Post-Transcriptional and Post-Translational Modifications

Post-transcriptional modifications are crucial in refining gene expression control. These modifications occur after the initial transcription of DNA into RNA and include:

1. RNA splicing: Removal of introns and joining of exons
2. Addition of 5' cap: Occurs as new RNA leaves the RNA polymerase
3. Addition of 3' polyadenylation: Adds a chain of adenine molecules to the 3' end

Definition: RNA splicing is the process of removing introns and joining exons to produce mature mRNA.

The 5' cap and 3' poly$A$ tail serve important functions:

• They help prevent endonuclease activity on the RNA
• Encourage translation
• Prevent RNA degradation

Post-translational modifications also play a significant role in gene expression control. One of the most important post-translational adaptations is phosphorylation.

Highlight: Phosphorylation activates or deactivates nearly half of all enzymes, making it a crucial mechanism for regulating protein function.

For example, cAMP activates an enzyme called PKA $Protein Kinase A$, which in turn activates other enzymes by adding a phosphate group. This process is involved in various cellular functions, including:

• Enzymes in glycogenolysis
• Enzymes promoting cardiac muscle contraction

Example: The activation of PKA by cAMP demonstrates how post-translational modifications can create a cascade of effects, amplifying the initial signal and leading to significant changes in cellular function.

These various levels of control – transcriptional, post-transcriptional, and post-translational – allow for precise regulation of gene expression, enabling cells to respond efficiently to their environment and maintain proper function.