* Process information from secondary sources to construct a model that demonstrates meiosis and the processes of crossing over, segregation of chromosomes and the production of haploid gametes
Meiosis - is a process by which a single parent diploid cell (Both homologous chromosomes) divides to produce four daughter haploids cells (One homologous chromosome of the pair), each containing half the chromosome number. Meiosis comprises two successive nuclear divisions with only one round of DNA replication. Four stages can be described for each nuclear division. One parent cell produces four daughter cells. The daughter cells have half the number of chromosomes found in the original parent cell and with crossing over, are genetically different. The segregation of chromosomes is a stage in cell division where chromosomes pair off with their similar homologous chromosome.
Interphase: Before meiosis begins, genetic material is duplicated. First division of meiosis
Prophase 1: Duplicated chromatin condenses. Each chromosome consists of two, closely associated sister chromatids. Crossing-over can occur during the latter part of this stage. Crossing over is the term referring to the instance when adjacent chromatids twist around each other, split where they touch, and then join up with different pieces. This allows for even further variation within the genotype of an individual. Metaphase 1: Homologous chromosomes align at the equatorial plate. Anaphase 1: Homologous pairs separate with sister chromatids remaining together. Telophase 1: Two daughter cells are formed with each containing only one chromosome of the homologous pair. Second division of meiosis: Gamete formation
Prophase 2: DNA does not replicate.
Metaphase 2: Chromosomes align at the equatorial plate.
Anaphase 2: Centromeres divide and sister chromatids migrate individually to each pole. Telophase 2: Cell division is complete and four haploid daughter cells are acquired.
Dot point 2
* Analyse information from secondary sources to outline the evidence that led to Beadle and Tatum’s ‘one gene – one protein’ hypothesis and to explain why this was altered to the ‘one gene – one polypeptide’ hypothesis
Whilst studying inheritance in the fruit fly (Drasophila), a scientist, George Beadle conducted an experiment in the laboratory of Thomas Morgan. His results indicated that the acquisition of eye colour in the fly is the outcome of a long series of chemical reactions, and that genes in some way affect these results. Beadle discovered that the mutant eye colour was caused due to an alteration in one protein, and concluded that genes must influence heredity chemically. Beadle then, with another scientist, Edward Tatum, set out together to provide experimental evidence of the linkage between genes and enzymes. They hypothesized that if there really was a one-to-one association between genes and specific enzymes, it should be possible to generate genetic mutants that are incapable of carrying out specific enzymatic reactions. To test this theory, they exposed spores of Neurospora crassa (a bread mould) to X-rays or UV radiation and observed the consequential mutations. The mutant moulds had an assortment of particular nutritional needs. Unlike the standard individuals, they could not survive without the addition of certain vitamins or amino acids to their food. The normal Neurospora requires only one vitamin (biotin), but the mutant moulds that were also required thiamine or choline. Genetic analysis illustrate that each mutant differed from the original type by only one gene. Biochemical studies showed that the mutants appeared to be unable to progress at specific stages in the ordinary metabolic pathways. Their cells contained large collections of the substance synthesised just before to the point of blockage. As Beadle and Tatum had predicted, they were able to form single gene mutations that debilitated...