RNA

• RNA contains the sugar ribose instead of deoxyribose found in DNA.
• About 10 base pairs of DNA are unwound or opened to allow the polymerase to work.
• Ribonucleic acid (RNA) is a polymer of nucleotides similar to DNA.
• RNA differs from DNA in the sugar moieties, having ribose instead of deoxyribose and, in one nitrogen base component, having uracil instead of thymine.
• RNA is synthesized as a single strand rather than as a double helix.
• RNA strands do not have complementary partner strands, but are not completely single-stranded.
• RNA species fold and loop upon themselves to take on as much of a double-stranded character as possible.
• RNA can also pair with complementary single strands of DNA or RNA and form a double helix.
• There are several types of RNAs found in the cell: ribosomal RNA, messenger RNA, transfer RNA, and small nuclear RNAs.
• RNA is copied, or transcribed, from DNA.
• Transcription is the copying of one strand of DNA into RNA by a process similar to that of DNA replication.
• Transcription occurs mostly in interphase nuclei.
• Evidence suggests that transcription takes place at discrete stations of the nucleus into which the DNA molecules move.
• One of these sites, the nucleolus, is the location of ribosomal RNA synthesis.
• The polymerization of RNA from a DNA template is catalyzed by RNA polymerase.
• After binding to its start site in DNA, a specific sequence of bases called the promoter, RNA polymerase and its supporting accessory proteins synthesize RNA using the base sequence of one strand of the double helix (the antisense strand) as a template.
• Uracil (U), the nitrogen base that replaces thymine in RNA, has the purine ring structure of thymine (dT) minus the methyl group.
• Uracil forms hydrogen bonds with adenine.
• Evolutionary theory places RNA as the original genetic material from which DNA has evolved.
• In most organisms, RNA is an intermediate between the storage system of DNA and the proteins responsible for phenotype.
• One family of RNA viruses, the retroviruses, which include leukemia viruses and the human immunodeficiency virus, have RNA genomes and, in order to replicate using host cell machinery, must first make a DNA copy of their genome by reverse transcription.
• DNA must be released locally from histones and the helix unwound in order for transcription to occur.
• As the polymerase proceeds past the polyA site, the nascent mRNA is released by an endonuclease associated with the carboxy terminal end of the polymerase.
• When the exonuclease catches up with the polymerase, transcription stops.
• Unlike DNA synthesis, RNA synthesis does not require a primer.
• Upon initiation of RNA synthesis, the first ribonucleoside triphosphate retains all of its phosphate groups as the RNA is polymerized in the 5' to 3' direction.
• Gene expression is the fundamental process for cell regulation, differentiation, and development.
• RNA synthesis proceeds along the DNA template until the polyadenylation signal is encountered. At this point the process of termination of transcription is activated.
• There are several types of RNAs found in the cell: ribosomal RNA, mRNA, transfer RNA, and small nuclear RNAs have distinct cellular functions.
• Subsequent ribonucleoside triphosphates retain only the alpha phosphate, the one closest to the ribose sugar. The other two phosphate groups are released as orthophosphate during the synthesis reaction
• RNA polymerase uses one strand of the double helix (the antisense strand) as a template for synthesis of RNA.
• There are also many more molecules of RNA polymerase than DNA polymerase in the cell.
• RNA synthesized beyond the site trails out of the RNA polymerase and is bound by another exonuclease that begins to degrade the RNA 5' to 3' toward the RNA polymerase.
• Compared with sites of initiation of DNA replication, there are many more sites for initiation of transcription (RNA synthesis) in both prokaryotes and eukaryotes.
• RNA polymerases work more slowly than DNA polymerases (50–100 bases/sec for RNA synthesis vs 1000 bases/sec for DNA replication) and with less fidelity.
• Signal transduction pathways that are the targets of several therapeutic strategies funnel internal and external signals to the nucleus where transcription factors bind to specific sequences in DNA and initiate or turn off transcription.
• The processes of gene expression involve numerous factors, including DNA binding proteins, transcription factors, histone modification enzymes, and RNA polymerase.
• The sense strand of the DNA template has a sequence identical to that of the RNA product (except for the U for T substitution in RNA), but it does not serve as the template for the RNA.
• there are three types of RNA poly- merases in eukaryotes: RNA polymerase pol I, pol II, and pol III. Pol I and III synthesize noncoding RNA. Pol II is responsible for the synthesis of messenger RNA (mRNA)
• RNA polymerase and its supporting accessory proteins assemble on DNA at a specific site called the promoter.