Describe the events of binary fission.

Ans 1. Binary fission is the form of asexual reproduction in single-celled organisms by which one cell divides into two cells of the same size, used by most prokaryotes. This process results in the reproduction of a living cell by division into two equal or near-equal parts.

Binary fission begins with DNA replication. DNA replication starts from an origin of replication, which opens up into a replication bubble (note: prokaryotic DNA replication usually has only 1 origin of replication, whereas eukaryotes have multiple origins of replication). The replication bubble separates the DNA double strand, each strand acts as template for synthesis of a daughter strand by semiconservative replication, until the entire prokaryotic DNA is duplicated.

Cell division in bacteria is controlled by the septal ring, a collection of about a dozen proteins that collect around the site of division. There, they direct assembly of the division septum. [1]

The cell membrane then invaginates (grows inwards) and splits the cell into two daughter cells, separated by a newly grown cell plate. This process is called cytokinesis.

Ans. 4. The spindle checkpoint blocks the entry of a cell undergoing mitosis into anaphase until all chromosomes are properly attached to the meiotic or mitotic spindle. To achieve proper segregation, the two kinetochores on the sister chromatids must be attached to opposite spindle poles. Only this pattern of attachment will ensure that each daughter cell receives one copy of the chromosome.

The spindle checkpoint is an active signal produced by improperly attached kinetochores. Unattached kinetochores trigger the spindle checkpoint. When sister kinetochores are properly attached to opposite spindle poles, forces in the mitotic spindle generate tension at the kinetochores. Kinetochores that are attached to the mitotic spindle but that are not under tension also trigger the spindle checkpoint. The mechanism by which kinetochores detect attachment and tension is unclear.

The spindle checkpoint blocks anaphase entry by inhibiting the anaphase-promoting complex.