🔤 Translation and Protein Synthesis

The Genetic Code

Codon: Three-nucleotide sequence in mRNA that codes for amino acid

Key features:

• 64 codons total (4³ = 64)
• 61 code for amino acids
• 3 are stop codons: UAA, UAG, UGA
• 1 start codon: AUG (also codes for methionine)

Properties:

1. Redundant (degenerate):

   • Multiple codons for most amino acids
   • Example: Leucine has 6 codons

2. Unambiguous:

   • Each codon specifies only ONE amino acid

3. Nearly universal:

   • Same in almost all organisms
   • Strong evidence for common ancestry

4. Non-overlapping:

   • Read in groups of 3, no overlap
   • Frame matters (reading frame)

tRNA Structure and Function

Structure:

• Cloverleaf shape (2D), L-shape (3D)
• Anticodon: three bases that pair with mRNA codon
• Amino acid attachment site: 3' end

Function:

• Brings correct amino acid to ribosome
• Anticodon pairs with codon (complementary, antiparallel)

Aminoacyl-tRNA synthetase:

• Enzyme that attaches amino acid to correct tRNA
• 20 different synthetases (one per amino acid)
• Ensures accuracy

Ribosome Structure

Two subunits:

• Small subunit: binds mRNA
• Large subunit: catalyzes peptide bond formation

Three binding sites:

• A site (aminoacyl): incoming tRNA
• P site (peptidyl): tRNA with growing chain
• E site (exit): empty tRNA leaves

Ribosomal RNA (rRNA):

• Catalytic component
• Ribozyme: RNA with enzymatic activity
• Forms peptide bonds

Translation Process

1. Initiation

Prokaryotes:

• Small ribosomal subunit binds mRNA at Shine-Dalgarno sequence
• Initiator tRNA (fMet-tRNA) binds start codon (AUG)
• Large subunit joins
• Initiation factors help

Eukaryotes:

• Small subunit binds 5' cap
• Scans for start codon (AUG)
• Initiator tRNA (Met-tRNA) binds
• Large subunit joins

2. Elongation

Three steps (repeating):

1. Codon recognition:

   • Aminoacyl-tRNA enters A site
   • Anticodon pairs with codon

2. Peptide bond formation:

   • rRNA catalyzes peptide bond
   • Amino acid transferred from P site to A site
   • Growing chain now on A-site tRNA

3. Translocation:

   • Ribosome moves 3 nucleotides (one codon)
   • tRNA in A site → P site
   • tRNA in P site → E site → exits
   • A site now empty for next tRNA
   • Requires GTP and elongation factors

3. Termination

• Stop codon enters A site (UAA, UAG, or UGA)
• Release factors bind (no tRNA for stop codons)
• Polypeptide released
• Ribosomal subunits dissociate

Polyribosomes (Polysomes)

• Multiple ribosomes on one mRNA
• Simultaneous translation
• Increases protein production efficiency

Mutations

Point Mutations

Silent mutation:

• Changes codon but NOT amino acid (due to redundancy)
• Usually no effect

Missense mutation:

• Changes codon → different amino acid
• May affect protein function
• Example: sickle cell (Glu → Val)

Nonsense mutation:

• Changes codon → stop codon
• Premature termination
• Nonfunctional protein (usually)

Frameshift Mutations

Insertion or deletion of nucleotides (not multiple of 3)

• Shifts reading frame
• All downstream codons changed
• Usually severe effects

Post-Translational Modifications

After translation, proteins may be modified:

• Cleaving signal sequences
• Adding chemical groups (phosphorylation, acetylation)
• Adding sugars (glycosylation)
• Folding with chaperones
• Forming disulfide bonds

Key Concepts

1. Genetic code: 64 codons, 61 for amino acids, 3 stop, 1 start (AUG)
2. tRNA: brings amino acids, anticodon pairs with codon
3. Ribosome: catalyzes peptide bonds (rRNA is ribozyme)
4. Three sites: A (incoming), P (peptide), E (exit)
5. Elongation: codon recognition → peptide bond → translocation
6. Stop codons: UAA, UAG, UGA (no tRNA, release factors bind)
7. Mutations: silent, missense, nonsense, frameshift