Which of the following is true of crossing over? It involves the exchange of chromosome segments between homologous chromosomes. It occurs in meiosis I. It increases genetic variability. All of the choices are correct.

All of the choices are correct.

Explanation:

Meiosis is a type of cell division of the nucleus that occurs in cells that will give rise to gametes (egg or sperm). These are haploid cells (n), that is, they possess only one chromosome of each of the pairs (23 in total). Through meiosis from a diploid cell (2n), four haploid cells are obtained, thanks to two successive divisions called meiosis I and meiosis II.  Both comprise prophase, metaphase,  anaphase and telophase.

Homologous chromosomes are a pair composed of a paternal and a maternal chromosome. They have the same DNA sequence arrangement from one end to the other, but different alleles. These chromosome copies have the same genes at the same loci where they contribute tips along each chromosome that facilitate a pair of chromosomes to align correctly with each other before they separate during meiosis. Sister chromatids are those that make up a chromosome after duplication of the genetic material.  

The homologous chromosomes of maternal and paternal origin, mate and this process is called "synapsis" and form what is known as a tetrad or bivalent chromosome, giving the impression that it is a chromosome with four chromatids. In these tetrads, junctions called chiasmata are formed, areas in which both homologues exchange genetic material or genes, a process known as crossing-over. The chiasmata allow the chromosomes to be held together until their separation in anaphase l. So, genetic recombination is the process by which a segment of genetic material from a maternal chromosome is cut and then joined to a paternal chromosome (or vice versa) during prophase I of meiosis. So it happens between non-sister chromatids of a paternal and a maternal chromosome.

After crossing over, the homologous chromosomes are arranged randomly at the cell's  the equator of the cell at random (methaphase), facing each other, still united in the areas where crossing over occurred. Each chromosome is associated with a meiotic spindle fiber of the meiotic spindle and they are pulled along the meiotic spindle and driven toward the respective poles (anaphase). The chromosomes are now arranged at the poles and the reorganization of the nuclear envelope and the nucleolus begins. Finally, cytokinesis occurs and the rearrangement of cellular components and reorganization of the cytoskeleton. As a result of meiosis I,  2 haploid cells are obtained and then meiosis II occurs, where the sister chromatids are separated, generating two more cells (a total of 4).

Genetic variability is generated because the gametes produced during meiosis will not inherit the same chromosomes as the mother cell, but due to crossing over, the inherited chromosomes will be genetically different.

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