topic 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 the amino acid sequence of a particular polypeptide and a functional RNA
• Locus


  The exact position that a particular gene is found on a chromosome
• Genetic code


  • Degenerate
  • Universal
  • Non-overlapping
• Triplet


  A sequence of three bases on DNA that codes for a particular amino acid
• There are 20 amino acids that exist
• Introns are sequences of DNA bases that do not code for polypeptides
• Exons are sequences of DNA bases that do code for amino acids
• Codon


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


  Three bases at the start of every gene that initiate translation
• Stop codon


  Three bases at the end of every gene that cause the ribosome to detach and stop translation
• Genome


  An organism's complete set of genes in a cell
• Proteome


  The full range of proteins that a cell is able to produce
• Bacteria have on average 600,000 DNA base pairs in their genome, while humans have 3 billion
• mRNA


  Shorter than DNA, single-stranded, found in the cytoplasm and nucleus, made during transcription
• tRNA


  Found in the cytoplasm, has an amino acid binding site and an anticodon that is complementary to a codon on mRNA
• Ribosome


  Holds tRNA molecules in place to enable the joining of amino acids during translation
• Transcription


  1. DNA helix unwinds

  2. One DNA strand acts as a template

  3. Free-floating mRNA nucleotides align with complementary DNA bases

  4. RNA polymerase joins the mRNA nucleotides

  5. mRNA is modified (splicing in eukaryotes)

  6. mRNA leaves the nucleus
• Translation


  1. mRNA binds to a ribosome

  2. tRNA molecules with complementary anticodons align to mRNA codons

  3. Ribosome holds two tRNA molecules at a time

  4. Amino acids are joined by peptide bonds

  5. tRNA molecules are released

  6. Ribosome moves along mRNA until it reaches the stop codon
• Gene mutation


  A change in the base sequence of DNA
• Chromosome mutation


  Changes in the number of chromosomes
• Non-disjunction

  Chromosomes or chromatids do not separate equally during meiosis
• Polyploidy


  Changes in the whole set of chromosomes (e.g. triploidy, tetraploidy)
• Aneuploidy


  Changes in the number of individual chromosomes (e.g. Down syndrome)
• 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
  • Introduces genetic variation through independent segregation of homologous chromosomes and crossing over
• 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 mutation

  2. Advantageous alleles more likely to be passed on

  3. Shifts in allele frequency over generations
• Types of natural selection

  • Directional selection: selects for extreme traits
  • Stabilizing selection: selects for middling traits
• Species


  • Organisms able to produce fertile offspring
  • Courtship behavior is unique and essential for successful mating and reproduction
• Phylogenetic classification


  • Arranging groups based on evolutionary origins and relationships
  • Closer branching indicates more recent common ancestor
• Taxonomic hierarchy


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


  • Genus and species names
  • Universal system for identifying organisms
• Biodiversity


  • Variety of habitats, genes, and species in a community
  • Decrease in biodiversity is a concern, often due to human activity
• Measuring biodiversity


  • Index of diversity formula: (Σn(n-1))/(N(N-1))
  • Higher values indicate greater biodiversity
• Genetic diversity can also be measured by comparing DNA, mRNA, or amino acid sequences