Transcription: Eukaryotes vs. Prokaryotes

Overview of Transcription

Transcription is the process by which genetic information encoded in DNA is copied into messenger RNA (mRNA) for subsequent protein synthesis. This process is fundamental to gene expression and differs in several key ways between prokaryotes and eukaryotes.

• Transcription occurs in the nucleus of eukaryotes and in the cytoplasm of prokaryotes.

• RNA polymerase is the enzyme responsible for synthesizing RNA from a DNA template.

• Transcription proceeds in the 5' to 3' direction, using the 3' to 5' DNA template strand.

Transcription in Prokaryotes

Prokaryotic transcription is relatively simple and rapid, involving a single type of RNA polymerase and minimal processing of mRNA.

• RNA polymerase core enzyme synthesizes RNA; the σ (sigma) subunit recognizes promoter sequences.

• Promoter regions are specific DNA sequences upstream of the gene, typically at -10 (TATAAT) and -35 (TTGACA) positions.

• The sigma subunit binds to these consensus sequences to initiate transcription.

• Unlike DNA polymerase, RNA polymerase does not require a primer to begin synthesis.

• Transcription proceeds rapidly (e.g., in E. coli, about 50 nucleotides per second at 37°C).

Prokaryotic Promoter Table

Transcription in Eukaryotes

Eukaryotic transcription is more complex, involving multiple RNA polymerases, transcription factors, and extensive mRNA processing.

• RNA polymerase II transcribes mRNA and requires transcription factors to bind to DNA.

• Transcription factors are classified as general (required for all genes) or specific (regulate particular genes).

• Core promoter elements and regulatory sequences (enhancers, silencers) control gene expression.

• Chromatin must be remodeled to allow access to DNA for transcription.

• Primary RNA transcripts (pre-mRNA) contain exons (coding regions) and introns (non-coding regions).

• Introns are removed by splicing to produce mature mRNA.

• mRNA is processed by addition of a 5' cap (7-methylguanosine) and a 3' poly-A tail (up to 250 adenines).

• Processed mRNA is exported from the nucleus to the cytoplasm for translation.

Key Steps in Eukaryotic mRNA Processing

1. Transcription produces a primary RNA transcript

2. 5' cap is added for stability and ribosome recognition

3. Polyadenylation signal (AAUAAA) triggers addition of the poly-A tail.

4. Splicing removes introns and ligates exons.

Splicing Mechanism

• Spliceosomes, composed of small nuclear RNAs (snRNAs) and proteins (snRNPs), recognize splice sites.

• Introns are excised as lariat structures and exons are joined to form mature mRNA.

Transcriptional Differences: Eukaryotes vs. Prokaryotes

Example: Transcription of a DNA Template Strand

Given the DNA template strand: 3'-TCAATAACTACA-5'

• The transcribed mRNA sequence is: 5'-AGUUAUUGAUGU-3'

• Base pairing rules: A → U, T → A, C → G, G → C

Calculating Maximum Number of Amino Acids

For a primary RNA transcript with two exons (519 and 450 nucleotides) and one intron (330 nucleotides):

• Total coding nucleotides: 519 + 450 = 969

• Subtract 3 nucleotides for the stop codon: 969 - 3 = 966

• Number of amino acids:

Key Terms

• Promoter: DNA sequence where RNA polymerase binds to initiate transcription.

• Exon: Coding region of a gene that remains in mature mRNA.

• Intron: Non-coding region removed during RNA splicing.

• Spliceosome: Complex responsible for removing introns from pre-mRNA.

• Polycistronic mRNA: mRNA that encodes multiple proteins (common in prokaryotes).

• Monocistronic mRNA: mRNA that encodes a single protein (typical in eukaryotes).

Additional info:

• Enhancers and silencers are regulatory DNA sequences that can be located upstream, downstream, or within introns, affecting transcription rates.

• Chromatin remodeling is essential in eukaryotes to allow transcription machinery access to DNA.