Untitled Quiz

Transcription Overview

• Transcription is the DNA-directed synthesis of RNA.
• It's the first step in gene expression, leading to protein synthesis.
• Each gene contains information specifying the final product and its regulated expression.
• To produce mRNA, the gene sequence and the exact start site are needed on DNA.

Structure of a Typical Eukaryotic Gene

• Composed of coding exons, non-coding introns, and non-coding consensus sequences (promoters).
• Promoters include the TATA box and CAAT box, serving as recognition sites for transcription factors.
• Splice acceptor and donor sequences are crucial for intron removal during RNA processing.
• Intron and exon numbers, sizes, and sequences differ between genes.

Promoters

• DNA sequences determining the RNA synthesis start site.
• A consensus sequence often includes "TATA" located within 15-30 base pairs upstream of the start site (TATA box).
• Other sequences required for promoter function include CAAT box and GC box.
• Transcription factors in eukaryotes bind to the TATA box, facilitating RNA polymerase attachment.

Splice Acceptor and Donor Sequences

• Consensus sequences found at the 5' and 3' ends of introns.
• Introns typically begin with GU and end with AG, preceding a pyrimidine-rich tract.
• Essential for splicing introns from the primary RNA transcript.

Features of Transcription

• Selective: Only specific parts of the genome are transcribed in a cell at a given time.
• Primary RNA transcripts undergo modification.

RNA Synthesis

• Occurs in the nucleus.
• Catalyzed by RNA polymerase.
• Protein-encoding genes generate mRNA as an intermediate.
• Regulatory mRNA sequences (UTRs) in the 5' and 3' untranslated regions affect process stability and translation efficiency but are not part of the final product.
• The initial mRNA is processed/edited.

RNA Polymerase, Transcription Initiation, and Regulatory Regions

• Eukaryotic cells contain multiple RNA polymerases.
• Proteins bind to the gene to be transcribed, initiating the process.
• Regulatory regions of the mRNA-producing gene include coding and regulatory regions.
• The regulatory regions are categorized into those ensuring basal and those regulating gene expression.
• Basal transcription complex formation requires several transcription factors.
• ssRNA is synthesized from dsDNA.

RNA Synthesis: Complementarity to DNA

• Newly synthesized RNA is complementary to the DNA template strand.
• The sequence of the template strand determines the sequence of the synthesized RNA.

Formation of the Transcription Complex

• RNA polymerase II assembly requires general and specific transcription factors.
• Enhancer sequences, coactivators, and TATA box elements contribute to the process.

RNA Processing

• Gene transcription produces larger RNA than mature mRNA.
• Primary transcripts include transcribed introns.
• Introns are removed, and exons are joined to form mature mRNA that's "capped and tailed."
• The 5' cap is a methyl guanosine residue added to protect mRNA from degradation and facilitate ribosome binding.

Addition of 5' Cap and Poly(A) Tail

• 5' cap: methylguanosine to prevent degradation and facilitate ribosome binding in protein synthesis.
• Poly(A) tail: a highly conserved AAUAAA sequence (polyadenylation signal) near the 3' end aids in mRNA stability and export.

Intron Removal

• Splice sites within genes delineate introns.
• The spliceosome removes introns and joins exons, forming mRNA.
• The spliceosome comprises the primary transcript, small nuclear RNAs (snRNAs), and snRNPs.
• Mature mRNA exits the nucleus through nuclear pores.

Alternative Splicing

• Genes can generate multiple proteins by joining different exon combinations of the primary transcript.
• Cell-specific regulation contributes to diverse protein production.

Inactivation of Prokaryotic RNA by Rifampin

• Rifampin inhibits prokaryotic RNA synthesis by binding to RNA polymerase and disrupting its conformation.
• Prokaryotic vs. eukaryotic RNA polymerase function differences are key.

Mutations in Splicing and Disease

• Mutations in splicing can lead to various diseases including thalassemias.
• Point mutations in TATA boxes, splice junctions, or intron-exon boundaries are potential contributors to thalassemias and other genetic diseases.