Textbook Question
Contrast the chromatin fiber with the mitotic chromosome. How are the two structures related?
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9. Mitosis and Meiosis
Mitosis
2:44 minutesProblem 26
Textbook Question
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., Am and Ap). 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.
During meiosis I, assuming no crossing over, what chromatid combination(s) will be present at the completion of prophase I? Draw all possible alignments of chromatids as migration begins during early anaphase.
Verified step by step guidance1
Understand the context: The problem involves a diploid cell with three pairs of homologous chromosomes (AA, BB, CC), each with a maternal (A^m, B^m, C^m) and paternal (A^p, B^p, C^p) member. The goal is to analyze the chromatid combinations during meiosis I, specifically at the completion of prophase I and early anaphase I, assuming no crossing over.
Step 1: Recall that during prophase I of meiosis, homologous chromosomes pair up through a process called synapsis. This forms structures called tetrads, where each homologous pair consists of two chromosomes (one maternal and one paternal), and each chromosome has two sister chromatids due to DNA replication. For example, the homologous pair AA will form a tetrad with chromatids A^m, A^m (sister chromatids from the maternal chromosome) and A^p, A^p (sister chromatids from the paternal chromosome). Repeat this for BB and CC.
Step 2: At the completion of prophase I, the homologous chromosomes are paired, and the chromatids are aligned in tetrads. The tetrads for the three chromosome pairs will look like this: (A^m, A^m, A^p, A^p), (B^m, B^m, B^p, B^p), and (C^m, C^m, C^p, C^p). These tetrads are held together by the synaptonemal complex and chiasmata (though no crossing over occurs in this problem).
Step 3: As meiosis I progresses into early anaphase I, homologous chromosomes begin to separate. This means that the maternal and paternal chromosomes of each homologous pair will migrate to opposite poles of the cell. For example, one pole will receive A^m, A^m (maternal chromatids), and the other pole will receive A^p, A^p (paternal chromatids). Repeat this separation for BB and CC.
Step 4: Draw all possible alignments of chromatids at the beginning of anaphase I. Since the orientation of homologous pairs is random (independent assortment), there are multiple possible combinations. For example, one possible alignment is: (A^m, A^m, B^m, B^m, C^m, C^m) at one pole and (A^p, A^p, B^p, B^p, C^p, C^p) at the other pole. Another possible alignment could be: (A^m, A^m, B^p, B^p, C^m, C^m) at one pole and (A^p, A^p, B^m, B^m, C^p, C^p) at the other pole. Repeat this process to account for all possible combinations of independent assortment.
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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 original cell. It involves several stages: prophase, metaphase, anaphase, and telophase. During prophase, chromosomes condense and become visible, and the nuclear envelope breaks down. Mitosis is crucial for growth, repair, and asexual reproduction in organisms.
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Meiosis
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. During meiosis I, homologous chromosomes pair and can exchange genetic material through crossing over, although the question specifies no crossing over. This process is essential for sexual reproduction and genetic diversity.
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Chromatid Pairing and Synapsis
Chromatid pairing occurs when replicated chromosomes align closely during cell division. In meiosis I, synapsis refers to the pairing of homologous chromosomes, forming structures called tetrads. This alignment is crucial for the proper segregation of chromosomes during anaphase. Understanding these concepts helps in visualizing the chromatid combinations and their behavior during the different stages of meiosis.
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