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.