Molecular Biology: Transcription Overview

Transcription Overview

• Transcription is the DNA-directed synthesis of RNA
• Gene expression begins with transcription
• To produce mRNA, a gene sequence on DNA and the exact start site are needed

Gene Structure

• Typically, each gene contains two types of information:
  • Specifies the primary structure of the final product
  • Critical for gene product expression regulation.
• Genes are composed of:
  • Coding exons
  • Non-coding introns
  • Non-coding consensus sequences (promoters and splice sites)

Promoters

• DNA sequences that determine the start site of RNA synthesis.
• Often contain a "TATA box" sequence located 15-30 base pairs upstream of the transcription start site.
• Other sequences, like CAAT and GC boxes, are also important for promoter function.
• In eukaryotes, transcription factors bind to the TATA box to facilitate RNA polymerase binding.

Splice Acceptor and Donor Sequences

• Consensus sequences at the 5' and 3' ends of introns
• Introns often begin with GU and end with AG, preceded by a pyrimidine-rich tract.
• Sequences are crucial for intron removal during RNA processing.

Features of Transcription

• Selective: Only specific parts of the genome are transcribed at any given time and cell.
• Primary RNA transcripts are modified

RNA Synthesis

• Occurs in the nucleus
• Catalyzed by RNA polymerase
• Protein-coding genes produce mRNA as an intermediate.
• Regulatory mRNA sequences (in the 5' and 3' UTRs) affect stability and translation efficiency.
• The initial mRNA is then processed/edited.

Transcription Complex Formation

• RNA polymerases: Several exist in eukaryotic cells.
• Pre-initiation complex formation: Proteins bind to the gene to be transcribed.
• This complex positions RNA polymerase on DNA.
• Regulatory regions: mRNA-producing genes can be divided into coding and regulatory regions. Regulatory regions include those for basal expression and where expression is regulated
• Basal transcription complex formation requires transcription factors.
• A single-stranded RNA is produced from double-stranded DNA.

Newly Synthesized RNA

• Complementary to the DNA template strand.

RNA Processing

• Gene transcription yields RNA that's larger than the cytoplasm's mRNA for translation.
• Primary transcript contains transcribed intron segments.
• Introns are removed, and exons are joined to form mature mRNA.
• mRNA is "capped" (methyl guanosine cap) at the 5' end and contains a polyadenine tail at the 3' end.

Intron Removal

• Splice sites within the gene define introns.
• Spliceosomes convert primary transcripts into mRNA.
• Spliceosomes are made of the primary transcript, small nuclear RNAs (snRNAs), and > 50 proteins (snRNPs).
• Mature RNA leaves the nucleus through nuclear pores and enters the cytoplasm.

Alternative Splicing

• Genes can create multiple proteins by joining different exon segments in the primary transcript.
• Cell-specific regulation may explain why the human genome has fewer genes than anticipated.

Rifampin

• Inactivates prokaryotic RNA by binding to RNA polymerase and altering its conformation, preventing RNA synthesis.
• Eukaryotic RNA polymerase isn't affected by rifampin.

Mutations and Disease

• Mutations in splice sites, the TATA box, or intron-exon boundaries can cause diseases like thalassemias.
• Thalassemias are hereditary anemias characterized by decreased hemoglobin production due to mutations affecting globin gene synthesis.

This quiz covers the essential aspects of transcription in molecular biology, including the structure and function of genes, promoters, and splice sequences. Test your understanding of how RNA is synthesized from DNA and the key elements involved in gene expression. Perfect for students learning about molecular genetics!