cba 2

• DNA


  A double helix comprising two strands of nucleotides, each nucleotide formed of a sugar, a phosphate group and one of four different bases (adenine, cytosine, guanine, thymine/uracil)
• Condensation reactions in joining the components of nucleotides and in the formation of di- and polynucleotides (nucleic acid)

  1. Nucleotides combine together to form macromolecules

  2. Condensation reaction between the phosphate group of one nucleotide and the sugar group of another nucleotide

  3. Alternating sugar and phosphate molecules form a backbone (sugar-phosphate backbone) with 1 of the bases attached to each of the sugar molecules along the strand (DNA stability)

  4. The bases vary, they represent a unique sequence that carries coded information held by the nucleic acid
• Dinucleotide


  Formed by a condensation reaction, a water molecule is lost in the process
• Complementary base pairing


  Adenine-thymine, guanine-cytosine
• The percentage of adenine is approximately equal to the percentage of thymine, while the percentage of guanine is approximately equal to the number of cytosine
• Semi-conservative DNA replication


  1. Separation of DNA strands

  2. Synthesis of DNA primer

  3. Synthesis of new daughter strands
• Gel electrophoresis


  • Nucleic acids are electrophoresed within a "gel"
  • Gel is immersed within an electrophoresis buffer that provides ions to carry a current and a buffer to maintain the pH at a relatively constant value
  • Nucleic acids have a consistent negative charge imparted by their phosphate backbone and migrate towards the anode
• Chemical elements which make up:

  • Carbohydrates: carbon, hydrogen, oxygen
  • Fats: carbon, hydrogen, oxygen
  • Protein: carbon, hydrogen, oxygen, nitrogen
• Roles of carbohydrates, fats and proteins in living organisms

  • Carbohydrates as an immediate source of energy
  • Fats for insulation and long-term storage of energy
  • Proteins for growth and repair of cells
• Starch-iodide test


  1. Add 1 drop of iodine solution on a white tile

  2. Followed by 1 drop of the tests solution and mix well

  3. Positive: mixture turned from yellowish-orange to blue-black (not brown) → starch is present

  4. Negative: mixture remained yellowish-orange → starch is absent
• Biuret test


  1. Add 1.0cm3 of test solution to a clean, dry test tube

  2. Add 1.0cm3 of Biuret reagent to the test solution & shake to mix

  3. Positive: mixture turned from blue to violet / lilac (not purple) → protein is present

  4. Negative: mixture remained blue → protein is absent
• Benedict's test


  1. Add 1.0cm3 of test solution in a test tube

  2. Add 1.0cm3 of Benedict's solution to the test tube and swirl

  3. Leave the test tube in a boiling water bath for 3 minutes

  4. Observe for formation of precipitate

  5. Positive: brick red precipitate was obtained from a blue mixture → a large amount of reducing sugar is present, orange precipitate was obtained from a blue mixture → a moderate amount of reducing sugar is present, yellow precipitate was obtained from a blue mixture → a low amount of reducing sugar is present, green precipitate was obtained from a blue mixture → a trace amount of reducing sugar is present

  6. Negative: mixture remained blue → reducing sugar is absent
• Ethanol emulsion test


  1. Add 2cm3 of ethanol to 2 drops of test solution in a test tube. mix well and allow it to stand for 2 minutes

  2. Decant the ethanol into another test tube containing 2cm3 of water. mix well

  3. Positive: a white emulsion was formed when water was added → fat / lipid is present

  4. Negative: mixture remained clear when water was added → fat / lipid is absent
• Large molecules are synthesised from smaller basic units
• Mitosis


  A type of cell division giving rise to genetically identical cells in which the chromosome number is maintained
• Importance of mitosis


  • Allows asexual reproduction to occur
  • New cells must be produced by mitosis in order for these organisms to grow
  • News cells are produced by mitosis to replace cells that are dead or damaged
• Behaviour of chromosomes during mitosis and the associated behaviour of the nuclear envelope, cell membrane and centrioles

  1. Prophase: chromatin condenses by supercoiling to become 2 sister chromatids held together at the centromere, nucleolus gradually disappears and nuclear envelope breaks down, in animals, the centrosome divides and the 2 centrioles replicate to form 2 centrosomes

  2. Metaphase: 2 centrosome move to opposite ends of the cell, microtubules of the cytoplasm start to form into a spindle, radiating out from the centrioles, each pair of sister chromatids is attached to a microtubule of the spindle and is arranged at the equator of the spindle

  3. Anaphase: centromeres separate and spindle fibres shorten, sister chromatids are pulled by centrosomes to opposite poles

  4. Telophase: nuclear envelope reforms around both groups of chromosomes at opposite ends of the cell, chromosomes decondense and become chromatin again, nucleolus reforms in each nucleus

  
• Nuclear division and cytokinesis

  • Animal cells: in-tucking of the cell surface membrane at the equator of the spindle, "pinching" the cytoplasm in half, cytokinesis occurs by cleavage
  • Plant cells: Golgi apparatus forms vesicles of new cell wall materials, which collect along the line of the equator of the spindle (cell plate), vesicles will merge, forming the new cell surface membranes and the cellulose cell walls between the 2 cells

  
• Causative factors of cancerous growth

  • Cell cycle operates without its normal controls (a molecular control system controlled by specific genes)
  • Rate of cell multiplication is much faster than the rate of cell death, and a tumour is formed
• Development of cancer as a multi-step process

  1. Normally dividing cells subjected to prolonged exposure to carcinogens

  2. The mutated cell undergoes repeated & rapid mitosis and the cells formed don't respond to signals from other cells and are not removed by the immune system

