Question

consider a diploid cell that contains three pairs of chromosomes designated \(\mathrm{AA}, \mathrm{BB}\), and \(\mathrm{CC}\). Each pair contains a maternal and a paternal member (e.g., \(A^{\mathrm{m}}\) and \(\mathrm{A}^{\mathrm{p}}\) ). 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. 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.

Short answer

Answer: There are 8 possible alignments of chromatids during early anaphase in meiosis I for a diploid cell with 3 pairs of chromosomes and no crossing over.

Step-by-step solution

Diploid cell with 3 pairs of chromosomes

At the beginning, the diploid cell contains three pairs of chromosomes – AA, BB, and CC, where each pair has a maternal and a paternal member (e.g., \(A^m\) and \(A^p\)). 2.

Prophase I

During prophase I of meiosis, the chromosomes condense and become visible. Homologous chromosomes pair up during a process called synapsis. For our diploid cell with 3 pairs of chromosomes, the possible alignments of homologous chromosomes are (\(A^m A^p\)), (\(B^m B^p\)), and (\(C^m C^p\)). 3.

Metaphase I

In metaphase I, the homologous chromosome pairs align at the center of the cell, called the metaphase plate. There are two possible alignments for each chromosome pair in metaphase I: either the maternal chromosome can be on the left and the paternal chromosome on the right, or vice versa. Since there are 3 chromosome pairs, we have \(2^3=8\) possible alignments at the metaphase plate. 4.

Anaphase I

During early anaphase of meiosis I, the homologous chromosomes migrate to opposite poles of the cell. Assuming no crossing over occurs, the chromatids remain intact and do not exchange any genetic material. We can now draw the 8 possible chromosome alignments at this stage (each example shows the chromosomes migrating to opposite poles): 1. \(A^m$$\rightarrow$$A^p$$\quad$$B^m$$\rightarrow$$B^p$$\quad$$C^m$$\rightarrow$$C^p\) 2. \(A^p$$\rightarrow$$A^m$$\quad$$B^m$$\rightarrow$$B^p$$\quad$$C^m$$\rightarrow$$C^p\) 3. \(A^m$$\rightarrow$$A^p$$\quad$$B^p$$\rightarrow$$B^m$$\quad$$C^m$$\rightarrow$$C^p\) 4. \(A^p$$\rightarrow$$A^m$$\quad$$B^p$$\rightarrow$$B^m$$\quad$$C^m$$\rightarrow$$C^p\) 5. \(A^m$$\rightarrow$$A^p$$\quad$$B^m$$\rightarrow$$B^p$$\quad$$C^p$$\rightarrow$$C^m\) 6. \(A^p$$\rightarrow$$A^m$$\quad$$B^m$$\rightarrow$$B^p$$\quad$$C^p$$\rightarrow$$C^m\) 7. \(A^m$$\rightarrow$$A^p$$\quad$$B^p$$\rightarrow$$B^m$$\quad$$C^p$$\rightarrow$$C^m\) 8. \(A^p$$\rightarrow$$A^m$$\quad$$B^p$$\rightarrow$$B^m$$\quad$$C^p$$\rightarrow$$C^m\) These are the chromatid combinations at the completion of prophase I, without crossing over, and the possible alignments during early anaphase in meiosis I.

Key concepts

Chromatid Combinations

Understanding how chromatids combine during meiosis is crucial to comprehending the genetic diversity resulting from sexual reproduction. In a diploid cell with three pairs of chromosomes labeled AA, BB, and CC, each chromosome consists of one maternal and one paternal chromatid—designated as A^m and A^p for example.

Upon replication, each chromosome forms two identical sister chromatids joined at the centromere, resulting in a total of four identical chromatids per chromosome pair. However, it's during meiosis that unique combinations arise. The first division, meiosis I, separates homologous chromosomes, so the two sister chromatids of each chromosome still remain together following anaphase I, while the homologous separate into different cells.

Without crossing over, the newly formed gametes at the end of meiosis I can have 8 distinct combinations of chromatids, arising from the different possible arrangements of the maternal and paternal homologues during metaphase I. Assessing these combinations helps us understand the potential genetic variability even before meiosis II further diversifies the genetic makeup of gametes.

Prophase I

Prophase I of meiosis is the stage where the preparatory steps for chromosomal separation occur, and it sets the stage for introducing genetic variability. During this phase, chromosomes start to condense and become visible under a microscope. A critical event in prophase I is synapsis, the pairing of homologous chromosomes. This intimate pairing facilitates the process of crossing over (recombination), although for the purpose of this exercise, we are considering no crossing over.

The special structure that holds the homologous chromosomes together is called the synaptonemal complex. The paired chromosomes, each consisting of two sister chromatids, appear as a tetrad due to the synapsis. If we were to illustrate this for our AA, BB, and CC pairs, we'd see each pair come together (A^mA^p, B^mB^p, and C^mC^p). This unique alignment during prophase I is crucial for the proper separation of homologous chromosomes during subsequent phases of meiosis.

Anaphase I

Anaphase I is marked by the physical separation of the homologous chromosomes. At this stage, the carefully orchestrated events of prophase I lead to the pivotal moment where homologues are pulled apart to opposite poles of the cell.

The previously aligned chromosome pairs at the metaphase plate are now on the move due to the shortening of the microtubules attached to the kinetochores. Notably, the sister chromatids remain attached at their centromeres throughout anaphase I; it's not until meiosis II that they'll be separated.

We can visualize this for our three pairs of chromosomes as either the maternal or paternal homologues migrating to opposite ends of the cell. When no crossing over occurs, the integrity of the chromatids is preserved, leading to 8 possible combinations at the completion of anaphase I for a cell with three chromosome pairs. These combinations lay the groundwork for the genetic composition of the resulting daughter cells.