inheritance, variation and evolution (b6)

• 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 mitosis, where two identical cells are formed from one cell

  3. No mixing of genetic information

  4. Leads to clones, genetically identical to each other and the parent
• Meiosis

  1. Cell makes copies of chromosomes, doubling genetic information

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

  3. Cell divides again producing four gametes, each with a quarter the chromosomes

  4. Gametes are genetically different due to shuffling of chromosomes
• Gametes with 23 chromosomes join at fertilisation to produce a cell with 46 chromosomes
• This cell divides by mitosis to produce many copies, forming an embryo which then undergoes differentiation
• Advantages of sexual reproduction

  • Produces variation in offspring
  • if the environment changes variation gives a survival advantage by natural selection
  • Allows selective breeding
• Advantages of asexual reproduction

  • Only one parent needed
  • Uses less energy and is faster
  • many identical offspring can be produced when conditions are favourable
• Organisms using both sexual and asexual reproduction

  • Malarial parasites reproduce asexually in the human host, but sexually in the mosquito
  • Many fungi reproduce asexually by spores but also reproduce sexually to give variation
  • Many plants produce seeds sexually, but also reproduce asexually by runners such as strawberry plants, or bulb division such as daffodils.
• DNA

  Genetic material in the nucleus of a cell, a polymer made up of two strands in a double helix structure
• Gene

  A small section of DNA on a chromosome 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, improving understanding of genes linked to diseases, treatment of inherited disorders, and tracing human migration patterns
• DNA structure

  1. Made up of nucleotides, each with a sugar, phosphate, and one of four organic bases

  2. Two DNA strands twisted together, with complementary base pairing (A-T, C-G)

  3. Each group of three bases codes for an amino acid

  4. The order of bases forms a code that determines amino acids and proteins
• Protein synthesis

  1. DNA in nucleus cannot leave, so mRNA is made as a template

  2. mRNA moves to ribosomes where amino acids are brought and joined to form a protein

  3. Protein folds into a unique 3D structure
• Mutations

  • Changes in the sequence of bases in DNA, can be insertions, deletions, or substitutions
  • Affect the amino acid sequence and protein structure
• Most mutations do not alter the protein or only do so slightly, but some can have a serious effect
• Variation between organisms arises from both coding DNA (determining proteins) and non-coding DNA (determining gene expression)
• Gamete

  An organism's reproductive cell, with half the number of chromosomes
• Chromosome

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

  A short section of DNA that codes for a protein, contributing to a characteristic
• Alleles

  The different forms of a gene, humans have two alleles (one from each parent)
• Dominant allele

  Only one is needed to be expressed and observed
• Recessive allele

  Two copies are needed to be expressed and observed
• Homozygous

  Both inherited alleles are the same
• Heterozygous

  One inherited allele is dominant, the other is recessive
• Genotype

  The combination of alleles an individual has
• Phenotype

  The physical characteristics that are observed in the individual
• Family trees show the inheritance of different phenotypes over generations
• A Punnett square diagram can be used to determine the probability of offspring genotypes and phenotypes from two parents
• 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