2.2 Inheritance, variation and evolution

• Meiosis


  The formation of four non-identical cells from one cell
• Mitosis


  The formation of two identical cells from one cell
• Sexual reproduction

  1. Joining of male and female gametes

  2. Each containing genetic information from the mother or father
• Gametes in animals

  • Sperm
  • Egg cells
• Gametes in flowering plants

  • Pollen
  • Egg cells
• Gametes


  Formed by meiosis, as they are non-identical
• A normal cell has 46 chromosomes
• There are two sets of chromosomes (i.e. 23 pairs)
• Each gamete has 23 chromosomes
• Gametes fuse in fertilisation

  Produces a cell with 46 chromosomes
• Asexual reproduction


  Involves one parent with no gametes joining
• Asexual reproduction

  1. Happens using the process of mitosis

  2. Two identical cells are formed from one cell
• There is no mixing of genetic information in asexual reproduction
• Asexual reproduction leads to clones, which are genetically identical to each other and the parent
• Organisms that reproduce asexually

  • Bacteria
  • Some plants
  • Some animals
• Meiosis

  Cells in the reproductive organs divide by meiosis to form gametes
• Gametes only have one copy of each chromosome
• Meiosis steps

  1. Cell makes copies of its chromosomes

  2. Cell divides into two cells

  3. Cell divides again producing four cells
• These cells are called gametes and they are all genetically different from each other
• The chromosomes are shuffled during the process, resulting in random chromosomes ending up in each of the four cells
• Fertilisation

  1. Gametes with 23 chromosomes join

  2. Produces a cell with 46 chromosomes

  3. Cell divides by mitosis

  4. Produces many copies

  5. Embryo forms

  6. Cells begin to take on different roles (differentiation)
• Advantages of sexual reproduction

  • Produces variation in offspring
  • Survival advantage
  • Decreases chance of extinction
• Advantages of asexual reproduction

  • Only one parent needed
  • Uses less energy
  • Faster reproduction
  • Identical offspring in favorable conditions
• Organisms that use both methods to reproduce

  • Malarial parasites
  • Some fungi
  • Some plants
• DNA


  The genetic material in the nucleus of a cell
• Genome


  All the genes coding for all of the proteins within an organism
• The whole human genome has now been studied
• This has improved our understanding of the genes linked to different types of disease
• Understanding the human genome will have great importance for medicine in the future
• DNA structure

  DNA is a polymer made up of two strands which wrap around each other like a rope
• Gene


  A small section of DNA on a chromosome that codes for a specific protein
• Each gene codes for a particular sequence of amino acids
• Protein synthesis

  The process of producing a protein from DNA
• Protein synthesis steps

  1. DNA contains genetic code

  2. Strands pull apart

  3. mRNA nucleotides match

  4. mRNA strand created

  5. mRNA moves to cytoplasm

  6. Bases on mRNA read in threes

  7. Amino acids brought to ribosomes

  8. Amino acids connect to form protein

  9. Protein folds to form unique 3D structure
• Proteins

  • Enzymes
  • Hormones
  • Structural proteins
• Mutations change the sequences of bases in DNA
• Types of mutations

  1. Base inserted

  2. Base deleted

  3. Base substituted
• A change in the type/sequence of amino acids will affect the way it folds
• Most mutations do not alter the protein or only do so slightly
• Some mutations can have a serious effect