In meiosis, crossing over occurs between non-sister chromatids of homologous chromosomes.

Crossing over, also known as recombination, is a key event that takes place during meiosis. Meiosis is a specialized cell division process that produces gametes (sex cells) with half the number of chromosomes as the parent cell. This process is essential for sexual reproduction and genetic diversity.

Homologous Chromosomes:
Homologous chromosomes are pairs of chromosomes that have the same genes at the same loci, although they may have different alleles. One homologous chromosome is inherited from the mother, while the other is inherited from the father. These chromosomes are similar in size, shape, and carry the same genetic information.

Non-Sister Chromatids:
Each chromosome is composed of two identical copies called sister chromatids, which are held together by a structure called the centromere. During meiosis, the homologous chromosomes pair up, and crossing over occurs between the non-sister chromatids of these homologous chromosomes.

Process of Crossing Over:
1. Homologous Pairing: In meiosis, homologous chromosomes come together and pair up along their entire length. This pairing is called synapsis and results in the formation of a protein structure called the synaptonemal complex.

2. Formation of Chiasmata: Within the synaptonemal complex, the non-sister chromatids of the homologous chromosomes break and exchange genetic material. These points of breakage and rejoining are called chiasmata. The exchange of genetic material leads to the formation of new combinations of genes on the chromatids.

3. Genetic Recombination: The exchange of genetic material during crossing over leads to the recombination of alleles between the non-sister chromatids. This process results in the production of genetically unique chromosomes in the gametes.

4. Independent Assortment: Once crossing over has occurred, the homologous chromosomes separate during meiosis I, and the sister chromatids separate during meiosis II. This leads to the formation of four haploid cells (gametes), each with a different combination of genetic material.

Significance of Crossing Over:
Crossing over plays a crucial role in increasing genetic variability within a population. It shuffles the genetic material between homologous chromosomes, creating new combinations of alleles. This genetic diversity is important for the survival of species, as it provides the potential for adaptation to changing environments and helps prevent the accumulation of harmful mutations.

In conclusion, crossing over occurs between non-sister chromatids of homologous chromosomes during meiosis. This process promotes genetic diversity and ensures the production of genetically unique gametes.

In meiosis crossing over occurs between non-sister chromatids of Homologous chromosomes.