Therefore, a nucleotide sequence thousands of nucleotides away can fold over and interact with a specific promoter. 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. More recently, elements have been identified that decrease transcription of neighboring genes, and these elements have been called silencers. Extensive analysis of enhancers have detected several features. First, these elements are functional over a large distance.
For example, an enhancer has been placed nt from the gene, and it can still increase expression. This article has been posted to your Facebook page via Scitable LearnCast. Change LearnCast Settings. Scitable Chat. 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. Like transcription, translation is controlled by proteins that bind and initiate the process.
In translation, before protein synthesis can begin, ribosome assembly has to be completed. This is a multi-step process. In ribosome assembly, the large and small ribosomal subunits and an initiator tRNA tRNA i containing the first amino acid of the final polypeptide chain all come together at the translation start codon on an mRNA to allow translation to begin. First, the small ribosomal subunit binds to the tRNA i which carries methionine in eukaryotes and archaea and carries N-formyl-methionine in bacteria.
Because the tRNA i is carrying an amino acid, it is said to be charged. Next, the small ribosomal subunit with the charged tRNA i still bound scans along the mRNA strand until it reaches the start codon AUG, which indicates where translation will begin. The start codon also establishes the reading frame for the mRNA strand, which is crucial to synthesizing the correct sequence of amino acids. A shift in the reading frame results in mistranslation of the mRNA.
The anticodon on the tRNA i then binds to the start codon via basepairing. The complex consisting of mRNA, charged tRNA i , and the small ribosomal subunit attaches to the large ribosomal subunit, which completes ribosome assembly. These components are brought together by the help of proteins called initiation factors which bind to the small ribosomal subunit during initiation and are found in all three domains of life.
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