Fertilization

The diploid chromosome number is restored at fertilization when two very different gamete nuclei fuse to form the zygote.

The variation present at fertilization is from three sources:

• Crossing over that occurs during prophase I.
• Random alignments at metaphase I that lead to millions of combinations of maternal and paternal chromosomes in each gamete.
• Of all the genetically diverse gametes produced, chance will determine which two will meet.

How Mitosis and Meiosis Differ

• Mitotic cell division produces identical cells (clones). This type of division is common in single-celled, asexually reproducing organisms and in the growth process of multicelled forms.
• Meiosis occurs only in the germ cells used in sexual reproduction. It gives rise to novel combinations of alleles in offspring.

END OF LECTURE 5