B6 - Inheritance, Variation and Evolution

• Meiosis


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


  The formation of two identical cells from one cell
• Sexual reproduction


  1. Joining of male and female gametes, each containing genetic information from the mother or father

  2. Sperm and egg cells in animals

  3. Pollen and egg cells in flowering plants
• Gametes are formed by meiosis, as they are non identical
• Normal cell


  Has 46 chromosomes, two sets of 23 chromosomes (one from each parent)
• Gamete


  Has 23 chromosomes, fuses in fertilisation
• The genetic information from each parent is mixed, producing variation in the offspring
• Asexual reproduction


  1. One parent with no gametes joining

  2. Happens using the process of mitosis, where two identical cells are formed from one cell

  3. No mixing of genetic information

  4. Leads to clones, which are genetically identical to each other and the parent
• Organisms that reproduce asexually

  • Bacteria
  • Some plants
  • Some animals
• Meiosis


  1. Cell makes copies of its chromosomes, has double the amount of genetic information

  2. Cell divides into two cells, each with half the amount of chromosomes (46)

  3. Cell divides again producing four cells, each with a quarter the amount of chromosomes (23)

  4. These cells are called gametes and they are all genetically different from each other because the chromosomes are shuffled during the process, resulting in random chromosomes ending up in each of the four cells

  
• 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, and an embryo forms
• The cells begin to take on different roles after this stage (differentiation)
• Advantages of sexual reproduction

  • Produces variation in offspring
  • Allows for 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
  • In favorable conditions lots of identical offspring can be produced
• Organisms that use both sexual and asexual reproduction

  • Malarial parasites
  • Some fungi
  • Some plants
• DNA


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


  A small section of DNA on a chromosome - a triplet of bases that codes for a specific protein
• Genome


  All the genes coding for all of the proteins within an organism
• The whole human genome has now been studied and this has improved our understanding of the genes linked to different types of disease, the treatment of inherited disorders and has helped in tracing human migration patterns from the past
• DNA structure


  • DNA is a polymer made up of two strands which wrap around each other like a rope - in a structure called a double helix
  • Between the two strands are the four nitrogenous bases lined up in single rows - these come together to form a series of complementary pairs
• Protein synthesis


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

  2. The two strands pull apart and mRNA nucleotides match to their complementary base on the strand

  3. The mRNA nucleotides are then joined together, creating a new strand called the mRNA strand

  4. The mRNA then moves out of the nucleus to the cytoplasm and onto structures called ribosomes

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

  6. The corresponding amino acids are brought to the ribosomes by carrier molecules

  7. These amino acids connect together to form a protein

  8. When the chain is complete the protein folds to form a unique 3D structure
• Protein functions


  • Enzymes - biological catalysts that speed up the rate of reaction
  • Hormones - chemical messengers that send signals around the body
  • Structural protein - strong proteins in order to form structures, such as collagen
• Mutations


  • Change the sequences of bases in DNA
  • Can be insertions, deletions or substitutions
  • Changes in the type/sequence of amino acids will affect the way the protein folds and therefore the structure
• Most mutations do not alter the protein or only do so slightly
• Some mutations can have a serious effect and can change the shape, so the substrate will not fit into the active site so it cannot act as a protein, or a structural protein may lose its shape
• There can also be mutations in the non-coding parts of DNA that control whether the genes are expressed
• Variation between two organisms arises because of the coding DNA that determines the proteins and their activity, and the non-coding DNA that determines which genes are expressed
• 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
• Dominant allele


  Only one (out of the two alleles) is needed for it to be expressed and for the corresponding phenotype to be observed
• Recessive allele


  Two copies are needed for it to be expressed and for the corresponding the phenotype to be observed

  
• Homozygous


  When both inherited alleles are the same (i.e. two dominant alleles or two recessive alleles)
• Heterozygous


  When one of the inherited alleles is dominant and the other is recessive
• Genotype


  The combination of alleles an individual has, e.g. Aa
• Phenotype


  The physical characteristics that are observed in the individual, e.g. eye colour
• Family trees show the inheritance of different phenotypes over generations in the same family
• A single gene cross looks at the probability of the offspring of two parents having certain genotypes and phenotypes, using a Punnett square diagram