Biology Chapter: Translation and Genetic Code

Translation Overview

• Translation is the synthesis of proteins from mRNA
• The process requires nucleic acids, DNA replication, and transcription
• It involves the structure of nucleic acids

The Genetic Code

• Proteins with 20 different amino acids are coded by a mRNA sequence with only four nucleotides
• A codon is a sequence of three nucleotides that encodes an amino acid
• There are 64 possible combinations of codons

Codon Chart

• A table showing the relationship between codons and amino acids
• Page 4 of the images shows a detailed codon table

Reading Frames

• A reading frame is a way of grouping nucleotides in threes to read the genetic message
• Three possible reading frames in the mRNA sequence
• These reading frames are non-overlapping on a single mRNA template

Mutations

• Mutations are changes in the nucleotide sequence
• Point mutations: changes in a single nucleotide, including a substitution, insertion, and deletion
• Affects the amino acid chain if substitutions occur
• Frame-shift mutations: addition or deletion of a base
• These frame-shift mutations can alter the reading frame and create different amino acid sequences

Trinucleotide Repeat Expansion Mutations

• Repeating sequences of three bases are amplified in the DNA
• Within the coding region results in extra copies of amino acids
• Within the non-coding region leads to a decrease in protein produced

Huntington's Disease

• A disease caused by a trinucleotide repeat expansion mutation, increasing glutamine residues in the huntingtin protein
• An example of triplet expansion disease

Cystic Fibrosis

• Caused by a deletion mutation in the CFTR protein.
• This results in the loss of a phenylalanine

Steps in Protein Translation

• Initiation: involves the assembly of components of the translation system before peptide bond formation
  • Two ribosomal subunits, mRNA, initiator tRNA, the aminoacyl-tRNA for the first codon, GTP, and initiation factors
• Elongation: addition of amino acids to the carboxyl end of the growing chain
  • Elongation factors bring in an aminoacyl-tRNA that pairs with the next codon, forming the next peptide bond.
  • GTP is used and is then exchanged for GDP plus a phosphate.
• Termination: occurs when one of the three termination codon moves into the A site. The termination codons are UAG, UAA, or UGA.
• This process requires a release factor, with GTP being hydrolyzed

Post-translational Modification

• Covalent alterations
• Phosphorylation (adds phosphate)
• Glycosylation (adds carbohydrates)
• Hydroxylation (adds hydroxyl functional groups)
• Biotinylation (adds biotin)
• Farnesylation (adds farnesyl groups)
• Trimming
• Example modifications of polypeptide chains
• Post translational modifications can happen after the peptide chain has been formed.

Energy Requirements of Translation

• Four high energy bonds are broken per amino acid (a.a.) during protein translation
  • Amino acid binding to tRNA
  • Aminoacyl-tRNA to ribosome
  • Ribosome translocation

Medical Relevance

• Antibiotics attach to translational machinery to target prokaryotes
• Diphtheria toxin affects eukaryotic translation

Explore the fascinating processes of translation and the genetic code in this quiz. Understand the intricacies of codons, reading frames, and mutations. Test your knowledge on how proteins are synthesized from mRNA and the implications of genetic variations.