DNA INHERITANCE AND EVOLUTION

• DNA


  The genetic material in the nucleus of a cell, composed of a chemical called DNA
• Genome


  The entire genetic material of an organism
• The whole human genome has now been studied and this will have great importance for medicine in the future
• Importance of understanding the human genome

  • Search for genes linked to different types of disease
  • Understanding and treatment of inherited disorders
  • Use in tracing human migration patterns from the past
• DNA


  A polymer made up of two strands forming a double helix
• Gene


  A small section of DNA on a chromosome that codes for a particular sequence of amino acids to make a specific protein
• The DNA polymer is made up of repeating nucleotide units
• DNA structure


  • Contains four bases: A, C, G and T
  • A sequence of three bases is the code for a particular amino acid
  • The order of bases controls the order in which amino acids are assembled to produce a particular protein
• Protein synthesis (HT only)


  1. Proteins are synthesised on ribosomes, according to a template

  2. Carrier molecules bring specific amino acids to add to the growing protein chain in the correct order

  3. When the protein chain is complete it folds up to form a unique shape
• Change in DNA structure


  May result in a change in the protein synthesised by a gene
• Mutations occur continuously
• Most mutations do not alter the protein, or only alter it slightly so that its appearance or function is not changed
• A few mutations code for an altered protein with a different shape
• Not all parts of DNA code for proteins. Non-coding parts of DNA can switch genes on and off, so variations in these areas of DNA may affect how genes are expressed
• Gamete


  A reproductive cell
• Chromosome


  A structure in the nucleus containing genetic material
• Gene


  A section of DNA that codes for a particular characteristic
• Allele


  Different forms of the same gene
• Dominant


  An allele that is always expressed, even if only one copy is present
• Recessive


  An allele that is only expressed if two copies are present (therefore no dominant allele present)
• Homozygous


  Having two identical alleles for a characteristic
• Heterozygous


  Having two different alleles for a characteristic
• Genotype


  The alleles present
• Phenotype


  The characteristics that can be expressed
• Most characteristics are a result of multiple genes interacting, rather than a single gene
• Students should be able to understand the concept of probability in predicting the results of a single gene cross, but recall that most phenotype features are the result of multiple genes rather than single gene inheritance
• Genetic cross


  Use a Punnett square diagram to make predictions using the theory of probability (HT only)
• Polydactyly (having extra fingers or toes) is caused by a dominant allele
• Cystic fibrosis (a disorder of cell membranes) is caused by a recessive allele
• Students should make informed judgements about the economic, social and ethical issues concerning embryo screening, given appropriate information
• Ordinary human body cells contain 23 pairs of chromosomes, 22 pairs control characteristics only, but one of the pairs carries the genes that determine sex
• In females the sex chromosomes are the same (XX), in males the chromosomes are different (XY)
• Differences in the characteristics of individuals in a population is called variation and may be due to differences in the genes they have inherited, the conditions in which they have developed, or a combination of genes and the environment
• There is usually extensive genetic variation within a population of a species
• All variants arise from mutations and that: most have no effect on the phenotype; some influence phenotype; very few determine phenotype
• Mutations occur continuously. Very rarely a mutation will lead to a new phenotype. If the new phenotype is suited to an environmental change it can lead to a relatively rapid change in the species
• There are links with this content to Speciation (biology only)
• Evolution


  A change in the inherited characteristics of a population over time through a process of natural selection which may result in the formation of a new species
• The theory of evolution by natural selection states that all species of living things have evolved from simple life forms that first developed more than three billion years ago
• Evolution by natural selection


  Occurs through natural selection of variants that give rise to phenotypes best suited to their environment