Question: 175: Bivalent or Tetrad formation is a characteristic feature observed during :
(1) Synaptonemal complex in zygotene stage
(2) Chiasmata in Diplotene stage
(3) Synaptonemal complex in Pachytene stage
(4) Chiasmata in zygotene stage
Answer: Option (1)
Explanation:
Meiosis I consists of prophase I, which is divided into leptotene, zygotene, pachytene, diplotene, and diakinesis stages.
During the zygotene stage, homologous chromosomes start pairing with each other in a process called synapsis.
This pairing is facilitated by the formation of a protein structure known as the synaptonemal complex.
As a result of synapsis, each pair of homologous chromosomes comes together to form a bivalent.
Since each bivalent consists of two homologous chromosomes and each chromosome has two chromatids, it is also called a tetrad.
Chiasmata formation occurs later during diplotene, not during zygotene.
Therefore, bivalent or tetrad formation is a characteristic feature of synaptonemal complex formation in the zygotene stage.