Скачать или смотреть 4. Genetic Code, Mutations & Translation | Medical Biochemistry | MBBS 1st Year | USMLE Step 1

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Genetic Code, Mutations & Translation | Medical Biochemistry | MBBS 1st Year | USMLE Step 1

👋 Hey future doctors and biochem learners! After transcription comes the next big step in the Central Dogma: Translation — converting the nucleotide language of mRNA into the amino acid language of proteins. But errors in the genetic code or mutations can alter this process, leading to genetic diseases and cancer.

This lecture covers the genetic code, types of mutations, and translation, with integrated clinical correlations that are favorites in MBBS, NEET PG, FMGE, and USMLE Step 1. 🧬📚

🌟 The Genetic Code

Triplet codons: 3 bases = 1 amino acid.

Universal (almost): same in all organisms.

Degenerate: multiple codons can code for the same amino acid.

Unambiguous: one codon = one amino acid.

Non-overlapping & comma-less: read continuously from start codon.

Start codon: AUG (Methionine).

Stop codons: UAA, UAG, UGA.

🌟 Mutations

1️⃣ Point Mutations:

Silent: no change in amino acid (due to degeneracy).

Missense: different amino acid (e.g., Sickle Cell Disease – Glu → Val).

Nonsense: codon changed to stop → truncated protein (e.g., Duchenne muscular dystrophy).

2️⃣ Frameshift Mutations:

Insertion/deletion not in multiples of 3 → altered reading frame (e.g., Tay-Sachs).

3️⃣ Splice Site Mutations:

Abnormal splicing (e.g., β-thalassemia).

4️⃣ Expanding Trinucleotide Repeats:

Fragile X, Huntington’s, Myotonic dystrophy.

🌟 Translation (Protein Synthesis)

Location: Ribosomes (cytoplasm; rough ER for secretory proteins).

Steps:
1️⃣ Initiation:

mRNA binds ribosome, start codon (AUG) recognized.

Initiator tRNA (Met) binds.

Requires initiation factors + GTP.

2️⃣ Elongation:

Aminoacyl-tRNA enters A site.

Peptidyl transferase forms peptide bond.

Ribosome translocates → growing peptide moves from A → P site.

3️⃣ Termination:

Stop codon recognized by release factors.

Polypeptide released, ribosome dissociates.

Energy: Both GTP and ATP are required.

🩺 Clinical Correlations

Antibiotics affecting translation:
• Tetracyclines – block tRNA binding.
• Chloramphenicol – inhibits peptidyl transferase.
• Macrolides – block translocation.
• Aminoglycosides – misreading of code.

Diphtheria toxin: inhibits eukaryotic elongation factor 2 (EF-2).

Ricin: inactivates ribosome.

🎯 Exam Integration

AUG = Start codon.

Stop codons = UAA, UAG, UGA.

Missense mutation → Sickle cell.

Nonsense mutation → truncated protein.

Frameshift = Tay-Sachs.
👉 A favorite exam area for both recall and case-based questions in USMLE & NEET PG.

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