Transcription helix unwinding

E Transcription is the process by which RNA is produced to carry genetic information from the nucleus to the ribosomes. A short segment of the DNA double helix unwinds, and complementary ribonucleotides line up to pro-... 

Transcription It involves copying of DNA base sequences into a mRNA. A small portion of DNA double helix unwinds and are of the two DNA strands act as the template for the synthesis of /wRNA. Ribonucleotides assemble along the uncoiled template in accordance with the base pairing principle. For example, U in the RNA being formed appears opposite A of DNA C opposite G A opposite T and G opposite C of DNA. Then occurs the bond formation between the various nucleotides thus assembled. 

The conversion of the information in DNA into proteins begins in the nucleus of cells with the synthesis of mRNA by transcription of DNA. Several turns of the DNA double helix unwind, forming a bubble and exposing the bases of the two strands. Ribonucleotides line up in the proper order by hydrogen bonding to their complementary bases on DNA, bond formation occurs in the 5 —> 3 direction, and the growing RNA molecule unwinds from DNA (Figure 28.9, p. 1170). 

RNA is formed by the transcription of DNA. On cell division, the two chains of the helix unwind, and each strand is used as a template for the construction of an RNA molecule. The complementary bases pair up, and the completed RNA (which corresponds to only a section of the DNA) then unwinds from DNA and travels to the nucleus. Unlike DNA, RNA remains a single strand of nucleotides. 

The process of transcription in eukaryotes is similar to that in prokaryotes. RNA polymerase binds to the transcription factor complex in the promoter region and to the DNA, the helix unwinds within a region near the startpoint of transcription, DNA strand separation occurs, synthesis of the RNA ffanscript is initiated, and the RNA transcript is elongated, copying the DNA template. The DNA strands separate as the polymerase approaches and rejoin as the polymerase passes. 

In rarer cases the ODNs also prevent normal gene transcription by directly forming triplex-helix structures with target DNA. This does not destroy a gene but prevents its unwinding or its binding to a gene promoter. 

To enable RNA polymerase to synthesize an RNA strand complementary to one of the DNA strands, the DNA duplex must unwind over a short distance, forming a transcription bubble. During transcription, the E. coli RNA polymerase generally keeps about 17 bp unwound. The 8 bp RNA-DNA hybrid occurs in this unwound region. Elongation of a transcript by E. coli RNA polymerase proceeds at a rate of 50 to 90 nucleotides/s. Because DNA is a helix, movement of a transcription bubble requires considerable strand rotation of the nucleic acid molecules. DNA strand rotation is restricted... 

Intercalating agents are compounds that insert themselves between the bases of the DNA helix (Figure 7.13(a)). The insertion causes the DNA helix to partially unwind at the site of the intercalated molecule. This inhibits transcription, which blocks the replication process of the cell. Although the mechanism of this inhibition is not known, inhibition of cell replication can lead to cell death and an improvement in the health of the patient. 

Chapter 22 describes how different chemical agents react with DNA and form interstrand crosslinks (ICLs) that covalently link the two strands together. This poses a special problem for the cell in that the replication and transcription machineries are unable to progress or unwind the helix at the sites where the two strands are covalently linked to each other. It also presents a challenge for DNA repair since the damaged sites occur in close proximity to each other on opposing strands. Hence it is difficult for one strand to serve as a template for repair of the other strand because they both contain damaged DNA. 

The anticancer activity of daunomycin (34) (O Scheme 14) is attributed to the intercalation of the drug with DNA. This pattern of effect is common to all anthracyclines [46]. Two daunomycin molecules intercalate at each of the two C G sites at either site of the duplex d(CGTACG). This binding results in an increase in base pair-separation from 3.4 to 6.8 A as the molecule associates, which in turn leads to an unwinding of the helix and the formation of a noncovalent complex with the DNA. This results in the inhibition of DNA replication and RNA transcription [47]. 

Cisplatin binds strongly to DNA in regions containing several guanidine units, binding in such a way as to form links within strands (intrastrand binding) rather than between them. Unwinding of the DNA helix takes place and transcription is inhibited. 

RNA Polymerase Must Unwind the Template Double Helix for Transcription to Take Place... 

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