ive

• Sexual reproduction


  Involves the fusion of male and female gametes (fertilisation) and the genetic information is combined
• Asexual reproduction


  Only involves one parent - the offspring are genetically-identical "clones" to the parent and there is no genetic variation within the offspring
• Organisms that rely on sexual/asexual reproduction

  • Most animals rely on sexual reproduction
  • Bacteria rely on asexual reproduction (binary fission)
  • Most plants rely on both
• Sexual reproduction


  • As there are 2 parents, the offspring contain a mixture of the two genotypes of the parents - this means that the offspring are genetically different to the parents AND there is genetic variation within the offspring
• Gametes


  • Sperm and egg cells in animals
  • Pollen and eggs in plants
• Gametes


  Haploid - only have half the genetic material of a normal (somatic) body cell
• Meiosis


  1. Produces gametes that are all genetically different to each other

  2. Because each gamete is genetically different, in sexual reproduction, all of the offspring are genetically different to each other and the offspring are genetically different to the parents
• Asexual reproduction


  Only involves one parent - this means that, because there is only one set of DNA as there is no fusion of gametes, the offspring are genetically-identical "clones" to the parent and there is no genetic variation within the offspring itself
• Meiosis

  1. A normal diploid cell (containing 46 chromosomes) grows and replicates its DNA to form a cell with 92 chromosomes

  2. This cell then divides into two diploid cells (each containing 46 chromosomes)

  3. These 2 diploid cells will then divide again into 4 genetically-varied gametes (each containing 23 chromosomes so are haploid)
• Fertilisation


  A male gamete (containing a single pair of chromosomes [23]) and a female gamete (also containing a single pair of chromosomes [23]) can fuse to form a zygote – a diploid cell, with 2 sets of chromosomes (46 chromosomes)
• Embryo development


  1. After fertilisation, the fertilised egg (zygote) will divide by mitosis to produce a ball of genetically identical cells called an embryo

  2. As the embryo develops, the cells will differentiate to form different types of specialised cell to form a whole new organism
• Chromosomes


  • Found in the nucleus of cells
  • Contain the molecule DNA (deoxyribonucleic acid)
• DNA


  • Contains genetic information that determines our inherited features
  • Consists of two polymer strands (each strand made by joining lots of smaller molecules)
  • Wrapped around each other in a structure called a double helix
  • Found in tightly packed coils called chromosomes in the nucleus
• Gene


  A small section of DNA on a chromosome that determines one protein (it determines a sequence of amino acids)
• Alleles


  Different versions of the same gene
• Genotype


  The alleles present
• Phenotype


  The characteristics caused by the person's alleles (genotype)
• Dominant allele


  Will always be expressed in the phenotype as long as there is one copy present
• Recessive allele


  Will only be expressed in the phenotype if there are two recessive alleles present (person needs to be homozygous recessive)
• Homozygous


  2 copies of the same allele
• Heterozygous


  2 different alleles
• Punnett Square


  Used to predict the probabilities of the genotypes and phenotypes of offspring
• Cystic fibrosis


  • An inherited disorder of cell membranes controlled by a single gene
  • The CF allele is recessive
• Polydactyly


  An inherited disorder that leads to having extra fingers or toes and is caused by a dominant allele
• Embryo screening


  1. Embryos are tested to see if they have the alleles for inherited disorders

  2. Embryos found not to have an defective allele will then be implanted into the woman and develop into healthy offspring
• Disadvantages of embryo screening

  • Expensive for the NHS, so money could be spent elsewhere to treat other patients
  • A large number of embryos are created but only a small number are implanted, meaning that healthy embryos with the potential for human life are destroyed and some think that this is unethical
  • Could lead to designer babies in the future, where we only screen for desirable features like intelligence or height. Many find this unethical
• Gene therapy


  Corrects faulty alleles to treat inherited disorders
• Protein synthesis occurs in the following stages: (1) Transcription - the base sequence of the gene is copied into a complementary template molecule called mRNA, (2) mRNA template molecule passes out of the nucleus and into the cytoplasm where it binds to a ribosome, (3) Translation - amino acids are brought to the ribosome on carrier molecules called tRNA, (4) The ribosome reads the base sequence and binds adjacent amino acid molecules together in the correct order, (5) The protein will then fold into its unique shape allowing it to function
• Mutation


  A random change in the DNA base sequence
• Mutations can sometimes be inherited but increased exposure to mutagens increase the likelihood of a mutation taking place (e.g., alpha radiation )
• Some mutations do not affect protein structure/function, but other mutations could affect DNA base sequence, leading to a different structure and function of the protein
• Insertions and deletions can also take place whereby a new base is simply added or an existing one taken away which would dramatically affect the entire protein
• Non-coding parts of DNA that do not code for a specific protein can control gene expression, so if a mutation occurs in a non-coding section, gene expression could be affected, causing uncontrolled mitosis leading to cancer