This lesson introduces the organization of prokaryotic DNA and explains how transcription begins in bacteria. Understanding initiation shows how RNA polymerase identifies the correct gene and starts making an RNA copy.
Prokaryotes, including Bacteria and Archaea, generally consist of single cells and lack a membrane-bound nucleus. Their DNA occupies a central area called the nucleoid region. A typical bacterial chromosome is a closed circle and is not organized around histone proteins in the same way as eukaryotic chromosomes.
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How do the template and coding DNA strands relate to mRNA?
RNA polymerase reads the template strand to make complementary mRNA. The mRNA resembles the coding strand, except RNA uses U instead of T.
What distinguishes the RNA polymerase core enzyme from the holoenzyme?
The core contains two α subunits, β, and β′. The holoenzyme is the core joined by sigma, which provides promoter-recognition specificity.
What do upstream and downstream mean relative to the +1 site?
Upstream positions have negative numbers; downstream positions have positive numbers.
What happens to DNA during transcription elongation?
DNA unwinds ahead of RNA polymerase and rewinds behind it.
How does rho-independent termination release the RNA transcript?
An RNA hairpin stalls polymerase, while weak RNA–DNA pairing helps destabilize the complex and release the transcript.
Quiz questions
Which description best distinguishes a plasmid from a typical bacterial chromosome?Answer: A short circular DNA molecule that may carry a few genes and transfer independently of the chromosome.
What is sigma's main role in the prokaryotic RNA polymerase holoenzyme?Answer: It helps RNA polymerase recognize promoters and appropriate transcription initiation sites.
In rho-dependent termination, how does rho help end transcription?Answer: It moves along the RNA and catches the polymerase after the polymerase stalls.
Why can bacterial ribosomes translate an mRNA before RNA polymerase has finished transcribing it?Answer: Bacteria lack a membrane-bound nucleus separating transcription from translation.
Why is the bacterial promoter −10 region relatively easy to unwind?Answer: Its A–T-rich sequence has base pairs that are relatively easy to separate.
Based on Biology 2e by Mary Ann Clark, Matthew Douglas, Jung Choi (OpenStax), licensed CC BY 4.0. Adapted into a summary, flashcards and quizzes.