Ch 7Cell Division — Class 11 Biology, concept-first.

Every multicellular organism, however large or complex it eventually becomes, begins life as a single cell — the zygote formed at fertilisation.

The life of every multicellular organism begins from a single cell -- the zygote -- and the growth of any living organism depends fundamentally on that cell, and the cells it produces, dividing again…

Not every cell divides in the same way. In animal cells three distinct kinds of cell division are recognised: amitosis (direct division), mitosis (indirect, equational division) and meiosis (reduction…

Amitosis is the simplest possible way a cell can divide, and it happens without the elaborate chromosome-condensation and spindle machinery that mitosis and meiosis rely on.

Mitosis is the type of division by which a single cell produces two daughter cells that are genetically identical both to each other and to the parent cell — it is an equational division, since the ch…

Prophase is the opening stage of karyokinesis, and it accomplishes four things at once: the chromatin material condenses into visibly distinct chromosomes, the centrioles migrate toward opposite poles…

By metaphase the chromosomes have reached their maximum condensation, so that they now look very short and compact, with their sister chromatids and shared centromere clearly visible.

Anaphase begins the moment the chromatids of every chromosome separate from each other, each becoming an independent daughter chromosome in its own right.

Telophase is the final stage of karyokinesis, and in many ways it reverses what happened in prophase. The chromosomes, now sitting at the two poles with their centromeres, begin to uncoil, lengthen, a…

Cytokinesis is the division of the cytoplasm that produces two separate daughter cells once karyokinesis is complete.

Because mitosis is an equational division, its most fundamental effect is that the chromosome number of a cell is kept exactly constant from one generation of cells to the next.

The term meiosis was coined by J. B. Farmer in 1905. Unlike mitosis, meiosis occurs only in specialised reproductive cells — called meiocytes — during the formation of gametes (or spores).

During the first meiotic division, a diploid cell divides to give two haploid cells — the chromosome number in each daughter cell is reduced to half of the parent cell's number, which is exactly why t…

Prophase-I lasts far longer than the prophase of mitosis, and it is where the features that make meiosis genetically special all occur. It is conventionally divided into five sub-stages:

In metaphase-I, the spindle fibres become fully developed, and the tetrads (paired homologous chromosomes) move to the equator of the cell.

In anaphase-I, the two homologous chromosomes of each pair are pulled apart from one another and carried toward opposite poles by the spindle apparatus — this separation of homologues is called disjun…

In telophase-I, once the haploid sets of chromosomes have reached their respective poles, they begin to uncoil and lengthen, and the nuclear membrane and nucleolus reappear around each set — forming t…

During the second meiotic division, the two haploid cells produced by the first meiotic division each divide further, yielding four haploid cells in total.

In prophase-II the chromosomes are already distinct, each still made up of two chromatids (carried over from meiosis-I).

In metaphase-II, the chromosomes move to and arrange themselves at the equator of the cell — but this time as single chromosomes, not paired tetrads.

In anaphase-II, the chromatids that have just separated from each other are now counted as independent daughter chromosomes, and they move toward opposite poles as the spindle fibres attached to their…

In telophase-II, the daughter chromosomes that have reached the poles begin to uncoil. A nuclear membrane forms around each group of chromosomes, and the nucleolus reappears inside each newly forming…

Meiotic division is what produces the gametes (or spores, in plants) needed for sexual reproduction, and its significance follows directly from what it achieves.