Notes (genetics)

• 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
  • Formed by meiosis, as they are non identical
  • Normal cell has 46 chromosomes, two sets of 23 pairs, one from each parent
  • 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
  • Happens using the process of mitosis, where two identical cells are formed from one cell
  • There is no mixing of genetic information
  • It leads to clones, which are genetically identical to each other and the parent
• Examples of organisms that reproduce asexually

  • Bacteria
  • Some plants
  • 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
  • The cell makes copies of its chromosomes, so it has double the amount of genetic information
  • The cell divides into two cells, each with half the amount of chromosomes (46)
  • The cell divides again producing four cells, each with a quarter the amount of chromosomes (23)
  • 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)
• DNA


  • A chemical that contains genetic material
  • It is a polymer that contains instructions for the body
  • It is made up of many small parts called nucleotides
  • Each nucleotide is made up of one sugar molecule, one phosphate molecule (which form the backbone) and one of the four types of organic bases (A, C, G, T)
  • Each DNA molecule is made up of two DNA strands which are twisted together
  • A bases only connect to T bases, and C bases only connect to G bases (complementary base pairing)
  • The order of the different bases forms a genetic code
• 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
• A white mass should precipitate at the top of the tube after 10 minutes; this is the DNA from the kiwi
• Bromelain


  An enzyme in pineapple juice that breaks down proteins attached to the DNA, helping to see the DNA more clearly
• Ethanol


  Causes the DNA to precipitate out of the solution, making it visible at the top of the container
• Protein Synthesis


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

  2. The mRNA nucleotides themselves are then joined together, creating a new strand called the mRNA strand. This is a template of the original DNA.

  3. An enzyme called RNA polymerase binds to non-coding DNA located in front of a gene on the DNA strand.

  4. The two strands of DNA pull apart from each other, and RNA polymerase allows mRNA nucleotides to match to their complementary base on the strand.

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

  6. At the ribosomes, the bases on the mRNA are read in threes (triplets) to code for an amino acid.

  7. The corresponding amino acids are brought to the ribosomes by carrier molecules called tRNAs - transport RNAs.

  8. These amino acids connect together to form a polypeptide (amino acids linked by peptide bonds).

  9. When the chain is complete the protein folds to form a unique 3D structure, which is the final protein.
• Genetic variants


  • Small changes in the order of bases that make up a strand of DNA
  • They can affect the structure of proteins in different ways, depending on whether they occur in coding DNA or non-coding DNA
• Genotype


  The genes present in the DNA of an individual
• Phenotype


  The visible effects of those genes (e.g the proteins that they code for)
• Coding DNA


  A genetic variant will alter the sequence of bases and therefore will change the sequence of amino acids, altering the final structure of the protein produced
• Non-Coding DNA


  A genetic variant can affect the amount of RNA polymerase that can bind, changing the structure of the final protein
• 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
• Some mutations can have a serious effect and can change the shape, so the substrate will not fit into the active site and 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
• 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
  • Came to the conclusions that offspring have some characteristics that their parents have because they inherit 'hereditary units' from each, one unit is received from each parent, and units can be dominant or recessive, and cannot be mixed together
• Mendel was not recognised till after his death as genes and chromosomes were not yet discovered, so people could not understand
• In the late 19th century chromosomes as a part of cell division were discovered, and Mendel's work was rediscovered and his principles of inheritance were confirmed
• 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 inheritance of 'hereditary units' from each parent, that can be dominant or recessive, and cannot be mixed together
• 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 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