10.1: Introduction
- Page ID
- 88954
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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. The differences between transcription in prokaryotes and eukaryotes are in the details. Here are some:
- E. coli uses a single RNA polymerase enzyme to transcribe all kinds of RNAs, while eukaryotic cells use different RNA polymerases to catalyze the syntheses of ribosomal RNA (rRNA), transfer RNA (tRNA), and messenger RNA (mRNA).
- In contrast to the eukaryotic genetic makeup, some bacterial genes are part of operons whose mRNAs encode multiple polypeptides.
- 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 eukaryotic 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). Then you will read about so-called eukaryotic split genes (with 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.
When you have mastered the information in this chapter, you should be able to do the following:
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.