Section 4

• Eukaryotic DNA


  • Longer
  • Linear
  • Associated with histones
• Prokaryotic DNA


  • Circular loops
  • Not associated with proteins
• Mitochondria and chloroplasts have their own circular DNA that is not wrapped around histones
• Gene


  Sequence of DNA that codes for a polypeptide or functional RNA
• Locus


  The exact position of a gene on a chromosome
• Genetic code


  • Degenerate
  • Universal
  • Non-overlapping
• Triplet


  A sequence of three bases on DNA that codes for an amino acid
• There are 20 amino acids that can be coded for by the 64 possible triplet combinations
• Introns


  DNA sequences that do not code for polypeptides
• Exons


  DNA sequences that do code for amino acids
• Codon


  Three bases on mRNA that code for a specific amino acid
• Start codon


  Three bases that initiate translation
• Stop codon


  Three bases that terminate translation
• Genome


  An organism's complete set of genes
• Proteome


  The full range of proteins a cell can produce
• Bacterial genomes contain around 600,000 DNA base pairs, while human genomes contain 3 billion base pairs
• mRNA


  Single-stranded, shorter than DNA, found in cytoplasm and nucleus
• tRNA


  Has an amino acid binding site and an anticodon that is complementary to an mRNA codon
• Transcription


  1. DNA helix unwinds

  2. One DNA strand acts as template

  3. RNA nucleotides align to complementary DNA bases

  4. RNA polymerase joins RNA nucleotides

  5. Pre-mRNA is modified by splicing out introns

  6. Mature mRNA leaves nucleus
• Translation


  1. mRNA binds to ribosome

  2. tRNA anticodons align to mRNA codons

  3. Amino acids are joined by peptide bonds

  4. Ribosome moves along mRNA

  5. Translation ends at stop codon
• Gene mutation


  Change in DNA base sequence
• Gene mutations


  • Base substitution
  • Base deletion
• Chromosome mutation


  Change in chromosome number
• Polyploidy

  1. Whole sets of chromosomes are duplicated

  2. Results in 3n or 4n cells
• Aneuploidy

  1. Individual chromosomes fail to separate properly

  2. Results in cells with extra or missing chromosomes
• Polyploidy is common in plants but fatal in humans
• Down syndrome is caused by non-disjunction of chromosome 21
• Meiosis


  1. Non-disjunction occurs

  2. Gametes are haploid (n) or haploid plus/minus one chromosome (n+1, n-1)

  3. Trisomy (3 copies of a chromosome) can occur
• Meiosis


  • Two nuclear divisions, creates genetically different haploid gametes
  • Independent segregation of homologous chromosomes
  • Crossing over between homologous chromosomes
• Identifying meiosis in life cycles


  Look for 2n to n transition
• Genetic diversity


  • Number of different alleles of genes in a population
  • Enables natural selection
• Natural selection


  1. New alleles created by random mutations

  2. Advantageous alleles more likely to be passed on

  3. Increases frequency of advantageous alleles over generations
• Directional selection


  Advantageous allele codes for an extreme trait
• Stabilizing selection


  Middling trait remains the selective advantage
• Species


  Organisms able to produce fertile offspring
• Courtship behavior


  • Unique sequence of actions for each species
  • Allows identification of own species
  • Synchronizes mating
  • Helps select for healthy mates
• Phylogenetic classification


  Arranging groups by evolutionary origins and relationships
• Taxonomic hierarchy

  • Domain
  • Kingdom
  • Phylum
  • Class
  • Order
  • Family
  • Genus
  • Species
• Binomial nomenclature


  Genus and species names
• Biodiversity


  Variety of habitats, genes, and species in a community