DNA

• Genome


  The entire genetic material found in an organism
• Chromosome


  Genetic structures that occur in functional pairs in the nucleus of cells (except gametes, where there is only one chromosome from each pair, and bacteria, which don't have a nucleus and only have a single chromosome)
• Gene


  A short section of DNA (chromosome) that codes for a particular characteristic
• DNA (deoxyribonucleic acid)


  The genetic material contained in chromosomes in the nucleus of the cell
• Structure of DNA


  1. Two phosphate and sugar (deoxyribose) strands held together by bases linked by hydrogen bonds

  2. Repeated along the length of the DNA molecule
• Base pairing


  Adenine-thymine, guanine-cytosine combinations
• DNA is the code-carrying part of genes and chromosomes that determines how individuals develop
• How DNA works


  1. Bases are read in sequences of three (base triplets)

  2. Each base triplet codes for a particular amino acid

  3. Amino acids are linked together to form a protein (protein synthesis)
• Only one strand of the DNA (the coding strand) is involved in the coding process
• Mitosis


  1. Cell division that produces genetically identical daughter cells

  2. Important for growth, replacing worn out cells and repairing damaged tissue

  3. Ensures new daughter cells have the same chromosome arrangement as the parent cell
• Meiosis


  1. Occurs in sex organs (testes and ovaries)

  2. Produces gametes with half the normal chromosome number

  3. One cell produces four genetically different, haploid cells in two divisions
• Mitosis vs Meiosis


  Mitosis produces identical daughter cells, meiosis produces genetically different gametes with half the chromosome number
• Gene


  A short section of chromosome that codes for a particular characteristic
• Allele


  A particular form of a gene
• Homozygous


  Both alleles of a gene are the same
• Heterozygous


  The two alleles of a gene are different
• Genetic cross


  1. Shows how two heterozygous parents produce offspring in a 3:1 ratio

  2. Gametes from each parent combine to form offspring genotypes and phenotypes
• Genotype


  Paired symbols showing the allele arrangement in an individual
• Phenotype


  Outward appearance of an individual
• Dominant


  In the heterozygous condition, the dominant allele overrides the non-dominant (recessive) allele
• Recessive


  The recessive allele is dominated by the dominant allele - it only shows itself in the phenotype if there are two recessive alleles
• Sex determination in humans

  1. Humans have 22 pairs of normal chromosomes and one pair of sex chromosomes (XY for males, XX for females)

  2. Equal numbers of boys and girls are produced
• Sex chromosomes can carry genes and alleles that control other characteristics (sex linkage)
• Test (back cross)


  To determine an unknown genotype
• Individuals that are homozygous dominant or heterozygous


  • Have different genotypes but the same phenotype
  • The test cross can be used to determine the genotype of an individual of dominant phenotype but unknown genotype
• Sex determination in humans

  • Humans have 22 pairs of normal chromosomes and one pair of sex chromosomes
  • The male sex chromosomes are XY and females have two XX chromosomes
  • Equal numbers of boys and girls are produced
• Sex linkage


  • Sex chromosomes can carry genes and alleles that control other characteristics
  • Recessive alleles present on the X chromosome in males cannot be masked by a dominant allele and therefore show in the phenotype
  • In females where there are two X chromosomes, the recessive condition can be masked by a dominant allele
• Sex-linked conditions


  • Red-green colour blindness
  • Haemophilia
• Genetic conditions


  • Conditions caused by a fault with genes or chromosomes (a genetic fault)
  • Some genetic conditions are inherited and passed down from parent to child
• Genetic conditions described


  • Haemophilia
  • Cystic fibrosis
  • Huntington's disease
  • Down's Syndrome
• Haemophilia


  • Caused by a problem with the blood clotting mechanism
  • Sufferers are at risk of excessive bleeding
  • It is a sex-linked inherited condition caused by a recessive allele on the X chromosome
• Cystic fibrosis


  • Individuals have frequent and serious lung infections and problems with food digestion
  • Caused by a recessive allele, so affected individuals must be homozygous recessive
• Huntington's disease


  • Individuals have progressive brain deterioration, which usually becomes apparent in middle age
  • It is fatal and there is no cure
  • Caused by the presence of a dominant allele
• Down's Syndrome


  • Caused by the presence of an extra chromosome so that affected individuals have 47 chromosomes rather than 46
  • Individuals have easily identified facial features, reduced muscle tone and reduced cognitive development
• Genetic screening


  • Can be used to identify the presence of genetic conditions
  • Can be used to test for the presence of Down's Syndrome and other conditions like cystic fibrosis in a foetus
• When screening for Down's Syndrome, pregnant mothers are offered a blood test after 10-14 weeks of pregnancy
• The blood test will determine if the possibility of having a Down's Syndrome child is higher than normal
• Amniocentesis


  • Normally offered to those mothers who appear to have a higher risk based on the earlier screening
  • While not as accurate as the blood test, it carries no risk to the foetus or the mother
• Ethical and moral issues with genetic screening

  • Is abortion the best thing to do?
  • Who decides on who should be screened?
  • Is there an acceptable risk associated with genetic screening?
  • Costs of screening compared with the costs of treating individuals with a genetic condition - should cost be a factor?
  • Should information from genetic screening be made public?
• Genetic engineering


  • The modification of the genome of an organism to introduce desirable characteristics
  • Usually involves adding a human gene to the DNA of another organism, e.g. bacteria