Consider the following statements regarding the pachytene substage of prophase I during meiosis and choose the correct answer:
1. Pachytene is said to begin when synapsis is completed
2. During pachytene the chromosomes are associated in bivalents or tetrads
3. Shortening and thickening of the chromosomes takes place due to longitudinal contraction
4. Exchange of groups of genes takes place as a result of crossing over

A. Only 1 and 2 are correct

B. Only 1, 2 and 3 are correct

C. Only 2, 3 and 4 are correct

D. 1, 2, 3 and 4 are correct

Answer: Option D