Divide and Conquer

Preface

To understand mitosis and meiosis you need to know and understand the difference between two key pairs of terms.

If you understand these four terms and how they apply to human cells, you will understand human cell division.

  • homologous chromosomes — a matching but non-identical chromosome pair with one of the pair originating from the father and the other from the mother (i.e. homologous pairs are, by definition, non-identical because they carry different versions (alleles) of the same gene, i.e. from each parent)
  • sister chromatid — one identical half of a duplicated chromosome (sister chromatids are identical because one is an exact copy of the other formed during DNA replication)

Note that a chromosome may have one chromatid or two, depending on whether it has been duplicated (DNA replication) or not. The image above shows four chromosomes — two with duplicated chromatids and two with single chromatids. The two chromosomes with duplicated chromatids are said to be a homologous pair, one of the pair originating from the mother and the other from the father. Similarly, it could be said that the two chromosomes with single chromatids are homologues — homologues (one from the mother and one from the father) prior to replication.

and

  • germ cell — an embryonic cell (i.e. diploid) with the potential to develop into a gamete
  • gamete — haploid cells, the products of germ-cell meiosis, that reproduce sexually

You will often see the terms “germ cell” and “gamete” used interchangeably, but that leads to much confusion. A primordial diploid germ cell begets a haploid gamete after meiosis (I and II).

That’s it! That’s all you need to know to understand mitosis and meiosis.

Cell Division

Cells divide to cater for an organisms need to grow — individual cells can only grow so large due to natural limitations — to allow it to repair and replenish, and finally to reproduce.

In the human, where a sperm and ovum meet, fertilisation forms a diploid zygote and, through two full cell cycles and mitosis, a four totipotential-celled morula.

Totipotent stem cells differentiate to cells of pluripotential, either along the somatic- or germ-cell line.

Pluripotent cell lineages. Somatic cell lineages. Essential Reproduction, Johnson, 2013.

Somatic cells differentiate along the lines of ectodermal, mesodermal, or endodermal lineage to undergo cell cycling and mitosis before differentiating further into specific progenitors of end-organ cells.

Germ cells also replicate by mitosis. But at some point in their relatively short lives, however, germ cells undergo meiosis, which instead involves two contiguous separate stages of cell division.

The first meiotic division separates duplicated homologous pairs of chromosomes, but not before they have had a chance to partake in crossing-over — whereby alleles mix. Then, without an intervening interphase, a second meiotic division splits sister chromatids from one another.

The end result is haploid gametes, in this case sperm and ova.

The Cell Cycle

One characteristic of the cell cycle is a period of interphase — a growth (G1) phase, a DNA replication (S) phase, and another growth phase (G2) — before a cell is ready to undergo mitosis.

Many cells are not actively involved in replicative cycles, preferring to carry out their normal functions in a resting state outside of the cell cycle, called the Gap (G0) phase.

Of those involved in active cell cycling, most of that time is spent in interphase and relatively small amount of time is required for mitosis (M phase) and its ultimate cytokinesis (cellular division).

Mitosis and meiosis end with cytokinesis as the cell divides into identical halves.

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