Genetics - Gene Expression and DNA Mutation

Key Concepts

• DNA provides the code to make proteins.
• Gene expression involves:
  • Transcription: mRNA copy of DNA.
  • Translation: mRNA to protein.
• Mutations change DNA sequence, affecting protein production.

Gene Expression

• A gene is a segment of DNA with a nucleotide base sequence that codes for a protein.

Transcription

• First step in gene expression: mRNA synthesis complementary to DNA.
  • RNA polymerase binds to the promoter sequence (start of gene).
  • mRNA synthesis involves complementary base pairing (U replaces T in RNA).
  • DNA base pairing:
  • Adenine (A) pairs with Uracil (U).
  • Thymine (T) pairs with Adenine (A).
  • Guanine (G) pairs with Cytosine (C).
  • Cytosine (C) pairs with Guanine (G).
• mRNA includes:
  • Initiation sequence: ribosome binding for protein synthesis.
  • Coding sequence: code for protein.
  • Termination sequence: stops protein production.

Translation

• Second step: protein synthesis using mRNA code at ribosomes.
• Codon: Three nucleotide bases on mRNA code for an amino acid.
• tRNA: Transfer RNA molecules provide the amino acid coded by a codon.
  • Anticodon: tRNA sequence that pairs with mRNA codon.
• Ribosomes:
  • Made of large and small subunits (proteins and rRNA).
  • Attach to mRNA; start codon (AUG) signals start, stop codons (UAA, UAG, UGA) signal end.
  • tRNA decodes mRNA, adding amino acids to form a protein via peptide bonds.

DNA Mutation

• Mutation: Change in nucleotide sequence of DNA, copied via replication.

Point Mutation

• Change in a single nucleotide.
  • Can cause a change in the transcribed mRNA and result in the wrong amino acid being added to a protein during translation.
  • Effects range from major changes to no effect on protein function.

Frameshift Mutation

• Nucleotide insertion or deletion alters the reading frame.
  • Results in multiple incorrect amino acids and a non-functioning protein.

Causes of Gene Mutations

1. Replication errors (rare, usually repaired).
2. Transposons: DNA pieces that move and disrupt gene function.
3. Mutagens: Environmental agents that damage DNA.
   • Examples: Ionizing radiation (X-rays, UV rays), some chemicals.
   • Organisms have repair systems.