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It is the process of synthesis of messenger RNA transcripts by copying the template strand of DNA in the prokaryotes. Messenger RNA is then translated to ...
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Introduction
Structure of gene A " gene " occurs over a particular physical region called locus of dsDNA molecule. It includes an RNA-coding region, where transcription occurs from the DNA template strand. It runs between the initiation and termination sites in 5 ‘ to 3 ' direction. Control regions for transcription, i.e., promoters and terminators occur immediately outside the coding sequence. By convention, the gene is written " left to right " in the 5 ' to 3 ' direction of the sense strand. The region to the ' left ' of the gene is called ' upstream ' and the region to the ' right ' is ' downstream '.
DNA to RNA – Transcription DNA transcription produces a single single-stranded RNA molecule that is complementary to one strand of DNA Molecular Biology of the Cell. 4 th edition. Poly-deoxyribonucleotide Poly-ribonucleotide
Structure of RNA Molecular Biology of the Cell. 4 th edition. RNA can fold into specific structures RNA is largely single-stranded, but it often contains short stretches of nucleotides that can form conventional base-pairs with complementary sequences and “nonconventional” base- pairs with modified bases on the same molecule that allow an RNA molecule to fold into a three-dimensional structure.
Types of RNAs Three main types of RNAs, viz. mRNA, rRNA and tRNA are found both in prokaryotes and eukaryotes. Transfer-messenger RNAs (tmRNA) are found only in bacteria. Small nuclear RNAs (snRNA) are found only in nucleated organism, such as eukaryotes. Some RNAs are involved in protein synthesis, some are in post-transcriptional modification, some are in replication and some are involved in regulation of gene expression and others.
Initiation of Transcription The RNA polymerase recognize and binds to Pribnow box in prokaryotes, whereas a TATA box binding protein (TBP) recognize and bind to TATA box in eukaryotes). In prokaryotes, there are no transcription factor In eukaryotes, there are six general transcription factors namely TFIIA, TFIIB (ortholog of Archeal TFB), TFIID (a subunit ortholog of Archeal TPB), TFIIE (ortholog of archeal TFE), TFIIF and TFIIH. Closing complex in prokaryotes Closing complex in eukaryotes
Initiation of Transcription Promoter region Region that contains specific DNA sequences recognized by a transcription factors. Transcription factor binds to promoter sequence and recruits RNA polymerase. It is of two types: o Core promoter: a minimal portion of the promoter required to properly initiate transcription. It has following characteristics:
Initiation of Transcription Difference between prokaryotic and eukaryotic promoters Prokaryotic promoters: In prokaryotic, the promoter consists of two short sequences at - 10 and - 35 positions from the transcription start sites (TSS). → The sequence at - 10 is called the Pribnow box, or the - 10 element, which usually consists of six nucleotides TATAAT. It is essential to start transcription in prokaryotes. → The sequence at - 35 is called - 35 element which usually consists of the six nucleotides TTGACA. Its presence allows a very high transcription rate. Eukaryotic promoters: In eukaryotic, the promoters are extremely diverse and are difficult to characterize. They lie upstream to gene and may have regulatory elements several kilobases away from the transcription start site. transcriptional complex can cause the DNA to bend back on itself, which allows for placement of regulatory sequences far from the actual site of transcription. → It contains a TATA box (sequence TATAAA), which binds to TATA binding protein that assists in the formation of the RNA polymerase transcriptional complex. TATA box typically lies very close to the transcription start site (often within 50 bases). http://www.scfbio-iitd.res.in/tutorial/promoter.html
Initiation of Transcription
Initiation of Transcription Transcription Cycle
Elongation of Transcripts The RNA polymerase transcribes the DNA (β subunit initiates the synthesis), but produces about 10 abortive (short, non-productive) transcripts which are unable to leave the RNA polymerase because the exit channel is blocked by σ-factor. The σ-factor eventually dissociates from the core enzyme and elongation proceeds. This enzyme has no exo/endonuclease activity and cannot repair the mistakes as DNA polymerase in replication.
Elongation of Transcripts Formation of closed complex by binding of the promoter sequence. Thereafter, the RNA polymerase holoenzyme separates 10 - 14 bases extending from - 11 to + 3 to form a open complex called melting. The changing from closed to open complex is called isomerization. RNA polymerase starts synthesizing RNA. RNA polymerase exhibits groove which have length of 55 Å and diameter of 25 Å. This groove fits well the 20 Å double strand of DNA. The 55 Å length can accept 16 nucleotides.
Elongation of Transcripts