Eukaryotic vs Prokaryotic Transcription Quiz

• Transcription is how genetic information passes from DNA to RNA, according to the central dogma of molecular biology.
• In cells, DNA undergoes transcription to produce RNA, which then undergoes translation to produce proteins.
• These proteins carry out biological functions, defining an organism's phenotype based on its genotype

Transcription Features

• It involves only specific regions of DNA, namely coding regions, which are DNA sequences coding for RNA such as mRNA, tRNA, and rRNA.
• Noncoding regions do not code for a gene product.
• Coding regions vary in when they are activated.
• Gene expression is regulated as coding regions are switched on and off over time
• The major control point of gene expression is transcription, specifically its initiation.
• RNA synthesis proceeds in a 5' to 3' direction.
• The three main events in transcription are initiation, elongation, and termination.
• The promoter is located at the 5' end and the terminator at the 3' end.
• The transcription initiation site is marked as +1.
• Upstream refers to the (-) direction of the gene, while downstream is the (+) direction.

RNA Polymerase

• RNA polymerase is multi-functional because it unwinds DNA, binds ribonucleoside triphosphates (rNTPs), and binds the DNA template (antisense).
• RNA polymerase also complements DNA with RNA and rewinds DNA.
• RNA polymerase lacks proofreading capabilities, resulting in an error rate of every 10^4 to 10^5 ribonucleotides incorporated.
• It can initiate chain growth without needing a primer.
• Prokaryotes have one kind of RNA polymerase, while eukaryotes posses 3.

Eukaryotic RNA Polymerase Types

Transcription Elements & Factors

• Promoters are key for transcription.
• In prokaryotes (E. coli), promoters include the -35 region with sequence TTGACA and the Pribnow box (-10 region) with sequence TATAAT.
• The -35 region is approximately 15-20 base pairs upstream from the Pribnow box, which is 5-8 base pairs from the initiation site.
• Eukaryotic promoters include the Goldberg/Hogness box (-25 region), with a -75 region sequence of GGCCAATCT, and a -25 region sequence of TATA, followed by +1 as transcription starts.
• Enhancers are DNA sequences activating transcription that can be located thousands of bases away from the promoter.
• They contain specific binding sites for gene regulatory proteins activating transcription, found in eukaryotes, but less common in prokaryotes.
• Terminators are sequences signaling the end of RNA polymerase activity that releases the DNA template and the RNA transcript, resulting in polymerase detachment.
• Transcription factors in eukaryotes assemble in a complex on the DNA at the promoter in order to recruit RNA polymerase to the site.

Transcription Stages

• Initiation begins at the startpoint within the transcription unit of DNA.
• RNA polymerase binds, initiating the process.
• Elongation involves the unwinding of DNA and addition of RNA nucleotides to the template strand.
• Termination occurs upon reaching the termination point, completing the RNA transcript

mRNA Processing

• Capping involves the addition of a methylated guanine nucleotide to the 5' end of the RNA molecule.
• It occurs after about 30 nucleotides have been added to the growing RNA transcript.
• Capping functions to protects the transcript from degradation and facilitates ribosome recognition.
• Tailing is adding 100-200 residues of adenylic acid to the 3' end of transcript.
• It depends poly A polymerase, which is a nuclear enzyme.
• A signal for cleavage consists of AAUAAA located 10-30 nucleotides upstream from the cleavage site
• Tailing aids mature mRNA export from the nucleus, promotes RNA stability, and serves as a ribosome recognition signal.
• Splicing involves processing hnRNA that is not yet fully developed (≠ mature RNA)
• Splicing is the process of cutting out introns and stitching together the exons to form mature RNA that codes a protein.
• Splicing signals tell the splicing machinery where to cut and paste.

rRNA Processing

• It begins with the start of a large RNA precursor gene.
• In E. coli, there are 7 RNA precursor genes encoding different types and numbers of tRNA.
• Ribonuclease III (RNase III) facilitates this processing.
• It starts with the start of a large RNA precursor gene which leads to 30S precursor RNA that results in the final products 16S, tRNA, 23S, 5S and tRNA
• In eukaryotes, RNA precursor genes exist in clusters.
• rRNA genes are located in the nucleolar part of the nucleus inside mammalian cells and include 100-2000 per cell
• Transcription units are separated by non transcribed spacers.
• Transcribed by RNA polymerase I into giant primary transcripts, consisting of 18S, 5.8S and 28S rRNA + ETS + ITS.
• Many such genes are transcribed together as part of the same transcription unit
• Nucleotide modification can be achieved via snoRNA-dependent mechanisms like pseudouridylation and methylation.
• Proper folding of rRNA in combination with the binding of riboproteins
• There are also exo- and endo-nucleolytic cleavages aided by snoRNA in complex with proteins.

tRNA Synthesis

• In prokaryotes, tRNA genes tend to be found in clusters requiring cleavage from a large tRNA precursor.
• In eukaryotes, tRNA are clustered and transcribed individually via RNA polymerase III.

Test your knowledge of transcription factors, mRNA processing, splicing, and rRNA synthesis in eukaryotic and prokaryotic cells. This quiz covers key differences and functions, including the role of enhancers and terminators.