Lab 8 summary
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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¶
1.1Interphase¶
- 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
1.2Mitosis¶
- 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
- Prophase: chromosomes condensing into visible threads, becoming shorter and thicker
- 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
- IN ANIMALS:
- No cell plate formed
- Instead, cytokinesis occurs by cleavage-furror, pinches inward around circumference of cell cytoplasm
- During telophase, fibrils called the phragmoplast form between the two daughter nuclei (called the cell plate) - IN PLANTS
1.3Meiosis¶
- 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
- Prophase I: two members of each homologous pair lie side by side (synapsis) to form a tetrad
- 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