Topic 3 - DNA and Inheritance

• How does the nucleus control the cell?

  • The nucleus contains chromosomes
  • Chromosomes are strands of DNA
  • Genes are sections of DNA molecules that determine the sequence of amino acids that make up the different types of proteins produced in cells
  • Some of these proteins are enzymes, which control processes
  • These enzymes affect the functioning of the cell and so the organism's inherited characteristics
• Structure of the DNA

  • DNA is made up of two long chains of alternating sugar and phosphate molecules connected by bases
  • The structure is twisted into a double helix shape
  • There are four bases - Adenine, Thymine, Guanine and Cytosine
  • Adenine and Thymine are complementary pairs
  • Guanine and Cytosine are complementary pairs
  • DNA is a triplet code where every three bases code for an amino acid, the order of the bases determine the order of amino acids, which determines the structure of the protein
• Discovery of DNA

  • Structure was discovered in 1953
  • By many scientists using a number of different techniques
  • James Watson and Francis Crick were scientists involved in the discovery
• Genetic Profiling

  • A genetic profile can be used to compare similarities between DNA samples

  Method:

  1. A sample of cells is collected - for example blood, hair, or skin cells
  2. These cells are broken up and DNA is extracted
  3. DNA is cut into short pieces by enzymes, so that it ends up in fragments of different sizes
  4. The fragments are then separated into bands
• Uses of genetic profiling

  • Solving criminal cases
  • Comparison between species for classification
  • Paternity cases
• Advantages of genetic profiling

  • Can be used to identify certain genes which may be associated with a particular disease
  • If symptoms have not yet become obvious, precautions could be taken to prevent or slow down the progression of the disease
  • Very useful in solving crimes
• Disadvantages of genetic profiling

  Raises several ethical issues:

  • Who owns the DNA sample and the information obtained from it?
  • Should information on an individual's DNA be kept on record?
  • Should third parties like insurance companies and potential employers have access to the information?
• Genetic crosses

  Example: Mendel's peas

  T = tall allele t = short allele

  Key points:

  • Choose a letter to represent the alleles (capital = dominant) (small case = recessive)
  • The phenotype is the characteristic you see (tall and short)
  • The genotype is always a pair of alleles (TT, Tt, tt)
  • The gamete is always a single allele (T, t)
  • All F1 offsprings have the genotype Tt. They are heterozygous
  • The recessive allele (short) is always hidden if a dominant allele is present (tall)
  • In the F2 cross there is a 25% chance of a short pea plant (Tt x Tt)
• George Mendel's work was ignored to begin with because no one knew about the existence of DNA and genes at the time
• Gene

  A section of the DNA which codes for a protein
• Allele

  A variety/different version of one gene
• Selfing

  A technique by which pollen from a plant is used to fertilise water in flowers of the same plant - self fertilisation
• Chromosome

  Length of DNA that contains many genes; found in the nucleus and visible during cell division
• Sex determination

  • Sex is determined in humans by chromosome pair 23
  • These chromosomes are either labelled as male(XY) or female(XX)
  • Each fertilisation has a 50% chance of the offspring being male or female by using genetic crosses
• Genetic Modification

  The transferring of genes artificially from one species to another
• Genetic Modification

  Case Study: Herbicide resistance in soya beans

  • Herbicides are used to kill plants that compete with crops

  Method to genetically modify soya bean plants to become resistant to Herbicide:

  1. Genes from resistant plants are "cut out" using restriction enzymes
  2. They are transferred into the chromosomes of soya bean plant cells
  3. The modified cells are cloned to produce GM plants resistant to herbicide
• The importance of carrying out extensive field trials of GM technology


  • To understand the possible effects on the environment
  • To identify any possible health problems
  • To check for possible transfer of genes to other species
• Advantages of genetic modification

  • Plants that are resistant to herbicides can be sprayed with that herbicide to kill weeds, leaving the plant unaffected
  • This leads to increased yields due to reduced competition for space and nutrients
• Disadvantages of genetic modification

  Concerns about:

  • The health effects of eating modified genomes
  • The escape of transferred genes into other plant species
  • The creation of super weeds resistant to herbicides
  • Unknown long term effects
• Structure of the DNA

  Diagram -