15.1: Introduction
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Transcription, the synthesis of RNA based on a DNA template, is the central step of the Central Dogma proposed by Crick in 1958. The basic steps of transcription are the same as for replication: initiation, elongation and termination. Differences between transcription in prokaryotes and eukaryotes are in the details.
- E.coli uses a single RNA polymerase enzyme to transcribe all kinds of RNAs while eukaryotic cells use different RNA polymerases to catalyze ribosomal RNA (rRNA), transfer RNA (tRNA) and messenger RNA (mRNA) synthesis.
- In contrast to eukaryotes, some bacterial genes are part of operons whose mRNAs encode multiple polypeptides.
- Bacterial mRNAs are typically translated as they are being transcribed.
- Most RNA transcripts in prokaryotes emerge from transcription ready to use
- Eukaryotic transcripts synthesized as longer precursors undergo processing by trimming , splicing , or both!
- DNA in bacteria is virtually ‘naked’ in the cytoplasm while eukaryotic DNA is wrapped up in chromatin proteins in a nucleus.
- In bacterial cells the association of ribosomes with mRNA and the translation of a polypeptide can begin even before the transcript is finished. This is because these cells have no nucleus. In our cells, RNAs must exit the nucleus before they encounter ribosomes in the cytoplasm.
In this chapter, you will meet bacterial polycistronic mRNAs (transcripts of operons that encode more than one polypeptide) and the split genes of eukaryotes (with their introns and exons ). We will look at some details of transcription of the three major classes of RNA and then at how eukaryotes process precursor transcripts into mature, functional RNAs. Along the way, we will see one example of how protein structure has evolved to interact with DNA.
Learning Objectives
1. Discriminate between the three steps of transcription in pro- and eukaryotes, and the factors involved in each.
2. State an hypothesis for why eukaryotes evolved complex RNA processing steps.
3. Speculate on why any cell in its right mind would have genes containing introns and exons so that their transcripts would have to be processed by splicing .
4. Articulate the differences between RNA vs. DNA structure .
5. Explain the need for sigma factors in bacteria.
6. Speculate on why eukaryotes do not have operons.
7. List structural features of proteins that bind and recognize specific DNA sequences.
8. Explain how proteins that do not bind specific DNA sequences can still bind to specific regions of the genome.
9. Formulate an hypothesis for why bacteria do not polyadenylate their mRNAs as much as eukaryotes do.
10. Formulate an hypothesis for why bacteria do not cap their mRNAs.