DNA inheritance and Cells

• DNA structure


  • Each strand consists of alternating sugar and phosphate molecules
  • The two strands are joined by a pair of bases
  • There are four bases: A (adenine), T (thymine), C (cytosine) and G (guanine)
  • A pairs with T and C pairs with G
• Gene


  A section of DNA that codes for a protein
• Triplet


  • A sequence of three bases in a gene
• Amino acid


  Each triplet codes for an amino acid
• Protein


  Order of amino acids determines the structure and function of the protein formed
• Genetic profiling


  1. Sample of cells collected

  2. DNA extracted from cells

  3. DNA sample cut into fragments using enzymes

  4. Fragments separated into bands, creating a genetic profile
• Genetic profiling


  Comparing DNA to view similarities in the DNA sequences

  
• Gene


  A length of DNA that codes for the production of a particular protein
• Allele


  A version of a gene
• Gamete


  Reproductive cells (e.g. egg and sperm cells) that contain a single copy of each chromosome
• Dominant


  Describes an allele that is always expressed, represented with a capital letter
• Recessive


  An allele that is only expressed in the absence of a dominant allele, represented with a small letter
• Homozygous


  Having two identical alleles of a gene
• Heterozygous


  Having two different alleles of a gene
• Genotype


  An organism's genetic composition, describes all alleles
• Phenotype


  An organism's observable characteristics
• F1 generation


  First generation in a genetic cross - the offspring produced when two organisms interbreed
• F2 generation


  Second generation in a genetic cross - the offspring produced when two organisms from the F1 generation are bred together
• Most characteristics of an organism are determined by multiple genes interacting, however some are determined by a single gene
• Monohybrid inheritance

  1. The inheritance of a single gene

  2. A punnett square can be used to illustrate this single gene inheritance

  
• Sex chromosomes


  One of the 23 pairs of chromosomes that determines sex
• Sex determination in humans

  • Males have an X and a Y chromosome (genotype XY)
  • Females have two X chromosomes (genotype XX)
  • Meiosis produces gametes with half the number of chromosomes and thus a single sex chromosome
  • All egg cells contain an X chromosome only whilst male sperm cells may contain an X or a Y chromosome
  • The baby's gender depends on which sperm cell fertilises the egg

  
• Sex determination


  A punnett square can be used to illustrate sex determination
• Genetic engineering


  The modification of the genome of an organism by the insertion of a desired gene from another organism
• Genetic engineering


  • Enables the formation of an organism with beneficial characteristics
  • Genetically engineered organisms are known as GM organisms
• Benefits of genetic engineering

  • Increased crop yields for growing population
  • Useful in medicine
  • GM crops produce scarce resources
  • GM crops can produce oils which can be used as biofuels
• Risks of genetic engineering

  • Long-term effects of consumption of GM crops are unknown
  • Negative environmental impacts
  • GM seeds are expensive, LEDCs may be unable to afford them or may become dependent on businesses that sell them
  • GM plants could become a pest themselves
• Chromosome


  Linear DNA molecule tightly coiled around proteins that carries genetic information in the form of genes
• Chromosomes are found in pairs (one from each parent) in all body cells. Human body cells contain 23 pairs of chromosomes (46 chromosomes in total)
• Chromosome pairs are not identical as they may contain different alleles (versions of a gene)
• Sex chromosomes

  A pair of chromosomes that determine sex. Males have an X and a Y chromosome, females have two X chromosomes.
• Mitosis


  Division of a cell to produce two genetically identical daughter cells with a full set of chromosomes (46 chromosomes)
• Mitosis


  • Important for growth, replacement of old cells, and repair of damaged tissues
• Meiosis


  Cell division that creates four genetically different daughter cells known as gametes. Involves two divisions and produces cells with half the number of chromosomes (23 chromosomes)
• Cancer


  Non-communicable disease in which uncontrolled mitosis (due to damaged DNA) leads to the formation of a primary tumour. Tumour cells break off and spread to other tissues forming secondary tumours.
• Cell differentiation


  Produces specialised cells with specific functions. Some genes are switched on or off, determining cell type. Once a cell differentiates, it cannot divide to make an unspecialised cell, nor a cell which has a different specialised function.
• Stem cells


  Unspecialised cells capable of differentiating into a range of different cell types
• Types of stem cells

  • Embryonic stem cells - unspecialised and capable of differentiating into any cell type, enable the growth and development of tissues in embryos
  • Adult stem cells - can differentiate into a limited range of cell types, enable the replacement of dead or damaged cells
• Advantages of using stem cell technology in medicine

  • Used to treat damage or disease e.g. type 1 diabetes, heart disease
  • Used to treat diseases that would otherwise be untreatable
  • Not rejected by the body
  • Do not have to be matched to the patient's tissue type
  • Can be used to grow organs for transplants
• Disadvantages of using stem cell technology in medicine

  • May become contaminated during preparation and when transplanted transmit infections to the patient
  • Difficult to find suitable stem cell donors
  • No guarantee that treatment will work
  • Transplanted stem cells could cause tumours
  • Long term risks are unknown
  • Potential side effects