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.
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 guidance

1. Begin by understanding the chromosome composition: the diploid cell has three pairs of chromosomes (AA, BB, CC), each with a maternal (e.g., A^{m}) and paternal (e.g., A^{p}) homolog. Before meiosis I, each chromosome replicates, so each consists of two sister chromatids joined at the centromere.
2. At the completion of prophase I, homologous chromosomes undergo synapsis, pairing tightly to form bivalents (tetrads). Each bivalent consists of four chromatids: two sister chromatids from the maternal chromosome and two sister chromatids from the paternal chromosome. Since no crossing over occurs, the chromatids remain unchanged in their original configuration.
3. For each chromosome pair (AA, BB, CC), represent the bivalent as two homologous chromosomes paired side-by-side, each with two sister chromatids. For example, the A bivalent will have A^{m} chromatids paired with A^{p} chromatids, each consisting of two sister chromatids (A^{m}_1 and A^{m}_2, A^{p}_1 and A^{p}_2).
4. As early anaphase I begins, homologous chromosomes separate and migrate to opposite poles. The key point is that sister chromatids remain attached; thus, each migrating unit is a chromosome with two sister chromatids. Draw all possible alignments showing that for each chromosome pair, either the maternal or paternal homolog can move to a given pole, reflecting independent assortment.
5. Summarize that at the end of prophase I, the cell contains three bivalents (each with four chromatids), and during early anaphase I, homologous chromosomes separate but sister chromatids stay together, resulting in chromatids aligned as pairs moving toward opposite poles without recombination.

Key Concepts

Chromosome Structure and Replication

Chromosomes consist of two sister chromatids joined at a centromere after DNA replication during the S phase. Each chromatid is an identical copy, and together they form a replicated chromosome. Understanding this structure is essential to track chromatids through cell division stages like mitosis and meiosis.

Synapsis and Homologous Chromosome Pairing in Meiosis I

During prophase I of meiosis, homologous chromosomes (maternal and paternal pairs) undergo synapsis, pairing tightly along their lengths to form tetrads. This pairing allows for alignment and potential crossing over, though in this question crossing over is excluded. Synapsis is key to understanding chromatid arrangements at this stage.

Chromatid Segregation During Meiosis I Anaphase

In meiosis I, homologous chromosomes (each with two sister chromatids) separate and migrate to opposite poles, but sister chromatids remain attached. This reductional division halves the chromosome number. Recognizing that chromatids stay paired until meiosis II clarifies the chromatid combinations present during early anaphase I.