Alevel bio-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 contain on average 600,000 DNA base pairs in their genome, whereas humans have 3 billion
• mRNA


  Messenger RNA, a short single-stranded copy of one gene found in the cytoplasm and nucleus
• tRNA


  Transfer RNA, found in the cytoplasm with an amino acid binding site and an anticodon
• 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. RNA nucleotides align with complementary DNA bases

  4. RNA polymerase joins the RNA nucleotides

  5. Pre-mRNA is modified by splicing out introns

  6. Mature mRNA leaves the nucleus
• Translation


  1. mRNA binds to a ribosome

  2. tRNA molecules with complementary anticodons align to mRNA codons

  3. Amino acids are joined by peptide bonds

  4. Ribosome moves along mRNA one codon at a time

  5. Ribosome detaches at stop codon
• Gene mutation


  A change in the base sequence of DNA
• Chromosome mutation


  Changes in the number of chromosomes
• Polyploidy


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


  Changes in the number of individual chromosomes, e.g. Down syndrome
• Non-disjunction during meiosis can lead to polyploidy or aneuploidy
• Meiosis


  1. Non-disjunction occurs

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

  3. Trisomy occurs (3 copies of a chromosome)
• Meiosis


  • Two nuclear divisions (unlike mitosis)
  • Creates genetically different haploid gametes
  • Introduces genetic variation through independent segregation of chromosomes and crossing over
• Meiosis


  Produces haploid gametes, unlike mitosis which produces diploid cells
• Meiosis can be identified in an unfamiliar life cycle by looking for the transition from diploid (2n) to haploid (n) cells
• Genetic diversity


  The number of different alleles of genes in a population
• Natural selection


  1. New alleles created by random mutation

  2. Advantageous alleles more likely to be passed on

  3. Allele frequency changes over generations
• Types of natural selection

  • Directional selection (favours extreme traits)
  • Stabilizing selection (favours middling traits)
• 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 according to evolutionary origins and relationships
• Taxonomic hierarchy


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


  Universal system using genus and species names
• Biodiversity


  Variety of genes, species, and habitats in an environment
• Decreasing biodiversity is a cause for concern, often due to human activities like farming