>
Mitosis - Meiosis
Review
Mitosis is the cell division process that generates genetically identical daughter cells. All somatic cells and stem germ cells (spermatogonia/oogonia) proliferate by mitotic cell division.

Meiosis involves a two stage cell division (Meiosis I followed by Meiosis II). By means of meiosis, primary spermatocytes & oocytes generate haploid gametes (sperm & ova) that are unique regarding genotype.


Beginning with the zygote, all cell divisions in an animal are mitotic excepts for the germ cell divisions that produce gametes by meiosis.

GLOSSARY:
      chromatid: a copy of a chromosome. Following mitosis (prior to synthesis during interface), each chromosome consists of one chromatid. Following synthesis, each chromosome is composed of two chromatids linked by a centromere.

      chromatin: refers to stained chromosomes within the nucleus of a eukaryotic cell; inactive chromosomes stain distinctly (heterochromatin) but active chromosomes are uncoiled and do not stain distinctly (euchromatin).

      chromosome: double-stranded DNA and associated histone proteins located within the nucleus of a eukaryotic cell.

      homologous chromosomes: refers to genotypically similar chromosomes that constitute a paternal-maternal pair; the two are linked together during Meiosis I (predisposing to crossover exchange of genes).

      diploid (2N): refers to the typical number of chromosomes found in all body cells except secondary germ cells and gametes; chromosomes are present as homologous pairs in diploid cells.

      haploid (N): refers to the number of chromosomes found in secondary germ cells and gametes; individual chromosomes are represented by either the paternal or maternal chromosome per homologous pair.

      interphase: refers to the period in the cell cycle when synthesis occurs in preparation for mitosis or meiosis I; synthesis produces two chromatids per chromosome during interphase.

      meiosis: refers to germ cell divisions that give rise to four gametes that are haploid with unique genotypes (meiosis I produces haploid daughter cells and meiosis II generates unique gametes).

      mitosis: refers to the typical cell division that gives rise to two daughter cells, each have the same genotype as the parent cell.

Chromosomes are double strands of DNA and associated histone proteins, evident as stained chromatin within the nucleus of a eukaryotic cell. Chromosomes become distinct during cell division (mitosis/meiosis), when they become tightly coiled. Chromosomes are indistinct during interphase (because they are uncoiled while DNA is being transcribed or synthesized).


A chromosome at different magnifications. A: DNA double strand. B: Double-strand DNA wrapped around histone proteins (nucleosomes). C: Following synthesis, two chromatids are joined by a centromere; each chromatid consists of highly coiled nucleosomes. D: Two chromatids of a chromosome are tightly coiled in preparation for cell division.

Following mitosis, at the start of interphase, each chromosome is composed of one chromatid (chromatin strand). Following DNA synthesis during interphase, each chromosome is composed of two chromatids connected by a centromere (required for mitosis).


A scanning electron micrograph of an X chromosome, composed of two chromatids in preparation for cell division.
Mitosis is the cell division process by which one parent cell produces two genetically identical daughter cells. Each daughter cell has a single chromatid per chromosome following mitosis. After DNA synthesis during interphase, each cell has two chromatids per chromosome and is prepared for mitosis.


Prior to undergoing mitosis, cells cycle through interface, during which DNA double strands separate and synthesize duplicate strands. Consequently, a cell enters interphase with one chromatid per chromosome and exits interphase with two chromatids per chromosome.

The four stages of mitosis are:
      — prophase: chromosomes become visible and the nuclear membrane disappears
      — metaphase: double-chromatid chromosomes align along the nuclear equator
      — anaphase: chromatids separate and each chromatid is pulled toward a daughter cell nucleus
      — telophase: nuclear membranes reappear and chromosomes become indistinct as interphase ensues.


Synthesis produces two chromatids per chromosome during the interphase that precedes mitosis. Chromosomes become evident and the nuclear membrane fades during mitosis. Individual chromosomes align at the equator during metaphase. Chromatids separate during anaphase and identical daughter cells form during telophase.
Meiosis is the cell division process by which one primary spermatocyte produces four spermatid gametes, each with a different genotype (also, one primary oocyte produces an ovum and three polar bodies).

Meiosis is a two stage process. In Meiosis I, the diploid primary germ cell divides into two secondary germ cells that are haploid. In Meiosis II, the haploid secondary germ cell (which has two chromatids per chromosome) divides into two haploid gametes each having one chromatid per chromosome.


Meiosis refers to the two-stage division of one diploid germ cell into four haploid gametes. In the first division (Meiosis I), a diploid primary germ cell becomes two haploid secondary germ cells. The second divisions (Meiosis II) involves separation of double chromatids per chromosome.

The four stages of meiosis I are:
      — prophase: chromosomes become visible and the nuclear membrane disappears
      — metaphase: homologous pairs of double-chromatid chromosomes align along the nuclear equator; homologous chromosomes can exchange material (crossover)
      — anaphase: chromosomes per pair separate, one chromosome goes to each daughter cell (secondary germ cell)
      — telophase: nuclear membranes reappear (each secondary germ cell is haploid and genetically different) and meiosis II proceeds.


Crossover refers to the exchange of chromosomal material between maternal and paternal chromosomes during metaphase of Meiosis I. Crossover occurs because paternal-maternal homologous chromosomes become paired during Meiosis I. Crossover makes the two chromatids of a chromosome different and thus individual gametes unique following Meiosis II.

The four stages of meiosis II are similar to mitosis:
      — prophase: chromosomes become visible and the nuclear membrane disappears
      — metaphase: double-chromatid chromosomes align along the nuclear equator
      — anaphase: the two chromatids per chromosome separate, each chromatid moves to a daughter cell (gamete)
      — telophase: nuclear membranes reappear (spermatids morph into spermatozoa; one oocyte gets all of the cytoplasm.)


Meiosis I (top) is a reduction division that produces haploid daughter cells. Meiosis II (bottom) produces single chromatid gametes that are genotypically unique.
During gametogenesis, diploid (2N) germ cells produce haploid (N) gametes by meiosis. Via fertilization, gamete fusion produces a diploid zygote, the start of a new individual. Thus germ cells provide continuity between generations through transfer of chromosomal DNA.


Note: Gametogenesis includes spermatogenesis and oogenesis. During oogenesis all cytoplasm goes to one daughter cell, the other becomes a polar body.

This Review is intended for veterinary students studying embryology (developmental anatomy). Its purpose is to remind students how mitosis and meiosis differ and the role that meiosis plays enabling DNA transfer across generations.

The Review web app was designed for smart phones but it may be viewed on tablet or computer browsers.


Note:
A screencast video regarding Mitosis and Meiosis is available for mobile devices at:
http://vanat.cvm.umn.edu/mEmbryoHL/

And a video for computer screens is at:
http://vanat.cvm.umn.edu/embryoHL/EarlyEmbryogenesisAppend.html

Visit our Veterinary Anatomy web site for links to these and other courseware sites:
http://vanat.cvm.umn.edu/