Microbial Genetics - Translation and Gene Regulation

Microbial Genetics

Translation

  • mRNA is converted into polypeptide (amino acids) during gene expression.

  • mRNA is read in triplets (codons) corresponding to amino acids.

  • Genetic code: 61 sense codons (amino acids) and 3 stop codons.

  • Redundancy in genetic code allows for misreading without affecting the protein.

Genetic Code Overview

  • Each nucleotide triplet on mRNA specifies an amino acid.

  • Proteins' primary structure determines their shape and function.

  • Proteins are responsible for the phenotype of organisms.

Ribosomes

  • Composed of rRNA (ribozymes) and ribosomal proteins.

  • Sites in ribosomes: A (aminoacyl), P (peptidyl), and E (exit).

  • Reading direction: mRNA from 5' to 3', synthesizing polypeptide from N terminus to C terminus.

Translation Steps

  1. Initiation: Large and small ribosomal subunits form a complex, tRNA binds to start codon (AUG).

  2. Elongation: tRNAs bring amino acids, peptide bonds are formed between amino acids.

  3. Termination: Stop codon recognized, polypeptide released, ribosomal components recycled.

Post-Translational Modifications

  • Chemical modifications, protein folding, protein cleavage for functionality.

  • Modifications can include adding phosphates, lipids, or sugars to proteins.

Gene Regulation

  • Not all genes are expressed at the same time; housekeeping genes are constitutively expressed (e.g., enzymes for basic life processes).

  • Regulated genes can be turned on (induced) or off (repressed) as needed.

  • Operons: groups of related genes regulated as a unit, controlled by promoter and operator.

Examples of Gene Regulation

  • Lac Operon: Inducible operon; transcription blocked in absence of lactose.

  • Trp Operon: Repressible operon; synthesis blocked in presence of tryptophan.

Terminology

  • Codon: Three-nucleotide sequence coding for an amino acid.

  • Sense Codons: Code for proteins; Nonsense Codons: Stop protein synthesis.

  • Inducer: Molecule that activates transcription; Repressor: Protein that inhibits transcription.