Sources of Genetic Variation

Mutations

  • An alteration in DNA sequence
  • 3 types
    • Substitution
    • A nucleotide is replaced by a different one
    • Insertion
    • Addition of one or more nucleotides into a DNA sequence
    • Deletion
    • The loss of one or more nucleotides from a segment of DNA
  • Caused by DNA replication errors and environmental mutations
    • (ie. radiation, chemicals, infections)

Crossing over

  • Homologous chromosomes

    • Each cell has 23 pairs of homologous chromosomes
    • Same genes but different alleles

  • Sister chromatids

    • DNA replicated before meiosis
    • Each chromosome duplicates into 2 identical “sister chromatids”
    • Chromatids between homologous chromosomes are non-identical “non-sister” chromatids

  • Synapsis and chiasmata

    • During prophase 1 of meiosis, homologous chromosomes pair up through synapsis
    • Non-sister chromatids bind to each other through bonds called chiasmata (aka chiasma singular)

  • Chiasmata allows non-sister chromatids to exchange segments of their DNA

  • Produces 4 genetically unique chromatids for each chromosome pair

  • Each gamete randomly receives one chromatid from each chromosome pair

When crossing goes wrong

Deletions/duplications

  • Loss or gain of genetic material

Translocation

  • A portion of one chromosome is transferred to another chromosome

Inversions

  • A section of the chromosome is broken away, twisted 180 degrees (end to end) and reinserted into the chromosome

Horizontal gene transfer

  • When gene transfer is between organisms

Conjugation

  • DNA shared through physical contact
  • Transferred using plasmids

Transformation

  • Bacteria uptake DNA from their surroundings
  • Either fragmented DNA or plasmid DNA

Transduction

  • When foreign DNA is introduced into a cell by a virus or viral vector

Why it’s important

  • Spreads antibiotic-resistant genes
  • Transformation and transduction are used in gene editing to insert foreign DNA into cells