Lab 8 summary CC-BY-NC

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Summary of Lab 8 for BIOL 112 (Winter 2011). Week of Feb 28 2011.

Mitosis and Meosis.

Genetic nomenclature and Mendelism.

Monohybrid and dihybrid crosses and genetics problems.

Sex-linked cross (Drosophila).

Microbiology II: Observation of lab 7 agar plates.

1Cell cycle


  • Events prior to mitosis
  • Phases:
    • Growth phase (G1): cells actively metabolising and growing but not replicating
    • Synthesis phase (S): each chromosome replicates, producing two identical copies (sister chromatids); still uncoiled
    • Second growth phase (G2): chromosomes begin to condense; tubulin synthesised, apparatus for moving chromosomes forms
  • At the end of interphase, nuclear membrane still intact and visible; one or more nucleoli also visible
  • In animal but not plant cells, there are two centrioles which begin to move apart just before mitosis
    • During prophase, they continue to move apart, have reached opposite poles of the nucleus by metaphase
    • Radiating from the centrioles are the spindle fibrils and astral fibrils


  • Mitosis: division of the nucleus; each daughter cell receives a homologous pair; for somatic cells
  • Phases:
    • Prophase: chromosomes condensing into visible threads, becoming shorter and thicker
      • Sister chromatids attached at a constriction point - centomere
      • Centromeres attached to protein disk called kinetochore
      • Microtubules (made of tubulin) form spindle apparatus, extends across cell as nuclear membrane breaks down
      • By the end of prophase, the nuclear membrane and nucleoli have disappeared
      • Centromeres of each pair of sister chromatids attached by microtubules to opposite poles of spindle
    • Metaphase: chromosomes drawn by microtubules to the equator of spindle (center of cell)
      • Spindle clearly visible, chromosomes arranged around equator
      • Ends when each chromosomal centromere divides
    • Anaphase: spindle poles move apart, pulling attached chromatids
      • Tubulin units of microtubules removed at poles --> shortening of microtubules
      • So sister chromatids are pulled apart for two reasons, each going to opposite poles
      • If you consider the centromere region of chromatids the head, they go head first towards the poles
      • By late anaphase, the two groups of chromatids are widely separated, almost at poles
    • Telophase: The two sets of chromatids, now termed chromosomes, are at poles
      • Spindle apparatus breaks down and the chromosomes become indistinct as they elongate into slender threads
      • Nuclear membrane reforms around each set of chromosomes, nucleoli reformed
      • Mitosis ends, cells enter interphase
  • Cytokinesis: division of the cytoplasm into two to form two daughter cells
    • During telophase, fibrils called the phragmoplast form between the two daughter nuclei (called the cell plate) - IN PLANTS
      • Grows outward until it reaches the wall of the dividing cell
      • Contains pectic substances, form middle lamella
      • Cellulose cell wall formed on each side of the middle lamella, around entire protoplast of each daughter cell
      • Original wall of parent cell is stretched and ruptured as daughter cell grows, gets larger
      • No cell plate formed
      • Instead, cytokinesis occurs by cleavage-furror, pinches inward around circumference of cell cytoplasm


  • Fundamental process in all organisms which reproduce sexually, to form gametes in animals, spores in most plants
  • Chromosome number is reduced by half, resulting in haploidy
  • Phases:
    • Prophase I: two members of each homologous pair lie side by side (synapsis) to form a tetrad
      • At the end, tetrad opens longitudinally, four chromatids visible
      • Points of contact (chiasmata) occur along length
      • Genetic recombination can occur at this point between chiasmata, exchange between chromosomes etc
      • This recombination provides the variability that is significant in the evolutionary process
    • Metaphase I: nuclear membrane disappears, spindle forms
      • Each tetrad, with two undivided centromeres, moves to equator
      • Centromere of one homologue faces one pole, centromere of other homologue faces the other
      • The arrangement of any pair of homologous chromosomes independent of the arrangement of any other
    • Anaphase I: non0sister chromatids of each tetrad (i.e. the homologues) separate, move to opposite poles of spindle
      • Tetrad no longer held together by chiasmata; sister chromatids connected only by undivided centromere
      • (In contrast to anaphase of mitosis, where the centromeres divide and sister chromatids separate)
      • This stage results in the halving of the number of chromosomes going to each pole
    • Telophase I: Same as in mitosis
      • Except the daughter nuclei have the haploid number of chromosomes
      • And the chromosomes are double-stranded (two chromatids) when they fade from view
    • Interkinesis - new cell wall formed between daughter nuclei, short interval, no replication of genetic material occurs
    • Prophase II: first phase of equational division (second division of meiosis)
      • Basically, each daughter cell divides again
      • Typical prophase, just shorter
    • Metaphase II: sister chromatids orient themselves at equators of daughter cell's spindles
      • Usually at right angles to the equator in metaphase I
    • Anaphase II: centromeres holding sister chromatids together divide, move to opposite poles of spindle, pulling chromatids along
    • Telophase II: haploid nuclei reform, new cell wall
  • Meiosis results in the production of four haploid cells
    • In animals, they may act as gametes
    • In plants, may divide by mitosis to form a haploid stage of the organism

1.4Feulgen reaction

  • Specific to DNA - stains chromosomes a deep violet
  • Fresh squash of an onion root tip taken from onion kept in water; young root tips show considerable mitotic activity
  • Root tips have been fixed for an hour in 1:3 acetic acid:alcohol
  • Procedure:
    • Hydrolyse in 1N (?) HCl, rinse in tap water
    • Stain in leucofuchsin (Schiff reagent)
    • After staining, the tip should be deep purple, now rinse in shit and place on cover slide, observe

2Genetic nomenclature and Mendelism

  • Pair of chromosomes: homologous pair; individual chromosome of a pair: homologue
  • Pair of genes: allelic pair; different forms of a gene: alleles
  • Allosomes - sex chromosomes; autosomes - non-sex chromosomes
    • Y-chromosome much smaller
    • Male for humans, female for chickens
  • Identical alleles - homozygous; non-identical - heterozygous
    • Hemizygous: when you have only one of each gene, e.g. human males and sex-linked traits on the X-chromosome
  • Wild-type: the common phenotype; has a + (e.g. w+)
  • Genes on the same chromosome separated by commas; different chromosome, semi-colon

3Monohybrid cross in corn

  • Production of chlorophyll in corn controlled by single gene
  • Since corn is diploid:
    • Homozygous chlorophyll-producer: c+/c+
    • Homozygous albino plant: c/c
  • F1 offspring all c+/c (phenotype: green)
  • F2 offspring: 1/4 albino, 3/4 green
    • Note: albino corn plants rarely actually seen in corn fields...
    • Since they can't produce chlorophyll, they're not very good at photosynthesising, so, will run out of food
    • And will soon DIE
  • Illustrates Mendel's law of segregation

4Sex-linked cross (Drosophila)

  • White eyes (w) and miniature wings (m) are both X-linked traits
  • For this experiment, we cross 5 WILD-TYPE MALES with 5 WHITE MINIATURE FEMALES
    • The F1 generation: females will all be WILD-TYPE (heterozygous); males will be all be WHITE-MINIATURE
    • The F2 generation: females will be wild type or white miniature; same for males
    • they should end up being around ~36 map units apart
    • Remember that crossover will only occur in the female (because they have two X-chromosomes)
    • I don't have the actual numbers for the results