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Lecture

3. Transcription and translation.pdf

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Department
Biology (Sci)
Course
BIOL 200
Professor
Richard Roy
Semester
Fall

Description
RNA polymerase start sie stop ste 1 Polymerase binds to 4 Polymerase advances 5 3 5 down template 3 5 At transcription stop site, RNA polymerase start sie stop ste 1 Polymerase binds to 4 Polymerase advances 5 3 5 down template 3 5 At transcription stop site,Naveen Sooknanan McGill Fall 2011 Transcription and translation: The central dogma represents the mechanisms through which a gene (DNA) is able to synthesize various proteins which carry out specific tasks in the cell. In prokaryotes (although the process is not so different in eukaryotes), the following ingredients are needed in transcription:  DNA template, which served as a blueprint  Ribonucleoside triphosphates (rNTPs) o Serve as the monomers of the RNA sequence during polymerization  RNA polymerase o An enzyme which catalyzes RNA synthesis  All three of these items are found in both prokaryotes and eukaryotes during transcription Take the following DNA template:  Transcription takes place only using the template strand o Transcription begins at the +1 site on the DNA template sequence  The promoter is a sequence of DNA directly upstream (5’  3’) from the +1 site  The coding sequence begins at the +1 site and is used to make the RNA sequence The transcription process must first be initiated by the following 3 steps  RNA polymerase locates and binds to the promoter sequence on the duplex DNA template, forming a closed complex  The duplex is melted an unwound by RNA polymerase forming a transcription bubble at the transcription site o This forms an open complex  RNA polymerases then catalyzes the incorporation of two initial rNTPs by forming phosphodiester linkages to the DNA template while removing a molecule of pyrophosphate (two PO ) 4 o The rNTP incorporation follows standard Watson and Crick base pairs The RNA strand can then be elongated in a process called elongation  RNA polymerase runs along the DNA template strand in the 3’  5’ direction while incorporating proper rNTPs to synthesize an RNA strand in the 5’  3’ direction o Phosphodiester linkages are created between the 3’ hydroxyl end of the template DNA and the 5’ phosphate group of the corresponding rNTP The transcription process is stopped one the RNA polymerase reaches the stop site in a process called termination 1Naveen Sooknanan McGill Fall 2011  RNA polymerase releases the DNA RNA hybrid at which point the DNA template dissociates from the RNA molecule and renatures into its double stranded form RNA polymerase (of E. coli in this example) is a holoenzyme which consist of several core subunits as well as a sigma factor  The core enzyme is made up of two α subunits, one β subunit and one β’ subunit o These subunits catalyze the RNA polymerization reaction  The sigma (σ) factor associates with the core enzyme to form the holoenzyme o Essential for binding the site of transcription initiation by scanning the DNA looking for the promoter region to which the holoenzyme can bind  It does this be recognizing a specific “signature DNA” with specific motifs essential for promoter activity (usually in the -10 to -35 regions of the E. coli gene)  Once the promoter is found, the sigma factor leaves the RNA polymerase before initiation In prokaryotes, their circular gene structure has many sites of transcription initiation. As a result of multiple proteins can be transcribed from one particular gene  Therefore, prokaryotic genes are typically polycistronic (code for multiple proteins) due to their multiple start points  Contrarily, linear DNA molecules are usually monocistronic, meaning one gene codes for the synthesis of one protein In eukaryotic DNA only, synthesized mRNA molecules are capped at the 5’ end with a methyl group to prevent degradation  They are also polyadenylated at the 3’ end, which means adenines are added at the 3’ end forming a 3’ poly (A) tail  Noncoding regions called introns spliced out leaving functional mRNA Next, the RNA is used as a template to synthesize polypeptides in a process called translation. For this, three types of RNA are needed:  Messenger RNA (mRNA) carries the genetic information and is used as a template in protein synthesis. It carries this information in the form of codons o Codons are 3 nucleotides long and each codon codes for the synthesis of one amino acids  Transfer RNA (tRNA) molecules are used to carry amino acids to the translation site. They have a 3 nucleotide sequence called a anticodon which each correspond to one codon on mRNA o Their unusual shape will be looked at in more detail  Ribosomal RNA (rRNA) is a ribozyme which associates with proteins in order to form ribosomes o Ribosomes are important structures in translation which will be looked at more closely 2//// 1111 ff// //玨// // Amie ー: //// 1111 ff // // 玨 // // Amie ー :Naveen Sooknanan
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