Final answer:
Chromosomes appear as packets of four chromatids, called tetrads, during prophase I of meiosis as the synaptonemal complex breaks down and chromosomes condense.
Step-by-step explanation:
During prophase I of meiosis, chromosomes appear as packets of four chromatids, which are known as tetrads. This occurs as prophase I progresses and the synaptonemal complex begins to break down, resulting in the condensation of chromosomes. The pairs of homologous chromosomes are held together at what are called chiasmata, and because we can see the four sister chromatids in each homologous pair, they are referred to as tetrads.