Transcription and RNA Processing in Genetics

Transcription: The Genetic Code and Transcription

Central Dogma of Molecular Biology

The central dogma describes the flow of genetic information within a biological system. DNA is transcribed into RNA, which is then translated into protein. This process is fundamental to gene expression and cellular function.

• DNA Replication: DNA makes a copy of itself for cell division.

• Transcription: DNA is used as a template to synthesize RNA.

• Translation: RNA is decoded to produce proteins.

• Key Terms: Codon (three-nucleotide sequence in mRNA), Anticodon (complementary sequence in tRNA).

• Example: The genetic code is read in non-overlapping triplets (codons) during translation.

Overview of Transcription

Transcription is the process by which RNA is synthesized from a DNA template. It involves several key elements in both DNA and mRNA.

• Regulatory Sequences: Sites for binding regulatory proteins that influence transcription rate.

• Promoter: Site for RNA polymerase binding; initiates transcription.

• Terminator: Signals the end of transcription.

• Ribosome-binding Site: In mRNA, where translation begins.

• Start Codon: Specifies the first amino acid in a polypeptide.

• Stop Codon: Specifies the end of polypeptide synthesis.

• Polycistronic mRNA: In bacteria, a single mRNA may encode multiple polypeptides.

Mechanism of Transcription

Transcription occurs in three main stages: initiation, elongation, and termination. Each stage involves specific molecular interactions and regulatory elements.

• Initiation: RNA polymerase binds to the promoter region, forming a closed complex. The DNA unwinds, forming an open complex, and transcription begins.

• Elongation: RNA polymerase synthesizes RNA in the 5' to 3' direction, using the template strand of DNA.

• Termination: Transcription ends when RNA polymerase encounters a terminator sequence, releasing the newly synthesized RNA.

RNA Polymerase Structure and Function

RNA polymerase is the enzyme responsible for synthesizing RNA from a DNA template. In bacteria, the holoenzyme consists of a core enzyme and a sigma factor, each with distinct functions.

• Core Enzyme: Composed of five subunits (α2ββ'ω); catalyzes RNA synthesis.

• Sigma Factor: One subunit (σ); recognizes promoter sequences and initiates transcription.

• No Primer Required: Unlike DNA polymerase, RNA polymerase does not require a primer.

Promoters and Consensus Sequences

Promoters are DNA sequences that direct RNA polymerase to the correct site for transcription initiation. In E. coli, promoters contain two consensus sequences: the -35 region and the -10 region (Pribnow box).

• -35 Region: TTGACA

• -10 Region (Pribnow box): TATAAT

• Consensus Sequence: DNA sequences homologous across different genes; essential for efficient transcription initiation.

Cis-acting and Trans-acting Elements

Gene expression is regulated by cis-acting DNA elements and trans-acting factors.

• Cis-acting Elements: DNA sequences located on the same molecule, often adjacent to the gene.

• Trans-acting Factors: Proteins or molecules that bind to cis-acting elements to influence gene expression (e.g., sigma factor).

Transcription Initiation in Bacteria

Initiation relies on the ability of RNA polymerase to bind to the promoter. The sigma factor recognizes the promoter, forms a closed complex, and then an open complex, allowing transcription to begin.

• Closed Complex: RNA polymerase binds to DNA without unwinding.

• Open Complex: DNA unwinds, and RNA synthesis starts.

• Sigma Factor Release: After initiation, the sigma factor is released, and the core enzyme continues transcription.

Transcription Elongation

During elongation, RNA polymerase moves along the template strand, synthesizing RNA in the 5' to 3' direction. The coding strand has the same sequence as the RNA transcript, except T is replaced by U.

• Template Strand: Used for RNA synthesis; also called the antisense strand.

• Coding Strand: Has the same sequence as RNA (except T/U).

• No Proofreading: RNA polymerase does not proofread as DNA polymerase does.

Transcription Termination in Bacteria

Termination releases the newly made RNA and RNA polymerase. There are two mechanisms in E. coli: rho-dependent and rho-independent termination.

• Rho-dependent: Requires the rho protein, which binds to the RNA and causes dissociation.

• Rho-independent: Involves a stem-loop structure and a uracil-rich sequence; does not require rho protein.

