Inheritance, Variation, and Evolution

• Gametes


  • Sex cells (sperm cells and egg cells in animals, pollen and egg cells in flowering plants)
  • Haploid (half the number of chromosomes)
• Interphase


  Copies of genetic information are made during this process
• Fertilisation


  1. Gametes join together to restore the normal number of chromosomes and the new cell then divides by mitosis (which increases the number of cells)

  2. As the embryo develops, cells differentiate
• Advantage of sexual reproduction

  • It creates genetic variation in offspring, increasing the probability of a species adapting to and surviving environmental changes
  • Natural selection can be speeded up by humans in selective breeding to increase food production
• Disadvantages of sexual reproduction

  • Two parents are required. This makes reproduction difficult in endangered populations or in species which exhibit solitary lifestyles
  • More time and energy is required so fewer offspring are produced
• Asexual reproduction


  • Type of reproduction
  • Involves mitosis only
  • Produces genetically identical offspring known as daughter cells
• Advantages of asexual reproduction

  • Only one parent is required
  • Lots of offspring can be produced in a short period of time, enabling the rapid colonisation of an area and reducing competition from other species
  • Requires less energy and time as do not need a mate
• Disadvantage of asexual reproduction

  • No genetic variation (except from spontaneous mutations) reducing the probability of a species being able to adapt to environmental change
• Malarial parasites


  • Sexual reproduction in the mosquito
  • Asexual reproduction in the human host
• Fungi


  • Asexual reproduction by spores
  • Sexual reproduction to give variation
• Plants


  • Sexual reproduction to produce seeds
  • Asexual reproduction by runners (e.g. strawberry plants) or bulb division (e.g. daffodils)
• DNA


  • A double-stranded polymer of nucleotides, wound to form a double helix
  • The genetic material of the cell found in its nucleus
• Genome


  The entire genetic material of an organism
• Importance of understanding the human genome

  • Searching for genes linked to different types of disease
  • Understanding and treating inherited disorders
  • Tracing human migration patterns from the past
• Chromosome


  A long, coiled molecule of DNA that carries genetic information in the form of genes
• Human body cells have 46 chromosomes (23 pairs)
• Human gametes have 23 chromosomes
• Gene


  A small section of DNA that codes for a specific sequence of amino acids which undergo polymerisation to form a protein
• Monomers of DNA


  Nucleotides
• DNA nucleotides


  • Common sugar
  • Phosphate group
  • One of four bases: A, T, C or G
• Four bases found in nucleotides

  • Adenine
  • Thymine
  • Cytosine
  • Guanine
• DNA nucleotides


  • Common sugar
  • Phosphate group
  • One of four bases: A, T, C or G
• Four bases found in nucleotides

  • Adenine
  • Thymine
  • Cytosine
  • Guanine
• How nucleotides interact to form a molecule of DNA

  1. Sugar and phosphate molecules join to form a sugar-phosphate backbone in each DNA strand

  2. Base connected to each sugar

  3. Complementary base pairs (A pairs with T, C pairs with G) joined by weak hydrogen bonds
• How a gene codes for a protein

  1. A sequence of three bases in a gene forms a triplet

  2. Each triplet codes for an amino acid

  3. The order of amino acids determines the structure (i.e. how it will fold) and function of protein formed
• Variation


  Differences in the characteristics of individuals in a population
• Causes of variation within a species

  • Genetics
  • Environment
  • A mixture of both
• Protein folding


  The folding of amino acids determines the shape of the active site which must be highly specific to the shape of its substrate
• Protein synthesis


  The formation of a protein from a gene
• Genetic variation


  Variations in the genotypes of organisms of the same species due to the presence of different alleles, creating differences in phenotypes
• What creates genetic variation in a species

  • Spontaneous mutations
  • Sexual reproduction
• Stages of protein synthesis

  • Transcription
  • Translation
• Mutation


  A random change to the base sequence in DNA which results in genetic variants, occurring continuously
• Transcription


  The formation of mRNA from a DNA template
• Transcription


  1. DNA double helix unwinds

  2. RNA polymerase binds to a specific base sequence of non-coding DNA in front of a gene and moves along the DNA strand

  3. RNA polymerase joins free RNA nucleotides to complementary bases on the coding DNA strand

  4. mRNA formation complete. mRNA detaches and leaves the nucleus
• How a gene mutation may affect an organism's phenotype

  • Neutral mutation does not change amino acid sequence, no effect on phenotype
  • Mutation may cause minor change in phenotype e.g. eye colour
  • Mutation may completely change amino acid sequence, resulting in non-functional protein, severe changes to phenotype
• Translation


  A ribosome joins amino acids in a specific order dictated by mRNA to form a protein
• Translation


  1. mRNA attaches to a ribosome

  2. Ribosome reads the mRNA bases in triplets. Each triplet codes for one amino acid which is brought to the ribosome by a tRNA molecule (carrier molecule)

  3. A polypeptide chain is formed from the sequence of amino acids which join together
• If a new phenotype caused by a mutation is suited to an environmental change, there will be a rapid change in the species
• Evolution


  A gradual change in the inherited traits within a population over time, occurring due to natural selection which may result in the formation of a new species