What is the number of helixes in a double chromosome? what is a centromere, and what is a homologous pair? what is the end result of meiosis with 3 pairs of chromosomes? what is crossing over, where dies it occur, and why is it important?

1) Four Helixes in a double chromosome [ 2 for each strand ]

2) Centromere is the region of a chromosome to which the microtubules of the spindle attach, via the kinetochore, during cell division.

3) A homologous pair is a pair of chromosomes containing a maternal and paternal chromatid joined to together at the centromere

4) End result = 12 pair of chromosome which is unindentical to each other.

5) Crossing over is the exchange of genes between homologous chromosomes, resulting in a mixture of parental characteristics in offspring.

Crossing over occurs during prophase I in nucleus of the cell [ as it is operation of chromosomes ]

Crossing over creates new combinations of genes in the gametes that are not found in either parent, contributing to genetic diversity. So, it is so important for Genetic diversity

Hope this helps!