Question

Lampbrush chromosome found in the oocytes of amphibians is seen in
A. Leptotene
B. Diplotene
C. Pachytene
D. None of the above

Answer 1

Hint: Lampbrush chromosomes are found in oocytes of amphibians. During the early stages of meiosis, synapsis occurs and each bivalent appears as a tetrad. Later, the synapsed chromosomes separate and are connected at the site of crossing over.

Complete answer:
Lampbrush chromosomes are the special type of large chromosomes found in some eukaryotes. They are found in the oocytes of most vertebrates and spermatocytes of some insects.

Oocytes and spermatocytes are produced by meiosis in the diploid germ cells. Meiosis includes two sequential nuclear divisions called meiosis I and meiosis II. The prophase I of meiosis I is specifically a long stage and is characterized by synapsis and crossing over.

During the leptotene stage of prophase I, chromatin condensation makes the chromosomes visible. The homologous chromosomes are paired during zygotene and appear as bivalents. The pairing is assisted by the synaptonemal complex. The non-sister chromatids of the paired homologous chromosomes undergo crossing over in pachytene. Pachytene is followed by diplotene during which the paired homologous chromosomes separate from each other but are attached at chiasmata.

Lampbrush chromosomes observed in oocytes appear as the paired homologous chromosomes held together at chiasmata. Each lampbrush chromosome has two axes from which lateral loops arise. Each axis consists of a pair of condensed chromatids while the lateral loops have a single chromatid. This arrangement gives the whole chromosome a look of the brush used to clean the lamp chimneys in older times.

Hence, the correct answer is option (B).

Note: Since lampbrush chromosomes are observed in the gametes, they are meiotic chromosomes. Each lateral loop of a lampbrush chromosome consists of a DNA molecule. It serves as an active site for the synthesis of RNA.