genetics

• Contents


  • 3.1B and 3.2B - Advantages and Disadvantages of Sexual and Asexual Reproduction
  • 3.3 - Role of Meiosis
  • 3.4 - The Structure of DNA
  • 3.5 and 3.6 - The Genome and Extracting DNA
  • 3.7B and 3.8B - The Stages of Protein Synthesis
  • 3.9B and 3.10B - Genetic Variants and their Effects
  • 3.11B - Mendelian Genetics
  • 3.12 - Alleles
  • 3.13 - Basic Genetics Terminology
  • 3.14 - Monohybrid Inheritance and Genetic Diagrams
  • 3.15 - Sex of Offspring
  • 3.16 - Outcomes and Pedigree Analysis
  • 3.17B - ABO Blood Group Inheritance
  • 3.18B - Sex-linked Inheritance
  • 3.19 - Multiple-Gene Inheritance and Causes of Variation
  • 3.21, 3.22 and 3.23 - Human Genome Project, Genetic Variation and Mutation affecting Phenotype
• 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
  • 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
  • Leads to clones, which are genetically identical to each other and the parent
• 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
• DNA

  • A chemical that contains genetic material
  • A polymer that contains instructions for the body
  • Made up of many small parts called nucleotides
  • Each nucleotide is made up of one sugar molecule, one phosphate molecule 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
• Extracting DNA from Fruit

  1. Gently mix together 50ml cold water, half a teaspoon of salt and 10ml washing up liquid. Gently heat this mixture at 50C for 5-10 minutes.

  2. Peel the skins of a kiwi and chop into small pieces. Pulverise the kiwis.

  3. Add the solution from Step 1 to the kiwi.

  4. Filter the solution using a few sheets of kitchen paper and a sieve. Pour the filtrate into a test tube.

  5. Add 10ml of pineapple juice to the filtrate and allow to rest for a few minutes.

  6. Add 2 teaspoons of cold ethanol to the solution and wait 10 minutes.
• Pineapple juice


  Contains an enzyme called bromelain which 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

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

  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

  8. These amino acids connect together to form a polypeptide

  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
  • 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)
• 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 of the protein</b>
• 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
• 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
• 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
• Zygote


  The stage of development immediately after fertilisation - a diploid (2n) cell formed from the fusion of two haploid gametes
• Alleles (different forms of the same gene) lead to differences in inherited characteristics
• Different alleles code for different forms of the same protein - an allele that codes for a damaged form of a protein can cause illness
• Monohybrid (single gene) cross


  Looks at the probability of the offspring of two parents having certain genotypes and phenotypes
• Punnett square diagram


  Used to determine the probability of the offspring of two parents having certain genotypes and phenotypes
• Uppercase letters are used to represent dominant characteristics. Lowercase letters represent recessive characteristics