Topic 3 bio genetics

• Sexual reproduction


  Type of reproduction involving the production of gametes by meiosis, where a gamete from each parent fuses to form a zygote, mixing genetic information
• Gametes


  Sex cells (sperm cells, egg cells), haploid (half the number of chromosomes)
• Meiosis


  1. Form of cell division involved in the formation of gametes

  2. Chromosome number is halved

  3. Involves two divisions
• Interphase must occur prior to meiosis
• First stage of meiosis

  1. Chromosome pairs line up along the cell equator

  2. The pair of chromosomes are separated and move to opposite poles of the cell (the side to which each chromosome is pulled is random, creating variation)

  3. Chromosome number is halved
• Second stage of meiosis

  1. Chromosomes line up along the cell equator

  2. The chromatids are separated and move to opposite poles of the cell

  3. Four unique haploid gametes are produced
• Importance of meiosis for sexual reproduction

  • It increases genetic variation
  • It ensures that the resultant zygote is diploid
• Advantage of sexual reproduction

  It creates genetic variation, increasing the probability of a species adapting to and surviving environmental changes
• Disadvantages of sexual reproduction

  • Two parents are required, making reproduction difficult in endangered populations or in species which exhibit solitary lifestyles
  • More time and energy is required so fewer offspring are produced
• Asexual reproduction


  Type of reproduction involving mitosis, producing genetically identical offspring known as daughter cells
• Advantages of asexual reproduction

  • Only one parent is required
  • Lots of offspring can be produced in a short period of time, enabling the rapid colonisation of an area and reducing competition from other species
  • Requires less energy
• Disadvantage of asexual reproduction

  No genetic variation (except from spontaneous mutations) reducing the probability of a species being able to adapt to environmental change
• DNA


  A double-stranded polymer of nucleotides, wound to form a double helix
• DNA nucleotides


  • Common sugar
  • Phosphate group
  • One of four bases: A, T, C or G
• Four bases found in nucleotides

  • Adenine
  • Thymine
  • Cytosine
  • Guanine
• How nucleotides interact to form a molecule of DNA

  1. Sugar and phosphate molecules join to form a sugar-phosphate backbone in each DNA strand

  2. Base connected to each sugar

  3. Complementary base pairs (A pairs with T, C pairs with G) joined by weak hydrogen bonds
• Genome


  The entire genetic material of an organism
• Chromosome


  A long, coiled molecule of DNA that carries genetic information in the form of genes
• Gene


  A section of DNA that codes for a specific sequence of amino acids which undergo polymerisation to form a protein
• Method to extract DNA from fruit

  1. Place a piece of fruit in a beaker and crush it

  2. Add detergent and salt, mix

  3. Filter the mixture and collect the liquid in a test tube

  4. Pour chilled ethanol into the test tube

  5. DNA precipitates forming a fibrous white solid

  6. Use a glass rod to collect the DNA sample
• Why detergent is added

  It disrupts the cell membranes, releasing DNA into solution
• Why salt is added

  Salt encourages the precipitation of DNA
• Why chilled ethanol is added

  DNA is insoluble in ethanol, encouraging its precipitation
• How a gene codes for a protein

  1. A sequence of three bases in a gene forms a triplet

  2. Each triplet codes for an amino acid

  3. The order of amino acids determines the structure (i.e. how it will fold) and function of protein formed
• Why folding of amino acids is important in proteins

  The folding of amino acids determines the shape of the active site which must be highly specific to the shape of its substrate
• Protein synthesis


  The formation of a protein from a gene
• Stages of protein synthesis

  • Transcription
  • Translation
• Transcription


  The formation of mRNA from a DNA template
• Outline of transcription

  1. DNA double helix unwinds

  2. RNA polymerase binds to a specific base sequence of non-coding DNA in front of a gene and moves along the DNA strand

  3. RNA polymerase joins free RNA nucleotides to complementary bases on the coding DNA strand

  4. mRNA formation complete. mRNA detaches and leaves the nucleus
• Differences between mRNA and DNA

  • mRNA is single stranded whereas DNA is double stranded
  • mRNA uses U whereas DNA uses T
• Why mRNA is used in translation rather than DNA

  DNA is too large to leave the nucleus so cannot reach the ribosome
• Translation

  A ribosome joins amino acids in a specific order dictated by mRNA to form a protein
• Outline of translation


  1. mRNA attaches to a ribosome

  2. Ribosome reads the mRNA bases in triplets. Each triplet codes for one amino acid which is brought to the ribosome by a tRNA molecule

  3. A polypeptide chain is formed from the sequence of amino acids which join together
• How tRNA is adapted to its function

  Each tRNA molecule has an anticodon which is specific to the codon of the amino acid that it carries
• Mutation


  A random change in the base sequence of DNA which results in genetic variants
• Effect of gene mutation in coding DNA

  • If a mutation changes the amino acid sequence, protein structure and function may change
  • If a mutation does not change amino acid sequence, there is no effect on protein structure or function
• Non-coding DNA


  DNA which does not code for a protein but instead controls gene expression
• Effect of gene mutation in non-coding DNA

  • A mutation may affect the ability of RNA polymerase to bind to non-coding DNA
  • This may affect protein production and the resulting phenotype of the organism
• How Mendel's work helped develop understanding of genetics

  1. Mendel studied the inheritance of different phenotypes of pea plants

  2. He established a correlation between parent and offspring phenotypes

  3. He noted that inheritance was determined by 'units' passed on to descendants

  4. Using gene crosses, he devised the terms 'dominant' and 'recessive'
• Why Mendel's work was initially overlooked

  Scientist's didn't understand Mendel's work as there was no knowledge of genes or DNA at the time