Synapsis or crossing over occuring in meiosis I at pachytene stage ; occurs between

Options

A. a male and a female gamete
B. mRNA and ribosomes
C. spindle fibres and centromere
D. two homologous chromosomes

Explanation

### Step-by-Step Solution:

1. **Understanding Synapsis and Crossing Over**:
- Synapsis is the process where homologous chromosomes pair up during meiosis. Crossing over is a key event that occurs during synapsis, where segments of DNA are exchanged between non-sister chromatids of homologous chromosomes.

2. **Identifying the Stage of Meiosis**:
- The question specifies that synapsis and crossing over occur during meiosis I, particularly at the pachytene stage. This is the stage where the homologous chromosomes are fully paired and crossing over can occur.

3. **Analyzing the Options**:
- **Option 1**: Male and female gametes - This is incorrect because crossing over does not occur between gametes; fertilization occurs instead.
- **Option 2**: mRNA and ribosomes - This is also incorrect as this pertains to protein synthesis, not to the process of crossing over in meiosis.
- **Option 3**: Spindle fibers and centromeres - This option is incorrect as well; spindle fibers help in chromosome movement, not in crossing over.
- **Option 4**: Non-sister chromatids of homologous chromosomes - This is the correct answer, as crossing over occurs between non-sister chromatids of homologous chromosomes during the pachytene stage of meiosis I.

4. **Conclusion**:
- Therefore, the correct answer to the question is that synapsis or crossing over occurs between the non-sister chromatids of homologous chromosomes during the pachytene stage of meiosis I.