Genetic Code, Mutations, and Translation

  >   Rahul's Noteblog   >   Notes on Biochemistry   >   Genetic Code, Mutations, and Translation

Proteins Modified Before Use:

• Proteins undergo post-translational modifications before they play their roles in cell function.

Genetic Mutations:

Silent:

• Asymptomatic.

Missense:

• Different amino acid specified; decrease in function.

Nonsense:

• Stop codon; shorter and nonfunctional.

Frameshift:

• Deletion/addition of base; shorter and nonfunctional.

Triplet repeat expansion:

• Protein product is longer and unstable; gets worse each successive generation.

Large Segment Deletions.

• Parts of chromosomes deleted during unequal crossover in meiosis I.

Alpha-Thalassemia:

• Deletion of alpha-globin genes from chromosome 16.

Beta-Thalassemia:

• Deficiency of beta-globin; malarial protection (?).

Cri-Du-Chat:

• Deletion of short arm of chromosome 5.

Genetic Mutations in Splice Sites:

• Inaccurate removal of introns.

• Cause of beta-thalassemia.

Trinucleotide Repeat Expansion:

• Defective allele has mutated trinucleotide.

• Trinucleotide may be a coding or noncoding region, and increases in size with each successive generations, exacerbating the disease symptoms.

• Huntington disease.

• Fragile X.

• Myotonic dystrophy.

Amino Acid Activation:

• tRNA correlates amino acids to their corresponding codons using their anti-codon sequences.

• Each amino acid activated by a different aminoacyl tRNA synthetase.

• Aminoacyl tRNA synthetase transfers amino acid to tRNA.

Eukaryotic Translation:

• In prokaryotes: transcription and translation occur simultaneously.

• In eukaryotes: transcription and translation occur consecutively.

• Translation occurs on ribosomes in the cytoplasm.

Initiation in Eukaryotes

• Met-carrying tRNA binds to AUG on mRNA; ribosome also binds to that AUG on mRNA.

Initiation in Prokaryotes

• 16S rRNA binds to Shine-Dalgarno sequence of 5' mRNA.

Elongation and Termination:

• Elongation: Ribosome moves in 5' to 3' direction and synthesizes protein.

• Termination: translation stops when stop/termination codon moves into A site of ribosome.

Cystic Fibrosis:

• Abnormal protein folding gives rise to defective CFTR.

• Increased sodium and water absorption; decreased chloride secretion.

Polyubiquinated Proteins:

• Proteins are marked for destruction.

N-terminal Hydrophobic Signal:

• Ensures protein translation on RER.

• Process occurs on free cytoplasmic ribosomes.

• Proteins destined to be secreted, eg., insulin, placed in cell membrane, or sent to lysosomes.

• New proteins acquire oligosaccharide chains while passing through ER and Golgi.

Lysosomal Enzymes:

• Pass through Golgi and their mannose residues are phosphorylated.

• Mannose-6-phosphate sends proteins to lysosomes.

• Genetic defects here cause I-Cell disease.

Alpha-1-Antitrypsin Deficiency:

Alpha-1-antitrypsin protein (inhibits proteases during an inflammatory response) is misfolded and accumulates in ER, where it damages the cell leading to cirrhosis.

• Point mutations produces Z and S variants.

• Liver cirrhosis.

Co- and Posttranslational Modifications of Proteins:

Glycosylation:

• Oligosaccharide addition in Golgi and ER.

Proteolysis:

• Peptide bond cleavage and activation.

Phosphorylation:

• Protein kinases add phosphate.

Gamma-Carboxylation:

• Ca2+ binding sites produced.

Prenylation:

• Farnesyl and geranylgeranyl lipid groups added.

Posttranslational Modifications of Collagen:

• Structure composed of repeating tripeptide.

In the Rough Endoplasmic Reticulum:

• Hydroxyproline produced by hydroxylation of prolyl residues.

• Hydrophobic prepro-alpha chains synthesized.

• Pro-alpha chains formed by signal sequence removal.

• Prolines and lysines hydroxylated; vitamin C required. [Scurvy]

• Hydroxylysines glycosylated.

• Triple helical structure (procollagen) formed.

In the Golgi:

• Further modification of oligosaccharide.

• Protein is secreted from the cell.

Outside of Cell:

• Cleavage of propeptides.

• Fibril formation and aggregation to form collagen fiber.

• Stabilized by Lysyl Oxidase (Cu+ and O2 required).

Menkes Disease:

• AKA Ehlers-Danlos type IX.

• Mutated ATP7A; copper absorbed but not transported in blood.

• Severe copper deficiency causes abnormal synthesis and processing of collagen.

Ehlers-Danlos Type IV:

• Mutations in gene for type-3 procollagen.

Additional Notes:

• Bacterial methylases: protect DNA from restriction enzymes.

• To start DNA synthesis, DNA polymerases require a primer, which is a made of RNA.

• Signal recognition particle: brings mRNA-ribosome complex into ER.

• Enhancer: enhances transcription of eukaryotic genes.

• Mg2+ removed from ribosomal proteins because it releases ribosomal proteins from the large subunit.

• Insulin undergoes proteolysis before activation: denatured then renatured.

• CAP gene makes glucose repress the lac operon.

• Bloom syndrome: defective DNA ligase I.

• Gene hotspot: more mutations occur here.

• p53 gene mutations determined at best by family history.

Additional Readings:

Basic Biochemistry

1. Nucleic Acid Structure and Organization
2. DNA Replication and Repair
3. Transcription and RNA Processing
4. Genetic Code, Mutations, and Translation
5. Genetic Regulation
6. Recombinant DNA
7. Amino Acids, Proteins, Enzymes
8. Hormones
9. Vitamins
10. Energy Metabolism
11. Glycolysis and Pyruvate Dehydrogenase
12. Citric Acid Cycle and Oxidative Phosphorylation
13. Glycogen, Gluconeogenesis, and Hexose Monophosphate Shunt
14. Lipid Synthesis and Storage
15. Lipid Mobilization and Catabolism
16. Amino Acid Metabolism Disorders
17. Purine and Pyrimidine Metabolism
18. Electron Transport
19. Citric Acid Cycle and Glyoxylate Cycle
20. Glycolysis
21. Pyruvate Metabolism
22. Mitochondrial ATP formation
23. Gluconeogenesis
24. Glycogen Metabolism
25. Nitrogen Fixation (Metabolism) reactions, and Heme Metabolism
26. Amino Acid Metabolism
27. What is Medium Chain Acyl-CoA Dehydrogenase Deficiency (MCADD)?

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