Transcription

The Molecular Basis of Inheritance (Ch. 16)
Gene Expression (Ch. 17)
  - From Gene to Protein (& Phenotype)
    - 17.1 Genes specify proteins via transcription and translation
    - 17.2 Transcription is the DNA-directed synthesis of RNA
    - 17.3 Eukaryotic cells modify RNA after transcription
    - 17.4 Translation is the RNA-directed synthesis of a polypeptide
    - 17.5 Mutations of one or a few nucleotides can affect protein structure and function
Regulation of Gene Expression (Ch. 18)

All organisms must regulate which genes are expressed at any given time:
- Indicates how organisms control gene expression to ensure proper cellular function.
Cell differentiation:
- Specialization in form and function due to differential gene expression.
Cell function:
- Involves expression of a unique subset of genes.
Key stages in gene expression regulation:
- Primarily occurs during transcription in eukaryotic cells.

The process of transcription involves:
- DNA → RNA transformation.
Key Components:
  - RNA Polymerase: An enzyme that synthesizes RNA by unwinding DNA and joining RNA nucleotides.
  - Template DNA Strand: The DNA strand that serves as a template for RNA synthesis.
  - Newly created RNA strand: Compiled from RNA nucleotides complementary to the template.

Initiation:
   - Occurs when RNA polymerase binds to the promoter region of DNA.
   - The TATA box is crucial for forming the initiation complex in eukaryotes.
   - Several transcription factors must bind to DNA to facilitate RNA polymerase binding.
Elongation:
   - RNA polymerase moves along the DNA, unwinding the double helix and synthesizing RNA.
   - Adds nucleotides to the growing RNA strand in the 5’ to 3’ direction.
   - Transcription proceeds at a rate of 20-40 nucleotides per second.
Termination:
   - For prokaryotes, transcription stops at a terminator sequence, with polymerase detaching.
   - In eukaryotes, transcription continues through a polyadenylation signal (AAUAAA), resulting in the cleavage of pre-mRNA.

Transcription results in pre-mRNA which undergoes modifications before becoming mature mRNA:
  - 5’ Cap: Modified nucleotide added to the 5’ end.
  - Poly-A Tail: 50-250 Adenine bases added to the 3’ end.
  - These modifications protect mRNA and facilitate transport from nucleus to cytoplasm.
  - RNA Splicing: Introns are removed and exons are joined together by the spliceosome to create a continuous coding sequence.

Some genes can encode multiple polypeptides through alternative RNA splicing, allowing different protein variants to be produced.

Chromatin Structure and Histone Modification:
- Methylation (adding -CH₃), acetylation (adding -COCH₃), phosphorylation (adding -PO₄).
Transcription Initiation:
- Transcription factors act as regulators, binding to enhancer/promoter regions and influencing RNA polymerase's ability to initiate transcription.
Post-Transcriptional Control:
- mRNA processing, stability, and transport from nucleus to cytoplasm are regulated.
Translation Efficiency:
- Factors involved in the speed of ribosome operation and subsequent protein modification can also regulate gene expression efficiency.

In prokaryotic cells (bacteria):
- mRNA is produced via transcription and translated immediately without processing.
In eukaryotic cells:
- Transcription and translation are compartmentalized, with RNA being processed post-transcriptionally before translation.

Ensures that genes are expressed in the correct cell type, during the proper time in development, and in response to environmental conditions.