A pair of homologous chromosomes in Drosophila has the following content (single letters represent genes):
Chromosome 1RNMDHBGKWU
Chromosome 2RNMDHBDHBGKWU
What term best describes the unusual structure that forms during pairing of these chromosomes?

Verified step by step guidance

1. Identify the gene sequences on both homologous chromosomes: Chromosome 1 has the sequence RNMDHBGKWU, and Chromosome 2 has the sequence RNMDHBDHBGKWU.
2. Compare the sequences of the two chromosomes to identify any differences. Notice that Chromosome 2 has an extra segment 'DHBG' inserted between 'D' and 'K'.
3. Understand that during meiosis, homologous chromosomes pair up and align gene by gene. However, the extra segment in Chromosome 2 will cause misalignment.
4. Recognize that this misalignment leads to the formation of a loop structure in the chromosome with the extra segment (Chromosome 2) to accommodate the additional genetic material.
5. Conclude that the term describing this unusual structure is a 'deletion loop' or 'insertion loop,' depending on the perspective, as it forms due to the presence of an extra segment in one homologous chromosome.

Key Concepts

Homologous Chromosomes

Homologous chromosomes are pairs of chromosomes in a diploid organism that have the same structure and gene sequence but may contain different alleles. In Drosophila, like in other organisms, these chromosomes pair during meiosis, allowing for genetic recombination and ensuring that each gamete receives one chromosome from each pair.

Chromosomal Pairing

Chromosomal pairing occurs during meiosis when homologous chromosomes align closely along their lengths. This process is crucial for the exchange of genetic material through crossing over, which increases genetic diversity. The unusual structure that forms during this pairing can lead to unique configurations, especially when there are differences in gene content or structure.

Synapsis

Synapsis is the process during prophase I of meiosis where homologous chromosomes become closely associated, forming a structure known as a synaptonemal complex. This complex facilitates the exchange of genetic material between chromatids, which is essential for proper segregation of chromosomes and genetic variation in offspring.