Genetic Code and Gene Expression

Introduction to Genetic Code

• The genetic code has redundancy; different codons can code for the same amino acid.
• Example: The codons AAA and AAG both code for the amino acid lysine.
• Use of codon charts (circular and rectangular) to illustrate codon relationships.

Translation Process

• Translation is the process where messenger RNA (mRNA) codons are used to synthesize proteins.
• Occurs at the ribosome in the cytoplasm.
• Ribosomes read the codon sequence in mRNA, where each codon consists of three nucleotides specifying an amino acid.
• The ribosome builds a polypeptide chain, eventually folding into a functional protein.
  • Proteins fold into specific shapes, which is crucial since many proteins act as enzymes that require specific structures to function properly.

Pathway from Genes to Phenotype

• The process from gene to physical traits (phenotype) follows these steps:
  • Gene → Messenger RNA → Protein → Function → Trait
• Proteins can serve various roles:
  • Enzymatic actions (e.g., creating pigments)
  • Structural proteins (e.g., building hair)
  • Hormones regulating growth.
• Phenotype is determined not only by the type of protein made but also the quantity and the cell types involved.
  • Examples of phenotypes include:
  • Morphological traits
  • Enzymatic functions
  • Structural components like ligaments and tendons
  • Behavioral traits and physical abilities.
• Gene expression influences observable traits, regulatory functions, and health outcomes.
  • Genetic predisposition (e.g., type 2 diabetes) exemplifies environmental interaction with gene expression, affecting trait manifestation.

Consideration of Gene Expression

• Gene expression can determine observable traits, regulate metabolic functions, and affect health outcomes.
• Variability in expression leads to differences like appearance influenced by genetics (e.g., nose shape inherited from parents).
• Certain genetic conditions can skip generations, raising questions about gene expression continuity (e.g., some cancers).

Case Study: Phenylketonuria (PKU)

• PKU is linked to a genetic mutation affecting the enzyme phenylalanine hydroxylase.
  • This enzyme is critical for converting phenylalanine into tyrosine; a mutation leads to toxic levels of phenylalanine.
  • Consequences of untreated PKU can include brain damage; newborns are screened for this condition within 24 hours.
• It is essential to understand both the normal function of the phenotype and the implications of genetic mutations for examination purposes.

Regulation of Gene Expression

• Gene expression is finely tuned and involves multiple layers of control:
  • Pretranscriptional Control
  • Ensures genes are ready for transcription.
  • Involves uncoiling DNA and chemical modifications of histones.
  • Acts as the initial checkpoint to determine whether a gene is ready for copying.
  • Transcriptional Control
  • Occurs during transcription and is an efficient regulation point.
  • Use of transcription factors to either promote or block messenger RNA (mRNA) production during transcription.
  • Post-transcription, pre-mRNA undergoes editing to remove non-coding regions (introns) by spliceosomes, resulting in mature mRNA.
  • Exons (coding regions) can be spliced together in different combinations, allowing one gene to produce multiple protein variants (alternative splicing).
    • Analogy: Similarity to sentence alteration (e.g., "The quick brown fox jumps" vs. "The fox jumps") demonstrating different but valid constructs.
  • Translational Control
  • Occurs in the cytoplasm, regulating protein synthesis by controlling mRNA availability and translation.
  • Role of microRNAs (miRNAs) in binding to mRNA to inhibit translation and small interfering RNAs (siRNAs) in marking mRNA for destruction.
  • Post-Translational Control
  • Involves modifications after protein synthesis that can include:
    • Addition of phosphate groups, sugars, or fats.
    • Proteolytic cleavage (cutting into smaller pieces).
    • Targeting proteins to specific locations within the cell for function.
  • Modifications activate proteins into their functional forms.

Differences Between DNA, RNA, and Protein

• Understanding the distinct roles and characteristics of DNA, RNA, and proteins is crucial in molecular biology.