Molecular Biology: Central Dogma and Transcription

Central Dogma of Molecular Biology

• The central dogma states that genetic information flows in one direction: DNA to RNA to protein.
• RNA can also be directly translated into protein in certain cases.

RNA Structure and Stability

• RNA is a single-stranded nucleic acid.
• RNA's backbone is made of alternating phosphate groups and ribose sugar.
• DNA is more stable than RNA due to its chemical properties and double-stranded structure.

Transcription

• Transcription is the process of copying DNA information into mRNA.
• DNA serves as a stable template for this process.
• Transcription occurs in three main steps: initiation, elongation, and termination.
• The process is always 5' → 3'.
• Template and non-template (coding) strands are relative to the gene.

Transcription Initiation: General Overview - Prokaryotes

• RNA polymerase binds to the promoter region.
• The promoter region (like the Pribnow box/TATA box) is not transcribed.
• Promoter strength dictates transcription rate.
• Strong promoters lead to high rates, while weak ones lead to low rates.

Transcription Elongation

• Core RNA polymerase carries out elongation by sequentially adding RNA nucleotides.
• A 17 base pair transcription bubble is maintained during elongation.

Transcription Termination

• Rho-independent: Termination involves a GC-rich stem-loop followed by a uracil run.
• Rho-dependent: Pause sites become termination sites with Rho factor presence.

Eukaryotic Polymerases

• Pol I transcribes ribosomal RNA
• Pol II transcribes protein-coding genes
• Pol III transcribes tRNA and other small RNAs

Eukaryotic Transcription Initiation

• General transcription factors and RNA polymerase II bind to the promoter.
• DNA unwinding occurs.
• The carboxyl-terminal domain (CTD) of RNA polymerase II is phosphorylated.
• The first 5-10 phosphodiester bonds are synthesized.
• RNA polymerase II and many general transcription factors are released.

Eukaryotic Promoter Elements

• TATA box: A promoter element similar to the prokaryotic Pribnow box, crucial for binding RNA polymerase II.
• Initiator Site: Another element that is bound by TAFs (proteins that bind to TBP) which further bind to the TATA box.

RNA Processing (Eukaryotes Only)

• Eukaryotic pre-mRNAs are larger than mature mRNAs.
• Introns (intervening sequences) are removed during processing, leaving only exons (expressed regions) for translation.
• Introns are removed by the spliceosome, a large RNA-protein complex. The spliceosome is composed of multiple subunits (U1, U2, U4/U6).

5' Cap

• Added immediately after transcription initiation.
• A 7-methylguanosine cap is added in a 5' to 5' linkage.
• Protects the mRNA from degradation and is essential for translation initiation.

Poly A Tail

• Added to the 3′ end of the mRNA.
• Not encoded in the DNA sequence.
• Assists in RNA transport outside the nucleus.

Alternative Splicing

• One gene can produce multiple proteins with varying functions due to alternative splicing.

Explore the fundamental concepts of molecular biology including the central dogma, RNA structure, and the transcription process. This quiz covers key mechanisms of how genetic information is expressed and processed in cells. Test your knowledge on prokaryotic transcription initiation and the roles of different RNA types.