Crossing over occurs between

Options

A. sister chromatids of the same chromosome
B. two different kinds of bivalents
C. two different kinds of chromosomes
D. nonsister chromatids of a bivalent

Answer

**Step-by-Step Solution:**

1. **Understanding the Process**: Recognize that crossing over is a process that occurs during meiosis, which is the type of cell division that leads to the formation of gametes (sperm and eggs).

2. **Identifying the Stage**: Note that crossing over specifically takes place during prophase I of meiosis. This is the stage where homologous chromosomes pair up.

3. **Homologous Chromosomes**: Understand that homologous chromosomes are pairs of chromosomes that have the same structure and carry genes for the same traits, but may have different alleles (versions of a gene).

4. **Bivalents Formation**: During prophase I, homologous chromosomes come together to form structures known as bivalents or tetrads. Each bivalent consists of four chromatids.

5. **Sister vs. Non-Sister Chromatids**: Identify the difference between sister chromatids (the two identical copies of a chromosome) and non-sister chromatids (the chromatids from different homologous chromosomes).

6. **Location of Crossing Over**: Conclude that crossing over occurs specifically between non-sister chromatids of homologous chromosomes. This exchange of genetic material increases genetic diversity.

7. **Final Answer**: Therefore, the correct answer to the question "Crossing over occurs between" is **non-sister chromatids of a bivalent**.

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