Which correctly describes crossing over?
the process whereby nonsister chromatids exchange genetic material
the process whereby homologous chromosomes are pulled to opposite poles of the cell
the process whereby sister chromatids are pulled to opposite poles of the cell
the process whereby homologous chromosome pairs align randomly along the metaphase plate

Crossing over is best describes by the process whereby non-sister chromatids exchange genetic material.

Further Explanation:

Crossing over is the exchange of chromosome segments during the production of gametes between non-sister chromatids. The increased genetic variation in the chromosomes is a result of crossover that occurs in daughter cell at the prophase of meiosis.

The chromosomal constitution of a mitotic cell chromosome behaving as a chromosome of meiosis I cell will be changed. The change in chromosomal constitution is because of occurrence of crossing over in the homologous chromosome at the prophase stage of meiosis I; this changes the genetic constitution of the chromosome.  

Also in meiosis I, the chromosome number gets halved because of separation of homologous chromosome to the two opposite pole of the cell. This also changes the ploidy of the cell as each daughter cell formed will have n number of chromosome. In meiosis crossing over occur in non-sister chromatid. So, the number of chromosome itself gets changed. However, in mitosis the crossing over of chromosome does not occur and only the sister chromatids of the chromosome get separated. This leads to equal distribution of chromosomes in the daughter cell and the ploidy level of the daughter cell will remain same as that of the parents.

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Answer Details:

Grade: High School

Subject: Biology

Chapter: Gene expression

Keywords:

Meiosis, haploid, daughter cell, chromosome, crossing over, separation, chromatid, genetic variation, mitotic cell.

the process whereby nonsister chromatids exchange genetic material

Explanation:

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cell regulation is conducted by special proteins.