Concepts Summary

• Early life used RNA as both the carrier of genetic information and as biological catalysts.
• RNA is a polymer, consisting of nucleotides joined together by phosphodiester bonds. Each RNA nucleotide consists of a ribose sugar, a phosphate, and a base. RNA contains the base uracil and is usually single stranded, which allows it to form secondary structures.
• Cells possess a number of different classes of RNA. Ribosomal RNA is a component of the ribosome, messenger RNA carries coding instructions for proteins, and transfer RNA helps incorporate the amino acids into a polypeptide chain.
• The template for RNA synthesis is single-stranded DNA. In transcription, RNA synthesis is complementary and antiparallel to the DNA template strand. A transcription unit consists of a promoter, an RNA-coding region, and a terminator.
• The substrates for RNA synthesis are ribonucleoside triphosphates.
• RNA polymerase in bacterial cells consists of a core enzyme, which catalyzes the addition of nucleotides to an RNA molecule, and other subunits. The sigma factor controls the binding of the core enzyme to the promoter.
• Eukaryotic cells contain several different RNA polymerases.
• The process of transcription consists of three stages: initiation, elongation, and termination.
• Transcription begins at the start site, which is determined by consensus sequences. A short stretch of DNA is unwound near the start site, RNA is synthesized from a single strand of DNA as a template, and the DNA is rewound at the lagging end of the transcription bubble. RNA polymerases are capable of proofreading.
• RNA synthesis ceases after a terminator sequence has been transcribed. Bacterial cells have two types of terminators: rho-independent terminators and rho-dependent terminators.
• The initiation of transcription in eukaryotes requires the modification of chromatin structure. Different types of RNA polymerases in eukaryotes recognize different types of promoters.
• For genes transcribed by RNA polymerase II, general transcription factors bind to the core promoter and are part of the basal transcription apparatus. Transcriptional activator proteins bind to sequences in regulatory promoters and enhancers and interact with the basal transcription apparatus at the core promoter.
• The three RNA polymerases found in all eukaryotic cells use different mechanisms of termination.
• Transcription in archaea has many similarities to transcription in eukaryotes.