Cell Biology Test - 16

G₀ of cell cycle is attained when the cell has undergone differentiation. The phase describes a cellular state outside of the replicative cell cycle. Classically, cells were thought to enter G₀ primarily due to environmental factors, like nutrient deprivation, that limited the resources necessary for proliferation. Thus it was thought of as a resting phase. G₀ is now known to take different forms and occur for multiple reasons.

All the above mentioned statements are correct regarding meiosis.

Crossing over takes place at a stage between zygotene and diplotene. Crossing over is the exchange of genes between two chromosomes, resulting in non-identical chromatids that comprise the genetic material of gametes (sperm and eggs).

Colchicine, a drug, which inhibits spindle fibre formation during mitosis and halts cell division at metaphase. This often will result in a doubling of the chromosome number since colchicine typically inhibits cytokinesis, but not karyokinesis. The doubled chromosomes will fuse within a single nucleus, thus diploidising the haploid cell.

Karyochoresis is that type of mitosis in which the nuclear membrane (karyotheca) does not disappear, but it persists throughout the cell division and it can be seen even at the telophase. During karyochoresis, the spindle formation is internal. After telophase, the parent nucleus undergoes centripetal constriction and divides into two daughter nuclei.

It stains regions having constitutive heterochromatin. Q-banding shows AT rich regions.

Spindles are formed during metaphase of M phase. These spindles are made of tubulin dimers. Cochicine disrupts tubulin dimers and cell cycle gets arrested. Thus, this option is the correct.

All these changes occur in G₂ phase and halts to complete this process.

Some allopolyploids form only bivalents and no multivalents or univalents are observed during meiosis. Generally, extreme genetical imbalance occurs among segregants of allopolyploids with numerous complex multivalents in meiosis resulting in reduced fertility.

During the S phase or synthetic phase of interphase, replication of DNA and synthesis of histone proteins occur. New histones are required in massive amounts immediately at the beginning of the S period of DNA synthesis to provide the new DNA with nucleosomes. At the end of S phase, each chromosome has two DNA molecules and a duplicate set of genes. S phase occupies roughly 35 to 45 per cent time of the cell cycle.