Question

Contrast the genetic content and the origin of sister versus nonsister chromatids during their earliest appearance in prophase I of meiosis. How might the genetic content of these change by the time tetrads have aligned at the equatorial plate during metaphase I?

Short answer

Answer: During the earliest appearance in prophase I, sister chromatids have identical genetic content and come from the same chromosome, copied during DNA replication. In contrast, nonsister chromatids have a similar but not identical genetic content and come from different homologous chromosomes (one maternal and one paternal). By metaphase I, the genetic content of nonsister chromatids has changed due to the crossing-over process that occurred during prophase I, resulting in a mixture of their original genetic content and new gene combinations, while sister chromatids still have the same genetic content.

Step-by-step solution

Understanding sister and nonsister chromatids

Sister chromatids are the identical copies of a single replicated chromosome, formed as the cell copies its DNA during DNA replication (usually during interphase). In contrast, nonsister chromatids come from homologous chromosomes – that is, chromosomes that have the same structure and carry the same sets of genes but can originate from different parents (e.g. one from the paternal side and one from the maternal side).

Genetic content and origin during prophase I

At the start of prophase I in meiosis, each homologous chromosome pair consists of a total of four chromatids – two sister chromatids in each homologous pair. The sister chromatids have the same genetic content and come from the same chromosome, copied during DNA replication. The nonsister chromatids, on the other hand, have a similar genetic content (since they carry the same set of genes) but come from different homologous chromosomes (one maternal and one paternal).

Changes in genetic content by metaphase I

During prophase I, homologous chromosomes pair up and form tetrads through a process called synapsis. The genetic content of chromatids can change as a result of crossing-over, a process where sections of DNA between homologous chromosomes are swapped and exchanged. Through this exchange, each nonsister chromatid ends up containing some genetic material from the other homologous chromosome, leading to new gene combinations. This is a crucial aspect of meiosis that creates genetic variation among offspring and is important for evolution and adaptation. By metaphase I, the tetrads with recombined chromatids have aligned at the equatorial plane of the cell, preparing for separation during anaphase I. The sister chromatids still have the same genetic content, while the genetic content of the nonsister chromatids is now a mixture of their original genetic content due to crossing-over events that happened during prophase I.