For Problems 25–30, consider a diploid cell that contains three pairs of chromosomes designated AA, BB, and CC. Each pair contains a maternal and a paternal member (e.g., A^m and A^p). Using these designations, demonstrate your understanding of mitosis and meiosis by drawing chromatid combinations as requested. Be sure to indicate when chromatids are paired as a result of replication and/or synapsis. You may wish to use a large piece of brown manila wrapping paper or a cut-up paper grocery bag for this project and to work in partnership with another student. We recommend cooperative learning as an efficacious way to develop the skills you will need for solving the problems presented throughout this text.
Assume that during meiosis I none of the C chromosomes disjoin at metaphase, but they separate into dyads (instead of monads) during meiosis II. How would this change the alignments that you constructed during the anaphase stages in meiosis I and II? Draw them.
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Identify the chromosome pairs in the diploid cell: AA, BB, and CC, with each pair having a maternal and paternal chromosome (e.g., A^m and A^p).
Understand that during meiosis I, homologous chromosomes are supposed to separate, but in this scenario, the C chromosomes do not disjoin at metaphase I.
Visualize the alignment at metaphase I: AA and BB pairs align normally, but the CC pair does not separate, remaining as a tetrad.
Proceed to anaphase I: AA and BB pairs separate into dyads, but the CC pair remains intact as a tetrad, moving to one pole.
In meiosis II, the CC tetrad separates into dyads instead of monads, resulting in gametes with an extra C chromosome.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Mitosis
Mitosis is the process of cell division that results in two genetically identical daughter cells, each with the same number of chromosomes as the parent cell. It involves several stages: prophase, metaphase, anaphase, and telophase. During mitosis, chromosomes are replicated and then evenly distributed to ensure that each daughter cell receives a complete set of chromosomes. Understanding mitosis is crucial for analyzing chromatid combinations and their behavior during cell division.
Meiosis is a specialized form of cell division that reduces the chromosome number by half, resulting in four genetically diverse gametes. It consists of two sequential divisions: meiosis I and meiosis II. In meiosis I, homologous chromosomes pair and can exchange genetic material through crossing over, while in meiosis II, sister chromatids are separated. The unique outcomes of meiosis are essential for understanding genetic variation and the implications of chromosomal behavior during the process.
Chromatid pairing occurs during the S phase of the cell cycle when DNA is replicated, resulting in sister chromatids that are joined at the centromere. In meiosis, the alignment and separation of these chromatids are critical for proper chromosome distribution. The scenario described, where C chromosomes do not disjoin during meiosis I but separate into dyads in meiosis II, alters the expected chromatid alignments and necessitates a reevaluation of the anaphase stages. Understanding these dynamics is key to accurately drawing and interpreting chromatid combinations.
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