Overview
This lecture covers the major steps of protein synthesis (gene expression) including transcription, mRNA processing, and translation, highlighting key molecular components and processes involved.
Protein Synthesis Overview
- Protein synthesis is how cells use DNA to make proteins, performing their functions.
- Gene expression involves two main steps: transcription (DNA to mRNA) and translation (mRNA to protein).
Genes and the Genetic Code
- A gene is a DNA sequence that codes for a specific protein or trait.
- DNA is made of four nucleotides: adenine (A), thymine (T), guanine (G), and cytosine (C).
- A triplet (three DNA nucleotides) codes for one amino acid; multiple triplets can code for the same amino acid (20 amino acids, 64 combinations).
- The mRNA codon is complementary to the DNA triplet.
- tRNA brings the corresponding amino acid, using the anticodon, which is complementary to the mRNA codon.
Transcription (Nucleus)
- Transcription is the process of copying DNA into mRNA in the nucleus.
- Steps: initiation (transcription factors and RNA polymerase bind to promoter), elongation (adding RNA nucleotides), and termination (ending transcription at a stop sequence).
- Transcription factors (e.g., activated by hormones) help RNA polymerase start transcription.
mRNA Processing
- After transcription, mRNA is modified before leaving the nucleus.
- Introns (non-coding regions) are removed; exons (coding regions) are joined together.
- A 5' cap and poly-A tail are added for stability and export.
- Processed mRNA exits the nucleus to the cytoplasm.
Translation (Cytoplasm)
- Translation converts mRNA into a sequence of amino acids to form a protein.
- Requires mRNA, ribosomes (small and large subunits), and tRNA.
- Initiation: ribosome assembles on mRNA; tRNA with methionine pairs with the start codon (AUG).
- Elongation: tRNAs bring amino acids, matching anticodons to mRNA codons, forming a growing peptide chain.
- The ribosome has three sites (A: aminoacyl, P: peptidyl, E: exit).
- Termination: translation ends at a stop codon; ribosome and new protein detach.
Post-Translational Modification
- New proteins may undergo folding, combining with other chains, or further chemical modifications.
- Proteins made on free ribosomes stay in the cell; those made on rough ER are packaged for export or insertion into membranes.
Key Terms & Definitions
- Transcription — synthesis of mRNA from DNA in the nucleus.
- Translation — synthesis of proteins from mRNA in the cytoplasm.
- Gene — a DNA sequence coding for a protein or trait.
- Triplet — a three-nucleotide DNA sequence coding for an amino acid.
- Codon — a three-nucleotide sequence on mRNA, complementary to DNA triplet.
- Anticodon — a three-nucleotide sequence on tRNA, complementary to mRNA codon.
- Exon — coding region of a gene, expressed in mRNA.
- Intron — non-coding region, removed from mRNA.
- Transcription Factor — protein (often activated by hormones) that enables transcription.
- RNA Polymerase — enzyme that synthesizes mRNA from DNA template.
- 5' Cap/Poly-A Tail — modifications added to mRNA for stability and export.
Action Items / Next Steps
- Review the slides and resources on protein synthesis if concepts are unclear.
- Prepare for the upcoming lecture on chapter 22.
- Contact Dr. Nerin for additional help if needed.