Polycistronic mRNA in Bacteria

Bacterial genes with related functions are often organized together and transcribed as a single polycistronic mRNA, encoding multiple proteins. This arrangement is rare in eukaryotes.

Transcription in Eukaryotes

Complexity of Eukaryotic Transcription

Eukaryotic transcription is more complex due to larger genomes, cellular complexity, multicellularity, and increased regulation. Nuclear DNA is transcribed by three different RNA polymerases.

• RNA Polymerase I: Transcribes all rRNA genes (except 5S rRNA).

• RNA Polymerase II: Transcribes all protein-encoding genes and some snRNA genes.

• RNA Polymerase III: Transcribes all tRNA genes and the 5S rRNA gene.

Initiation of Transcription in Eukaryotes

Initiation involves cis-acting DNA elements and trans-acting factors. The core promoter includes the TATA box and transcriptional start site. Enhancers and silencers regulate transcription.

• Core Promoter: TATA box plus transcriptional start site.

• Proximal-promoter Elements: Located near the core promoter.

• Enhancers/Silencers: Increase or decrease transcription rates.

• Transcription Factors: General (GTFs) and regulatory (activators/repressors).

Transcription Factors and Basal Transcription Apparatus

Transcription factors facilitate RNA polymerase binding and are essential for transcription initiation. The basal transcription apparatus consists of RNA polymerase II and five GTFs.

• General Transcription Factors (GTFs): Required for all protein-coding gene transcription.

• Regulatory Factors: Bind to enhancers and silencers.

Termination of Transcription in Eukaryotes

Termination occurs after the polyadenylation signal sequence is transcribed. Two models explain termination: allosteric and torpedo.

• Allosteric Model: RNA polymerase becomes destabilized after the polyadenylation signal.

• Torpedo Model: An exonuclease degrades the RNA downstream of the polyadenylation signal, causing RNA polymerase to dissociate.

Posttranscriptional RNA Processing in Eukaryotes

mRNA Modification

Eukaryotic mRNA is modified before leaving the nucleus. Modifications include poly-A addition at the 3' end, capping at the 5' end, and splicing.

• Poly-A Tail: String of adenine nucleotides added enzymatically after transcription; important for mRNA stability and translation.

• 5' Cap: 7-methyl guanosine added to the 5' end; recognized by cap-binding proteins for translation and splicing.

• Splicing: Removal of introns from pre-mRNA by the spliceosome.

Splicing and the Spliceosome

Splicing ensures colinearity of gene expression. Coding sequences (exons) are interrupted by noncoding sequences (introns), which are removed from pre-mRNA by the spliceosome.

• Introns: Non-coding regions within pre-mRNA; defined by specific sequences at intron-exon boundaries.

• Spliceosome: Multicomponent structure composed of snRNPs; catalyzes intron removal and exon ligation.

• Alternative Splicing: Allows production of different mature mRNAs from the same pre-mRNA, increasing protein diversity.

RNA Editing

RNA editing is a posttranscriptional process that alters the nucleotide sequence of RNA. It includes insertion/deletion editing and substitution editing, prevalent in mitochondrial and chloroplast RNAs in plants.

Comparison of Transcription and RNA Modification in Bacteria and Eukaryotes

Bacterial and eukaryotic transcription differ in promoter structure, initiation, elongation, termination, splicing, capping, and polyadenylation.

Component	Bacteria	Eukaryotes
Promoter	-10 and -35 sequences	TATA box and transcriptional start site
Initiation	Sigma factor required	Transcription factors required
Elongation	RNA polymerase moves along template strand	RNA polymerase II moves along template strand
Termination	Rho-dependent or independent	Polyadenylation signal, allosteric/torpedo models
Splicing	Rare	Common, spliceosome required
Capping	Does not occur	7-methyl guanosine cap
Polyadenylation	Does not occur	Poly-A tail added

Key Equations and Concepts

• Transcription Direction: RNA is synthesized in the 5' to 3' direction.

• Complementarity Rule: AT/GC rule, except U replaces T in RNA.

• Example Equation: $\text{DNA (template strand)} \rightarrow \text{mRNA (5' to 3')} \rightarrow \text{Protein}$

Summary

Transcription is a fundamental process in genetics, converting DNA information into RNA, which is then processed and translated into proteins. The mechanisms and regulation of transcription differ between bacteria and eukaryotes, reflecting their cellular complexity and gene expression needs.