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| Transcription |
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| DNA contains the information required for the synthesis of cells specific proteins. DNA is located in the nucleoid (prokaryotes) or nucleus (eukaryotes) and protein synthesis occurs in the cytoplasm. DNA does not move to the site of protein synthesis (ribosomes) to directly guide the process. Instead, it transfers its information to mRNA molecules which move to the ribosomes to direct protein synthesis. The process of the formation of RNA from the DNA template is called transcription. It involves rewriting the genetic message coded in DNA into an RNA molecule. |
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| Transcription occurs in the nucleus during the G1 and G2 phases of cell cycle. DNA has the promoter and terminator sites. Transcription starts at the promoter site and stops at the terminator site. |
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| RNA transcription requires the following components. |
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 The enzyme RNA polymerase |
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 A DNA template |
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 All four types of ribonucleoside triphosphates (ATP, GTP and UTP) |
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 Divalent metal ions Mg++ or Mn++ as a co-factor |
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| No primer is needed for RNA synthesis |
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| RNA transcription is a process that involves the following steps. |
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| The histone coat protecting the DNA double helix of the gene to be transcribed is removed, on a signal from the cytoplasm, exposing the polynucleotide sequences in this region of DNA. The RNA polymerase enzyme binds to a specific site, called promoter, in the DNA double helix. This site is located on the 5 side of the gene to be transcribed. It signals the beginning of RNA synthesis. The promoter also determines the DNA strand that is to be transcribed. |
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| The RNA polymerase moves along the DNA and causes local unwinding and splitting of the DNA double helix into two chains in the region where the gene to be transcribed is located. This exposes the A, T, C and G bases that project into the karyoplasms from the phosphatedeoxyribose sugar backbone. Only one strand, called sense strand, of DNA functions as a template, the other strand is complementary. The mechanism which selects the template is not known. |
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| The ribonucleoside triphosphates, namely, adenosine triphosphate (ATP), guanosine triphosphate (GTP), cytidine triphosphate (CTP) and uridine triphosphate (UTP), floating free in the nucleus, serve as the raw material for RNA synthesis. They are formed by activation (phosphorylation) of ribonucleoside monophosphates, viz., adenosine monophosphate (AMP), guanosine monophosphate (GMP), cytidine monophosphate (CMP) and uridine monophosphate (UMP) as a result of their combining with ATP. The enzyme phosphorylase catalyses this activation process. The ribonucleotide triphosphates are joined to the bases of the DNA template chain one by one by hydrogen bonding according to the base pairing rule i.e., A U, U A, C G, G C. This base pairing is brought about by the RNA polymerase. |
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| The nucleotides are added one by one. A=Adenine, T=Thymine, C=Cytosine, G=Guanine, U=Uracil, R=Ribose sugar, P=Phosphate |
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| fig. 22.2 Synthesis of mRNA from DNA |
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| The various ribonucleoside triphosphates on linking to the DNA template chain break off their high-energy bonds. This changes them to ribonucleoside monophosphates which represent the normal components of RNA, and sets free pyrophosphate groups (P~P). Pyrophosphate contains a high-energy bond (~). It undergoes hydrolysis by the enzyme pyrophosphotase, releases energy and sets free inorganic phosphate Pi. The first ribonucleotide phosphate retains all the three phosphates and is, thus, chemically distinct from the other nucleotides added after it. |
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| Each ribonucleoside monophosphate attached to the DNA template chain then combines with the ribonucleotide arrived earlier, making the RNA chain become longer. The process is catalysed by the enzyme RNA polymerase and requires a divalent ion Mg++ or Mn++. |
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| The RNA chain, thus formed, contains nitrogenous bases that are complementary to those of the template DNA chain. |
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| As transcription proceeds, the hybrid DNA-RNA molecule dissociates, partly releasing the RNA molecule under synthesis. When polymerase reaches a terminator signal on the DNA, it leaves the DNA. The fully formed RNA chain is now totally released by this process, one gene forms several molecules of RNA, which get released from the DNA template one after the other. |
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| In some cases, such as in E. coli, a specific chain terminating protein, called rho factor (P), stops the synthesis of RNA chain. In most cases, the enzyme RNA polymerase on its own can stop transcription. |
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| As the RNA chain grows, the transcribed region of the DNA molecule gets hydrogen bonded to the opposite strand and the two become spirally coiled to assume the original double helical form. When the last ribonucleotide is added, the RNA polymerase and RNA chain are completely released from the DNA, and now the DNA completes its winding into a double helix. The protective protein coat is added again to the DNA duplex. |
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| The sequence of nitrogen bases from the promoter to the terminator sites form a transcription unit. It may include one or more genes. An entire transcription unit gets transcribed into a single RNA chain. |
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| The forms of RNAs originally transcribed from DNA are called primary transcripts. These undergo extensive changes, termed processing or post-transcriptional modification of RNAs, before they can become functional in both prokaryotes and eukaryotes. |
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| In RNA processing, |
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 Larger RNA precursors are cut into smaller RNAs by a ribonuclease-P cleaving enzyme. |
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 Unwanted nucleotides are removed by enzymes called nucleases (splicing). |
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 Useful regions are rejoined by ligase enzyme. |
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 Certain nucleotides are added at the terminal ends enzymatically (terminal addition). |
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 The RNA molecule may fold on itself to assume proper shape (folding) and |
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 Some nucleotides may be modified (nucleotide modification). |
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| The entire process of RNA transcription may be summed up in the equation- |
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| (ATP + GTP +CTP +UTP) n |
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