms the transcription initiation complex. In addition to the general transcription factors, other transcription factors can bind to the promoter to regulate gene transcription.
These transcription factors bind to the promoters of a specific set of genes. They are not general transcription factors that bind to every promoter complex, but are recruited to a specific sequence on the promoter of a specific gene. There are hundreds of transcription factors in a cell that each bind specifically to a particular DNA sequence motif.
When transcription factors bind to the promoter just upstream of the encoded gene, they are referred to as cis-acting elements because they are on the same chromosome, just next to the gene.
The region that a particular transcription factor binds to is called the transcription factor binding site. Transcription factors respond to environmental stimuli that cause the proteins to find their binding sites and initiate transcription of the gene that is needed. Enhancers increase the rate of transcription of genes, while repressors decrease the rate of transcription. In some eukaryotic genes, there are regions that help increase or enhance transcription.
These regions, called enhancers, are not necessarily close to the genes they enhance. They can be located upstream of a gene, within the coding region of the gene, downstream of a gene, or may be thousands of nucleotides away. Enhancer regions are binding sequences, or sites, for transcription factors.
This shape change allows the interaction between the activators bound to the enhancers and the transcription factors bound to the promoter region and the RNA polymerase to occur. Whereas DNA is generally depicted as a straight line in two dimensions, it is actually a three-dimensional object. Therefore, a nucleotide sequence thousands of nucleotides away can fold over and interact with a specific promoter.
Enhancers : An enhancer is a DNA sequence that promotes transcription. Each enhancer is made up of short DNA sequences called distal control elements.
Activators bound to the distal control elements interact with mediator proteins and transcription factors. Like prokaryotic cells, eukaryotic cells also have mechanisms to prevent transcription.
Transcriptional repressors can bind to promoter or enhancer regions and block transcription. Like the transcriptional activators, repressors respond to external stimuli to prevent the binding of activating transcription factors. A corepressor is a protein that decreases gene expression by binding to a transcription factor that contains a DNA-binding domain. The corepressor is unable to bind DNA by itself. The corepressor can repress transcriptional initiation by recruiting histone deacetylase, which catalyzes the removal of acetyl groups from lysine residues.
This increases the positive charge on histones, which strengthens the interaction between the histones and DNA, making the DNA less accessible to the process of transcription.
Both the packaging of DNA around histone proteins, as well as chemical modifications to the DNA or proteins, can alter gene expression. Discuss how eukaryotic gene regulation occurs at the epigenetic level and the various epigenetic changes that can be made to DNA.
The human genome encodes over 20, genes; each of the 23 pairs of human chromosomes encodes thousands of genes. The DNA in the nucleus is precisely wound, folded, and compacted into chromosomes so that it will fit into the nucleus. It is also organized so that specific segments can be accessed as needed by a specific cell type. The first level of organization, or packing, is the winding of DNA strands around histone proteins.
Histones package and order DNA into structural units called nucleosome complexes, which can control the access of proteins to the DNA regions. Under the electron microscope, this winding of DNA around histone proteins to form nucleosomes looks like small beads on a string. These beads histone proteins can move along the string DNA and change the structure of the molecule.
These nucleosomes control the access of proteins to the underlying DNA. When viewed through an electron microscope b , the nucleosomes look like beads on a string. Nucleosomes can move to open the chromosome structure to expose a segment of DNA, but do so in a very controlled manner. Nucleosomes can change position to allow transcription of genes : Nucleosomes can slide along DNA. When nucleosomes are spaced closely together top , transcription factors cannot bind and gene expression is turned off.
When the nucleosomes are spaced far apart bottom , the DNA is exposed. Transcription factors can bind, allowing gene expression to occur. Modifications to the histones and DNA affect nucleosome spacing. How the histone proteins move is dependent on signals found on both the histone proteins and on the DNA.
These signals are tags, or modifications, added to histone proteins and DNA that tell the histones if a chromosomal region should be open or closed. These tags are not permanent, but may be added or removed as needed.
They are chemical modifications phosphate, methyl, or acetyl groups that are attached to specific amino acids in the protein or to the nucleotides of the DNA. The tags do not alter the DNA base sequence, but they do alter how tightly wound the DNA is around the histone proteins.
DNA is a negatively-charged molecule; therefore, changes in the charge of the histone will change how tightly wound the DNA molecule will be. When unmodified, the histone proteins have a large positive charge; by adding chemical modifications, such as acetyl groups, the charge becomes less positive.
Modifications affect nucleosome spacing and gene expression. The DNA molecule itself can also be modified. This occurs within very specific regions called CpG islands. These are stretches with a high frequency of cytosine and guanine dinucleotide DNA pairs CG found in the promoter regions of genes.
When this configuration exists, the cytosine member of the pair can be methylated a methyl group is added. This modification changes how the DNA interacts with proteins, including the histone proteins that control access to the region.
