Question

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Describe a day in the life of a chromosome. Include descriptions of the processes of independent assortment, crossing over, and mutations.

Answer 1

The cell divides twice (without an intervening chromosome duplication) to produce four cells with half the number of chromosomes as the original parent cell. For diploid cells (i.e. those containing two copies of each chromosome—one from the organism's mother and one from its father), meiosis produces four haploid cells containing one copy of each chromosome. (In some cases, such as the formation of mammalian eggs, only one of the haploid daughter cells survives, while the others become polar bodies.) The haploid cells resulting from meiosis are gametes. Each of the chromosomes in the gamete cells is a unique mixture of maternal and paternal DNA, resulting in offspring that are genetically distinct from either parent. This gives rise to genetic diversity in sexually reproducing populations. This genetic diversity can provide the variation of physical and behavioural attributes (phenotypes) upon which natural selection can act.

Prior to the meiosis process the cell's chromosomes are duplicated by a round of DNA replication, creating from the maternal and paternal versions of each chromosome (homologs) two exact copies, sister chromatids, attached at the centromere region. In the beginning of meiosis, the maternal and paternal homologs pair with each other. Then they typically exchange parts by homologous recombination leading to crossovers of DNA between the maternal and paternal versions of the chromosome. Spindle fibers bind to the centromeres of each pair of homologs and arrange the pairs at the spindle equator. Then the fibers pull the recombined homologs to opposite poles of the cell. As the chromosomes move away from the center the cell divides into two daughter cells, each containing a haploid number of chromosomes composed of two chromatids. After the recombined maternal and paternal homologs have separated into the two daughter cells, a second round of cell division occurs. There meiosis ends as the two sister chromatids making up each homolog are separated and move into one of the four resulting gamete cells. During this process, chances of a mutation occurring could lead to chromosomes being unevenly distributed to daughter cells thus resulting in polyploidy individuals. This may result in genetic defects and ultimately death for new borns.

Answer 2

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