Grade 9 Detail

• Sexual reproduction involves the fusion of male and female gametes produced by meiosis

  • e.g. egg + sperm or egg + pollen
  • mixes DNA from each parent creating variation in offspring
  • gametes are non identical and the fusion of each pair is random
• asexual reproduction

  • All bacteria, some plants and few animals do this
  • no gametes involved, only 1 parent cell
  • no mixing of DNA, no variation
  • relies on mitosis to produce genetically identical clones of the parent cell
• Meiosis - cell division that produces gametes / sex cells. Occurs in reproductive organs e.g. testes and ovaries

  • one diploid cell divides twice
  • produces four non identical haploid cells
• • during fertilisation, the nuclei of two haploid gametes fuse and the full chromosome number is restores to form a diploid zygote with 46 chromosomes
  • cell starts dividing by mitosis grows into a ball of stem cells which is an embryo
  • continues dividing by mitosis, but cells go through differentiation to create a foetus
• some organisms can reproduce sexually and asexually depending on environmental conditions:

  • plants - reproduce sexually using flowers to make them seeds and asexually using runners and bulbs
  • protists e.g. malaria parasite - produces asexually in human liver and blood cells but sexually in the mosquito vector
  • fungi - reproduce sexually to create genetic variation and asexually by budding of spores
• sexual reproduction pros and cons:

  • need to find a mate as 2 parents needed - slower and uses energy
  • allows natural selection to occur if environment changes as genetic variation means some offspring are more likely to survive
  • allows humans to selectively breed organisms for desired characteristics
• asexual reproduction pros and cons:

  • only one parent needed so fast and less energy needed
  • no genetic variation so they are less likely to be able to survive environmental change e.g. a new pathogen
  • can produce many identical offspring in favourable conditions so humans can produce lots of plants for sale
• chromosome is made up of coiled DNA
• DNA is a polymer made up of two strands twisted to form a double helix and the strands are made up of 4 different nucleotides (monomers)
• genes are short sections of DNA which code for proteins by coding for a specific sequence of amino acids
• genome - all the genetic material in an organism
• human genome project allows us to:

  • search for genes linked to different diseases
  • understand and treat inherited disorders
  • trace human migration patterns from the past through human evolution
• All nucleotides contain a phosphate group, a sugar and a base
• a sequence of 3 bases codes for a particular amino acid so the order of bases in the DNA code controls the order of amino acids joined together to make a particular protein
• Protein synthesis:

  • a template copy of the DNA sequence of a gene leaves the nucleus and travels to a ribosome to produce a protein
  • carrier molecules bring amino acids to build a protein chain in the right order
  • bases pair with their complementary base
• • if the DNA sequence is changed, the order of amino acids in the protein may change
  • after a protein is formed it folds out produce a unique shape which is specific to its function as an enzyme, hormone, antibody or structural protein such as collagen
  • if the sequence is changed a different shape would form
• • mutations are changes to the DNA base sequence which occur continuously and some do not alter proteins or affect them only slightly but do not change their shape
  • however rarely a mutation will code for an altered protein shape which will affect its function
• non coding parts of DNA control gene expression by switching genes on or off

  • if a mutation occurs in a non coding part, the mutation could affect gene expression
• allele - different versions of a gene that give a certain characteristic
• genotype - alleles an organism has for a particular gene
• phenotype - observable characteristics an organism has based on genotype
• • homozygous recessive - need 2 alleles to be expressed
  • homozygous dominant - need 1 allele to be expressed
  • heterozygous - dominant is expressed
• we can us ratios or percentages to show phenotype probabilities
• inherited disorders are caused by mutations in certain alleles which are passed down from parents to offspring
• cystic fibrosis (recessive inherited disorder)

  • if someone is heterozygous for cystic fibrosis, they are a carrier
  • there is no cure however people can have antibiotics or gene therapy as short term solutions or an organ transplant
• polydactyly (dominant recessive disorder)

  • no carriers
  • not serious but can be treated by surgery
• Embryo screening - done using IVF:

  • IVF forms embryo
  • at the 8 cell stage a cell is removed
  • the cell’s DNA is extracted and analysed
  • only embryos without the disorder are implanted into the uterus
• Embryo screening pros & cons:

  • Pros - helps parents have a healthy child and reduces suffering
  • Cons - issues with embryo destruction and ethical objections to interfering
• Sex determination - pair 23 are sex chromosomes which are XX (female) and XY (male)
• In most populations there is variation which is differences in characteristics between organisms
• genetic variation is caused by mutations
• variation can be the result of inherited genes, the environment or both
• evolution - change in the inherited characteristics of a population over time, due to natural selection as only some organisms with certain characteristics survive and reproduce in their environment
• Charles Darwin’s theory of evolution by natural selection suggests all species alive today evolved from simple life forms that first developed over 3 billion years ago
• Natural selection:

  • variation in a population due to random mutations
  • some organisms have characteristics that enable them to survive and reproduce
  • pass on alleles for these characteristics to their offspring so there are more in the population
  • leads to the population having phenotypes that are best adapted to their environment
• if natural selection happens in 2 populations of the same species they can become so different they can no longer interbreed to produce fertile offspring so new species are formed
• Selective breeding - humans breed organisms for certain desired characteristics

  • select 2 parents with desired characteristics
  • breed these together
  • select individuals with desired characteristics from offspring and breed these together
  • repeat over many generations until all offspring have desired characteristics
• desired characteristics in animals: more meat, more milk, increased yield of eggs and wool, gentle nature for dogs
• desired characteristics in plants: disease resistance (in food crops) and large, unusual or scented flowers
• Issues with selective breeding: inbreeding reduces genetic variation making organisms more likely to suffer from recessive genetic disorders and are more susceptible to new pathogens