Topic 3

• Sexual reproduction


  Involves the joining of male and female gametes, each containing genetic information from the mother or father
• Gametes


  • Sperm and egg cells in animals
  • Pollen and egg cells in flowering plants
• Gametes are formed by meiosis, as they are non identical
• A normal cell has 46 chromosomes, there are two sets of chromosomes (i.e. 23 pairs), one from the father and one from the mother
• Each gamete has 23 chromosomes and they fuse in fertilisation
• The genetic information from each parent is mixed, producing variation in the offspring
• Asexual reproduction


  Involves one parent with no gametes joining, it happens using the process of mitosis, where two identical cells are formed from one cell
• There is no mixing of genetic information in asexual reproduction, it leads to clones which are genetically identical to each other and the parent
• Organisms that reproduce asexually


  • Bacteria, some plants and some animals
• Advantages of sexual reproduction


  • Produces variation in offspring
  • Decreases the chance of the whole species becoming extinct
  • Allows selective breeding
• Advantages of asexual reproduction


  • Only one parent is needed
  • Uses less energy and is faster as organisms do not need to find a mate
• Meiosis


  The formation of four non-identical cells from one cell
• Meiosis


  1. Cell makes copies of its chromosomes

  2. Cell divides into two cells, each with half the amount of chromosomes

  3. Cell divides again producing four cells, each with a quarter the amount of chromosomes

  4. These cells are called gametes and they are all genetically different from each other
• Gametes with 23 chromosomes join at fertilisation to produce a cell with 46 chromosomes, the normal number
• This cell divides by mitosis to produce many copies, an embryo forms, and the cells begin to take on different roles (differentiation)
• DNA


  A chemical that contains genetic material, found in the nucleus
• Nucleotides


  The small parts that DNA is made up of, each containing one sugar molecule, one phosphate molecule and one of the four types of organic bases (A, C, G, T)
• DNA molecule


  Made up of two DNA strands twisted together, with each base connected to another base in the other strand (complementary base pairing)
• Genetic code


  The order of the different bases in DNA
• Double helix


  The structure of DNA, a polymer made up of two strands wound around each other
• Gene


  A short section of DNA that codes for many amino acids, which are joined together to make a specific protein
• Genome


  All the genetic information (DNA) of a single organism
• Extracting DNA from fruit


  1. Gently mix together water, salt and washing up liquid, heat the mixture

  2. Pulverise the fruit

  3. Add the solution to the fruit

  4. Filter the solution

  5. Add pineapple juice

  6. Add ethanol
• Pineapple juice contains bromelain which breaks down proteins attached to the DNA, and ethanol causes the DNA to precipitate out of the solution
• Protein synthesis


  The process of producing a protein from DNA
• Protein synthesis


  1. DNA contains the genetic code for making a protein, but it cannot move out of the nucleus

  2. RNA polymerase binds to non-coding DNA and allows mRNA nucleotides to match to their complementary base on the DNA strand

  3. The mRNA moves out of the nucleus to the ribosomes

  4. At the ribosomes, the bases on the mRNA are read in threes to code for an amino acid

  5. Carrier molecules called tRNAs bring the corresponding amino acids to the ribosomes

  6. The amino acids connect together to form a polypeptide, which then folds to form the final protein
• Genetic variants


  Small changes in the order of bases that make up a strand of DNA
• Genotype


  The genes present in the DNA of an individual
• Phenotype


  The visible effects of the genes (e.g. the proteins that they code for)
• Types of mutations


  • A base is inserted into the code
  • A base is deleted from the code
  • A base is substituted
• Most mutations do not alter the protein or only do so slightly, but some can have a serious effect and change the shape of the protein
• Mendelian genetics


  The study of how characteristics are passed on from parents to offspring, based on the work of Gregor Mendel
• Mendel observed the characteristics passed on to the next generation in plants and concluded that offspring inherit 'hereditary units' from each parent, which can be dominant or recessive
• Mendel's work was not recognised until after his death, as genes and chromosomes were not yet discovered
• Gregor Mendel


  • Trained in mathematics and natural history in Vienna
  • Worked in the monastery gardens and observed the characteristics passed on to the next generation in plants
  • Carried out breeding experiments on pea plants
  • Used smooth peas, wrinkled peas, green peas and yellow peas and observed the offspring to see which characteristics they had inherited
  • Came to conclusions about hereditary units and how they are inherited
• Mendel was not recognised till after his death as genes and chromosomes were not yet discovered, so people could not understand
• Gamete


  An organism's reproductive cell (egg in female and sperm in males), which has half the number of chromosomes (23)
• Chromosome


  A structure found in the nucleus which is made up of a long strand of DNA
• Gene


  A short section of DNA that codes for a protein, and therefore contribute to a characteristic
• Alleles


  The different forms of the gene - humans have two alleles for each gene as they inherit one from each parent