Comparison of mitosis and meiosis

Mitosis consists of a single cell division resulting in the production of two identical daughter cells.  Meiosis entails two successive cell divisions:  reductional division, (Meiosis I), reducing chromosome number by one-half; and equational division (Meiosis II), reducing the DNA content by one-half.  Meiosis results in four dissimilar daughter cells.

Interphase is similar in meiosis and mitosis.  The first stage of interphase, the G1 stage, is seen here, which is followed by S phase, when both the centrioles of the centrosome are duplicated and the DNA of the chromosomes is replicated.  For diagrammatic purposes, chromosomes are shown in a condensed state.

Prophase of meiosis I is much longer than prophase of mitosis and is marked by side-by-side linkage of homologous chromosomes.  This linkage, and the proximity it provides, allows for the exchange of chromosomal segments between chromatids (crossing over).  Crossing over increases greater genetic diversity of the daughter cells.  A similar linkage does not occur in mitosis.

During metaphase of meiosis I, replicated homologous chromosomes remain linked, with one member of the pair aligning on either side of the metaphase plate.  In mitosis, replicated chromosomes (sister chromatids) align at the equatorial plate but each member of a pair of homologous chromosomes is distributed independently within the plate.

During anaphase of meiosis I, one duplicated chromosome of each homologous pair moves toward a pole; sister chromatids remain attached.  This process halves the number of chromosomes, resulting in haploid daughter cells.  Anaphase of mitosis separates sister chromatids.  Each sister chromatid, now referred to as a chromosome, moves to a pole, resulting in diploid daughter cells.

In both meiosis and mitosis, the nuclear envelope reforms during telophase.  In meiosis I, the resultant nuclei contain one replicated chromosome of each homologous pair (haploid, 1N); sister chromatids remain attached.  In mitosis, resultant nuclei contain an entire set of paired, homologous chromosomes (diploid, 2N).

Cytokinesis proceeds similarly in mitosis and meiosis I.  Meiosis I results in the formation of two dissimilar daughter cells, each with a single set of chromosomes (haploid, 1N).  Mitosis results in the formation of two identical daughter cells, each with two sets of homologous chromosomes (diploid, 2N).

Cells enter prophase of meiosis II rapidly after cytokinesis of meiosis I without any intervening DNA synthesis.  Prophase of meiosis II is not prolonged as in prophase of meiosis I; crossing over does not occur; and events closely parallel those of mitosis.

During metaphase of meiosis II, the haploid set of replicated chromosomes (sister chromatids) align at the metaphase plate.  The cohesive protein complex attaching the chromatids is cleaved, allowing for furture separation at the centromere during anaphase.

Anaphase of meiosis II begins abruptly as sister chromatids separate and each is drawn towards one pole of the cell.  The end of anaphase is marked by the segregation of a single set of chromosomes to each spindle pole.

Telophase of meiosis II is marked by the reformation of the nuclear envelope, decondensation of the chromosomal DNA and the reappearance of the nucleolus.  A cleavage furrow develops, indicating the initial formation of the daughter cells (cytokinesis).

Cytokinesis produces gametes (ova or sperm), each with a haploid (1N) set of chromosomes.  However, while nuclear events producing ova (oogenesis) and sperm (spermatogenesis) are similar, cytoplasmic stages are different between the two.  In males, a single germ cell produces four sperm; in females, only a single ovum is formed, along with two to three non-functional polar bodies.