DNA & Genetics

• DNA


  Deoxyribonucleic acid
• DNA


  • Contained in the nucleus of all cells
  • Same structure in all organisms
  • Made up of smaller molecules called nucleotides
  • Arranged in a double helix
• Nucleotides


  Nucleotide molecules have 3 parts: phosphate group, deoxyribose sugar, one of four nitrogen-rich bases
• Double helix


  • Uprights are made of alternating phosphate and sugar groups
  • Rungs of the ladder are made of the nitrogen rich bases
  • Bases: adenine (A), thymine (T), guanine (G), cytosine (C)
• Complementary base pairing


  The nitrogen rich bases can only pair up in one way
• If our genetic code is ACTG, the complementary pair will be TGAC
• Chromosomes


  • Long, thin, thread like structures found in the nuclei of all cells
  • One very long DNA molecule wrapped around proteins called histones
• Karyotype


  Chromosomes viewed under a microscope, arranged and numbered in order from longest to shortest
• Autosomes and sex chromosomes

  • Humans have 46 chromosomes - 44 autosomes and 2 sex chromosomes (X and Y)
  • Females have XX, males have XY
• Homologous chromosomes


  • Are the same length
  • Have the centromere in the same position
  • Have genes for particular characteristics at the same location
• Genes


  Sections of DNA arranged along chromosomes
• Proteins


  • The order of bases along the DNA strand is the genetic code, each gene codes for a specific protein
  • Proteins may be structural, enzymes, or regulatory
• Individuals have 2 copies of each gene, one on each homologous chromosome
• DNA Replication


  1. Strands of the double helix separate

  2. Free nucleotides pair up with exposed bases following complementary base pairing

  3. Sugar and phosphate molecules bond with neighbouring nucleotides to form new strands

  4. Results in 2 identical DNA molecules
• Chromatids


  The identical DNA molecules after replication
• Cell Division


  1. Mitosis: Produces 2 identical daughter cells

  2. Meiosis: Produces gametes with half the number of chromosomes
• Mitosis


  • DNA coils up and separate chromosomes become visible
  • Each chromosome has 2 chromatids
  • Chromosomes line up and separate to opposite poles
  • New nuclear membrane forms around each set of chromosomes
• Diploid number


  The number of chromosomes in body cells, 2 sets of autosomes and 2 sets of sex chromosomes
• Asexual reproduction


  Unicellular organisms reproduce by mitosis, producing identical offspring
• Meiosis


  1. Produces gametes (sperm and eggs) with half the diploid number of chromosomes

  2. Allows for genetic variation in offspring
• Fertilisation


  A single egg and sperm fuse to create a zygote, which then divides by mitosis to develop into a new individual
• Alleles


  The two varieties of a gene on a pair of homologous chromosomes
• Dominant vs. recessive


  Dominant traits are seen in the outward appearance, recessive traits are hidden
• Homozygous vs. Heterozygous


  Homozygous has two of the same alleles, Heterozygous has one dominant and one recessive allele
• Genotype vs. Phenotype


  Genotype is the actual genetic information, Phenotype is the observable characteristics
• Punnett squares show the possible offspring genotypes and phenotypes from a genetic cross
• Red


  Dominant trait, seen in the outward appearance
• White


  Recessive trait, hidden in the outward appearance
• The white trait has not disappeared, as the offspring still carry the gene for this trait
• Homozygous


  An allele for a dominant trait is represented by an upper case (R) letter, and a recessive trait a lower case letter (r)
• Heterozygous


  An allele for a dominant trait is represented by an upper case (R) letter, and a recessive trait a lower case letter (r)
• Possible allele combinations for the red/white pea plants

  • RR
  • Rr
  • rr
• RR and rr are homozygous as the two alleles are the same
• Rr is heterozygous as it has a combination of dominant and recessive
• RR and Rr would produce red flowers (both allele combinations contain a dominant gene)
• rr would produce white flowers, as there is a lack of the dominant gene
• Genotype


  The actual genetic information carried by the individual (i.e. not visible as its carried on the chromosomes)
• Phenotype


  The observable characteristics or traits of the individual (i.e. what the individual looks like)
• Punnett Square


  A way to show all the types of offspring that could result in a cross
• A Punnett Square represents a probability and is interpreted as a ratio or a percentage