Highly-methylated hypermethylated DNA regions with deacetylated histones are tightly coiled and transcriptionally inactive. These changes to DNA are inherited from parent to offspring, such that while the DNA sequence is not altered, the pattern of gene expression is passed to the next generation.
This type of gene regulation is called epigenetic regulation. Instead, these changes are temporary although they often persist through multiple rounds of cell division and alter the chromosomal structure open or closed as needed. A gene can be turned on or off depending upon the location and modifications to the histone proteins and DNA.
If a gene is to be transcribed, the histone proteins and DNA are modified surrounding the chromosomal region encoding that gene. This opens the chromosomal region to allow access for RNA polymerase and other proteins, called transcription factors, to bind to the promoter region, located just upstream of the gene, and initiate transcription.
If a gene is to remain turned off, or silenced, the histone proteins and DNA have different modifications that signal a closed chromosomal configuration. In this closed configuration, the RNA polymerase and transcription factors do not have access to the DNA and transcription cannot occur. RNA splicing allows for the production of multiple protein isoforms from a single gene by removing introns and combining different exons.
Gene expression is the process that transfers genetic information from a gene made of DNA to a functional gene product made of RNA or protein. In order to ensure that the proper products are produced, gene expression is regulated at many different stages during and in between transcription and translation.
In eukaryotes, the gene contains extra sequences that do not code for protein. These pre-mRNA transcripts often contain regions, called introns, that are intervening sequences which must be removed prior to translation by the process of splicing.
The regions of RNA that code for protein are called exons. Splicing can be regulated so that different mRNAs can contain or lack exons, in a process called alternative splicing. Alternative splicing allows more than one protein to be produced from a gene and is an important regulatory step in determining which functional proteins are produced from gene expression. Thus, splicing is the first stage of post-transcriptional control.
Alternative Splicing : There are five basic modes of alternative splicing. Alternative splicing is a process that occurs during gene expression and allows for the production of multiple proteins protein isoforms from a single gene coding.
Alternative splicing can occur due to the different ways in which an exon can be excluded from or included in the messenger RNA. This results in what is called alternative splicing. The pattern of splicing and production of alternatively-spliced messenger RNA is controlled by the binding of regulatory proteins trans-acting proteins that contain the genes to cis-acting sites that are found on the pre-RNA.
Some of these regulatory proteins include splicing activators proteins that promote certain splicing sites and splicing repressors proteins that reduce the use of certain sites. Some common splicing repressors include: heterogeneous nuclear ribonucleoprotein hnRNP and polypyrimidine tract binding protein PTB.
Proteins that are translated from alternatively-spliced messenger RNAs differ in the sequence of their amino acids which results in altered function of the protein. This is one reason why the human genome can encode a wide diversity of proteins. Alternative splicing is a common process that occurs in eukaryotes; most of the multi-exonic genes in humans are spliced alternatively.
Unfortunately, abnormal variations in splicing are also the reason why there are many genetic diseases and disorders. Mechanism of Splicing : Alternative splicing can result in protein isoforms. The splicing of messenger RNA is accomplished and catalyzed by a macro-molecule complex known as the spliceosome.
Interactions between these sub-units and the small nuclear ribonucleoproteins snRNP found in the spliceosome create a spliceosome A complex which helps determine which introns to leave out and which exons to keep and bind together. Once the introns are cleaved and removed, the exons are joined together by a phosphodiester bond. As noted above, splicing is regulated by repressor proteins and activator proteins, which are are also known as trans-acting proteins.
Equally as important are the silencers and enhancers that are found on the messenger RNAs, also known as cis-acting sites. These regulatory functions work together in order to create splicing code that determines alternative splicing. These promoters take interest in floating some companies.
Entrepreneur promoters. Financial promoters. Discovery of a business idea. Detailed investigation. Assembling the factors of production. Entering into preliminary contracts. Naming a company. Is TATA box a promoter?
It is a type of promoter sequence, which specifies to other molecules where transcription begins. Is promoter transcribed? A promoter is a sequence of DNA needed to turn a gene on or off. The process of transcription is initiated at the promoter. Usually found near the beginning of a gene, the promoter has a binding site for the enzyme used to make a messenger RNA mRNA molecule.
Are introns transcribed? In most eukaryotic genes, coding regions exons are interrupted by noncoding regions introns. During transcription, the entire gene is copied into a pre-mRNA, which includes exons and introns. During the process of RNA splicing, introns are removed and exons joined to form a contiguous coding sequence. Is a promoter a protein?
A promoter is a region of DNA where transcription of a gene is initiated. What is translation in DNA? Translation is the process that takes the information passed from DNA as messenger RNA and turns it into a series of amino acids bound together with peptide bonds. How does Promoter work? In genetics, a promoter is a region of DNA that leads to initiation of transcription of a particular gene.
Promoters are located near the transcription start sites of genes, upstream on the DNA towards the 5' region of the sense strand. Promoters can be about — base pairs long. Does the TATA box get transcribed? What is a strong promoter?
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