  3. The carcinogen causes mutation in a cell

  4. A benign tumour absorbs nutrients, enlarges, may compress surrounding tissues, but does not spread from its site of initiation

  5. Malignant tumour cells may have unusual chromosomes and their metabolism is disabled

  6. Angiogenesis: consists of cells that secrete signals triggering growth of blood and lymph vessels to serve the tumour cells at the expenses of other tissues

  7. Metastasis: spread of cancer cells to other locations
• Haploid


  Has 1 set of chromosomes
• Diploid


  2 sets of chromosomes
• Genetically dissimilar gametes can be produced

  
• Homologous pairs of chromosomes

  Matching pairs of chromosomes that can possess different versions of the same genes, 1 member from male parent and the other from female parent
• Meiosis


  A type of cell division that gives rise to genetically dissimilar cells in which the chromosome number is halved due to the separation of homologous chromosomes
• Meiosis is used in the formation of gametes

  
• Behaviour of chromosomes during meiosis, and the associated behaviour of the nuclear envelope, cell membrane and centrioles

  1. Meiosis I - Prophase I: homologous chromosomes commence pairing as they continue to shorten and thicken by coiling, breakages occur in parallel non-sister chromatids at identical points, rejoining of non-sister chromatids form chiasmata -> new combinations of genes, positions of the chiasmata become visible as tight pairing of homologous chromosomes ends

  2. Meiosis I - Metaphase I: spindle forms, homologous pairs become attached to individual microtubules of the spindle by their centromeres, homologous pairs are arranged at the equator of the spindle

  3. Meiosis I - Anaphase I: chromosomes of each homologous pair move to opposite ends of the spindle but with the individual chromatids remaining attached by their centromeres, separation of homologous pairs of chromosomes

  4. Meiosis I - Telophase I: end of meiosis I: homologous chromosomes arrived at opposite poles, chromosomes uncoil, nuclear envelopes reform around both nuclei, spindle breaks down

  5. Meiosis II - Prophase II: nuclear envelope breaks down again, chromosomes shorten and rethicken by coiling, centrioles duplicate and move to opposite poles of the cell, spindle apparatus reformed at right angles to original spindle

  6. Meiosis II - Anaphase II: centromeres divide, sister chromatids move to opposite poles of the spindle, centromeres first

  7. Meiosis II - Telophase II: nuclear envelopes form around the four groups of sister chromatids -> four nuclei are formed -> 4 cells, each with half the chromosome number of the original parent cell, chromosomes uncoil and become dispersed as chromatin, nucleoli reform

  
• How meiosis and fertilisation can lead to genetic variation


  • Random assortment of maternal and paternal homologous chromosomes: the way the chromosomes of each homologous pair line up at the equator of the spindle in meiosis I is entirely random, which chromosome of a given pair goes to which pole is unaffected by the behaviour of the chromosomes in other pairs, 223 = over 8 million possible combinations in human
  • Crossing over of segments of individual maternal and paternal homologous chromosomes: results in new combinations of alleles on the chromosomes of the haploid cells produced, generates an almost unimaginable degree of variation / unlimited recombination
  • Fusion of male and female gametes in sexual reproduction
• Functions of the following cell structures (including organelles) of typical plant and animal cells

  • Cell wall: encloses the entire plant cell, surrounds the cell surface membrane, protects the cell from injury and gives the plant cell a fixed shape
  • Cell surface membrane: controls movement of substances in and out of the cell
  • Cytoplasm
  • Nucleus: controls cell growth, repair of worn-out parts, cell division
  • Cell vacuoles: animal - temporary stores of water and food substances, plant - stores dissolved substances (sugar, mineral salts, amino acids)
  • Chloroplasts: contains chlorophyll for photosynthesis
• Functions of the following membrane systems and organelles

  • Endoplasmic reticulum: rough - ribosomes attached on the outer surface are sites of synthesis of proteins that are usually transported out of the cell, smooth - site of synthesis of fats and steroids and detoxification
  • Golgi body: chemically modifies substances made by the endoplasmic reticulum, stores and packages these substances in vesicles for secretion out of the cell
  • Mitochondria: site of aerobic respiration and release of adenosine triphosphate
  • Ribosomes: sites where proteins are synthesised in cells
• Comparison of the structure of typical animal and plant cells

  
• what does each nucleotide contain

  deoyribose, phosphate group, nitrogenous base
• 4 phases 


  prophase, metaphase, anaphase, telophase
• Semi-conservative DNA replication


  1. Separation of DNA strands

  2. Begins at the ori

  3. Helicase unwinds the double helix by breaking the hydrogen bonds between the complementary base pairs in the parental strands

  4. Each parental strand acts as the template for the synthesis of a new DNA strand

  5. Synthesis of DNA primer

  6. Prime attaches to the unwound chain and catalyses the synthesis of RNA primer to provide free 3'OH ends for DNA polymerase III

  7. Synthesis of new daughter strands

  8. DNA polymerase III elongates the new daughter strand in the 5' to 3' direction by catalysing phosphodiester bond formation between the incoming deoxyribonucleotides and free 3'OH end of the daughter strand

  9. Free deoxyribonucleotides are incorporated by complementary base pairing to the parental DNA strands (A-T, C-G)

  10. In a replication fork, the leading strand is synthesised continuously

  11. The lagging strand is synthesised discontinuously to form okazaki fragments

  12. RNA primers are removed and replaced by deoxyribonucleotides by DNA polymerase I

  13. Nicks between the okazaki fragments are filed in by DNA ligase by forming phosphodiester bonds between okazaki fragments