NB2S summary

• Mitosis


  • Growth & repair
  • Produces 2 genetically identical cells
  • Cells are diploid (full chromosome set), 2n
  • Occurs in somatic (body) cells
• Meiosis


  • Production of gametes (sperm & egg)
  • Produces 4 genetically different cells
  • Cells are haploid (half chromosome set), n
  • Occurs in testes & ovary
• Chromosomes are made up of DNA
• The chromosomes carry genes along them
• Genes code for proteins
• DNA


  • Triplet code; 3 bases = one amino acid
  • Some bases are stop / start, others are nonsense (don't code for anything we know!)
  • Sequence of bases determines the protein produced – which gives the characteristic / trait
• Allele


  • Small differences in DNA sequence of a gene
  • Alternative version of a gene
• Genetics terms


  • Homozygous – 2 alleles the same
  • Heterozygous – 2 different alleles
  • Pure breeding – individual which always produces offspring which all resemble the parent / is homozygous (for a trait)
  • Dominant – characteristic that always shows if the dominant allele is present
  • Recessive – characteristic that only shows if there is no dominant allele present
  • Dominant allele masks the recessive allele
• How meiosis causes variation / new combinations of genes in the gametes

  1. Crossing over – bits of homologous chromosomes are exchanged (swapped) during meiosis

  2. Independent assortment; it is random which one of a pair of homologous chromosomes goes into a gamete after they line up in pairs during meiosis
• Punnett squares


  • Gametes written at side / top – single letters
  • Genotype – combination of alleles
  • Phenotype – physical appearance/feature
  • Ratio 1 BB : 2 Bb : 1 bb or 3 brown : 1 blue
  • Punnet square predicts "possible" outcomes and probability but NOT exact numbers
  • Fertilisation is random (which sperm & which egg) so we don't EXACTLY get predicted ratios
  • The larger the number of offspring the closer the results will be to the predicted ratios
• DNA structure & replication

  1. Sugar & phosphate backbone

  2. "ladder' of bases

  3. A=T and C≡G / complementary base pairing

  4. Nucleotide = sugar & phosphate + base

  5. DNA unwinds and opens up, exposing BASES, new nucleotides come in (join A=T and C≡G, nucleotides joined by enzymes, DNA winds up again

  6. DNA has to be accurately copied to ensure that the daughter cells created have the same DNA as the parent cell
• Mutation


  • Permanent change in DNA sequence or number of chromosomes
  • Difference in base sequence → (may produce a) different amino acid sequence → different protein
  • Change in order = slightly altered gene
  • Alternative form of same gene is called an allele
  • Mutations can only be passed on if they are in a gamete / sex cell
• Natural selection – e.g. disease resistance

  1. Genetic diversity is due to meiosis

  2. Disease which affects one individual may not affect all the individuals because of the different combination of genes

  3. Some individuals are more resistant to disease than others due differences in genetic material

  4. These individuals can survive & breed

  5. Resistance may be passed on

  6. If it is useful it can lead to evolution

  7. Survival of the fittest (better suited = survival)