Topic 3 ~ Genetics

• Topics covered in Edexcel Biology GCSE - Genetics

  • Advantages and Disadvantages of Sexual and Asexual Reproduction
  • Role of Meiosis
  • The Structure of DNA
  • The Genome and Extracting DNA
  • The Stages of Protein Synthesis
  • Genetic Variants and their Effects
  • Mendelian Genetics
  • Alleles
  • Basic Genetics Terminology
  • Monohybrid Inheritance and Genetic Diagrams
  • Sex of Offspring
  • Outcomes and Pedigree Analysis
  • ABO Blood Group Inheritance
  • Sex-linked Inheritance
  • Multiple-Gene Inheritance and Causes of Variation
  • Human Genome Project, Genetic Variation and Mutation affecting Phenotype
• Sexual reproduction


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

  2. Gametes are formed by meiosis, as they are non-identical

  3. Each gamete has 23 chromosomes and they fuse in fertilisation

  4. The genetic information from each parent is mixed, producing variation in the offspring
• Asexual reproduction


  1. Involves one parent with no gametes joining

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

  3. Leads to clones, which are genetically identical to each other and the parent
• Advantages of sexual reproduction

  • Produces variation in offspring
  • Allows us to use 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
• The Importance of Meiosis

  1. Meiosis is used to produce haploid gametes (sperm and egg cells)

  2. The cell makes copies of its chromosomes, so it has double the amount of genetic information

  3. The cell divides into two cells, each with half the amount of chromosomes (46)

  4. The cell divides again producing four cells, each with a quarter the amount of chromosomes (23)

  5. 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

  6. These gametes with 23 chromosomes join at fertilisation to produce a cell with 46 chromosomes, the normal number
• Formation of gametes


  Random chromosomes end up in each of the four cells
• Fertilisation


  Gametes with 23 chromosomes join to produce a cell with 46 chromosomes
• Mitosis


  The cell with 46 chromosomes divides to produce many copies
• Embryo formation


  More and more cells are produced, and an embryo forms
• Differentiation


  Cells begin to take on different roles after embryo formation
• DNA


  Chemical that contains genetic material
• Deoxyribonucleic acid (DNA)


  A polymer that contains instructions for the body
• Nucleotides


  Small parts that make up DNA
• Complementary base pairing


  A bases connect to T bases, and C bases connect to G bases
• Genetic code


  The order of different bases in DNA
• Double helix


  Structure formed by two twisted DNA strands
• Gene


  Short section of DNA that codes for amino acids to make proteins
• Amino acids


  • Joined together to make proteins, there are 20 types
• Genome


  All the genetic information of a single organism
• Human genome study has improved understanding of genes linked to diseases, treatment of inherited disorders, and tracing human migration patterns
• Pineapple juice contains bromelain enzyme which helps in seeing DNA more clearly
• Ethanol causes DNA to precipitate out of the solution, making it visible
• Protein synthesis


  Process of producing a protein from DNA
• If a gene is coded to make a protein, it has been expressed
• Non-coding DNA


  Parts of DNA that do not code for proteins, responsible for switching genes on or off
• Protein synthesis


  The process of producing a protein from DNA
• Expressed gene


  If a gene is coded to make a protein, it has been expressed
• DNA contains the genetic code for making a protein, but it cannot move out of the nucleus as it is too big
• Creating mRNA strand


  The mRNA nucleotides are joined together, creating a new strand called the mRNA strand, which is a template of the original DNA
• RNA polymerase binding


  An enzyme called RNA polymerase binds to non-coding DNA located in front of a gene on the DNA strand
• Matching mRNA nucleotides


  The two strands of DNA pull apart, and RNA polymerase allows mRNA nucleotides to match to their complementary base on the strand
• Movement of mRNA


  The mRNA moves out of the nucleus to the cytoplasm and onto ribosomes
• Reading bases on mRNA

  At the ribosomes, the bases on the mRNA are read in threes (triplets) to code for an amino acid
• Formation of polypeptide


  Amino acids connect together to form a polypeptide (amino acids linked by peptide bonds)
• Protein folding


  When the chain is complete, the protein folds to form a unique 3D structure, which is the final protein
• Genetic variants are small changes in the order of bases that make up a strand of DNA, affecting the structure of proteins
• Genotype


  Refers to the genes present in the DNA of an individual
• Phenotype


  Refers to the visible effects of those genes, such as the proteins they code for
• Coding DNA


  DNA that codes for proteins