Pairing of homologous chromosomes in prophase of meiosis I appears to be critical for proper alignment, crossing over, and subsequent separation. This pairing is facilitated by the sharing of sequence homology between homologous chromosomes. If X and Y chromosomes are so different, how can they achieve the necessary pairing?

A. They share short homologies at their respective tips.

B. They do not pair because they are not homologous.

C. They are only different in size, most of the DNA sequences match.

D. A special function of the spindle apparatus forces X and Y chromosomes together.

A) They share short homologies at their respective tips

Explanation:

The cells of human contain two types of chromosomes that is autosomal and sex chromosomes.

The 22 pairs of autosomal chromosomes in humans are formed by the pairing of these chromosomes during the meiotic phase via the formation of a structure called synaptonemal complex between homologous sequences.

However, the two types of sex chromosomes called X and Y are not similar in their length but they pair via the formation of synapse which involves very short homologous sequences called "pseudoautosomal regions". The formation of the synapse through this region is called the X-Y body.

Thus, Option-A is the correct answer.