Why are there two sets of faces during meiosis but only one during mitosis

Mitosis and Meiosis

In order for cells to divide, the entire DNA sequence must be copied so that each daughter cell can receive a complete complement of DNA. The growth phase of the cell cycle terminates with the separation of the two sister chromatids of each chromosome, and the cell divides during mitosis. Before cell division, the complete DNA sequence is copied by the enzyme DNA polymerase in a process called DNA replication. DNA polymerase is an enzyme capable of the synthesis of new strands of DNA using the exact sequence of the original DNA as a template. Once the DNA is copied, the old and new copies of the chromosomes form their respective pairs, and the cell divides such that one copy of each chromosome pair belongs to each cell (Fig. 1.1.4). Mitotic cell division produces a daughter cell that is an exact replica of the dividing cell.

Meiotic cell division is a special type of cell division that results in a reduction of the genetic material in the daughter cells, which become the reproductive cells—eggs (women) and sperm (men). Meiosis begins with DNA replication, followed by a pairing of the maternal and paternal chromosomes (homologous pairing) and an exchange of genetic material between chromosomes by recombination (Fig. 1.1.5). The homologous chromosome pairs line up on the microtubule spindle and divide such that the maternal and paternal copies of the doubled chromosomes are distributed to separate daughter cells. A second cell division occurs, and the doubled chromosomes divide, which results in daughter cells that have half the genetic material of somatic (tissue) cells.

Meiosis

Gametes are derived from primordial germ cells specific to the ovary and testes. These primordial germ cells have 2n (46) chromosomes (diploid) but give rise to gametes, which have half that number, n (23) chromosomes (haploid). The process leading to this reduction division is termedmeiosis. Meiosis is divided into meiosis I and II. One important distinction is that total DNA goes from 4n to 2n during meiosis I and 2n to n during meiosis II. The configuration of DNA (e.g., tetrad and sister chromatids) represented by chromosomes is also unique (Table 1.1). There are characteristic phases (e.g., prophase, metaphase, anaphase, and telophase) within meiosis I and II. Prophase of meiosis I has five distinct stages (leptotene, zygotene, pachytene, diplotene, and diakinesis). During zygotene, homologous chromosomes (maternal and paternal chromosomes) align at the synaptonemal complex giving way to a bivalent (two homologous chromosomes) tetrad (each chromosome has two sister chromatids). Homologous recombination occurs during pachytene, when sister chromatids of maternal and paternal homologs exchange segments of DNA resulting in genetic variability among offspring from the same parents.

An important distinction between male and female gamete development is the time in life that meiosis is initiated and the time course to completion. In males, this is a short process (approximately 64 days), has its onset at puberty, and is continuous throughout a man's reproductive life. In females, oogenesis begins in utero but stops during prophase I and is dormant by 8 months' gestation (seeTable 1.1). This arrested state occurs during diplotene. The dormant stage, dictyotene, occurs only in oogenesis. Meiosis I resumes at puberty, and each month, another one or more oocytes (a function of follicular recruitment) resumes this reduction division. Meiosis I is completed at the time of ovulation (first polar body is formed), and meiosis II begins but is once again halted, this time during metaphase. Meiosis II is completed only if fertilization occurs (second polar body is formed). Fertilization most often takes place in the fallopian tube.

Suppression of DNA Replication Between Meiosis I and Meiosis II

Meiosis is unique in that it involves two M phases with no intervening S phase. On exit from meiosis I, Cdk1 kinase is reactivated immediately. This blocks assembly of prereplication complexes (see Fig. 42.8), thereby blocking DNA replication. At least two pathways contribute to reactivation of Cdk1.

The first involves downregulation of translation of Wee1 protein kinase in meiosis. Wee1 is a mitotic inhibitor (see Fig. 43.3) that inactivates Cdk1 by phosphorylation at Tyr15. The absence of Wee1 in meiosis I was first observed in Xenopus laevis but this seems to be a universally conserved way of reactivating Cdk1 without an S phase. Ectopic expression of Wee1 in mature X. laevis oocytes prevents reactivation of Cdk1 immediately after the meiosis I division. As a result, the oocytes reenter interphase and replicate their DNA. Meiotic cells also express a specialized isoform of Cdc25, the phosphatase that counteracts Wee1 (see Fig. 43.1).

Meiosis

Somatic cells replicate by mitosis, in which genetically identical daughter cells are formed.Germ cells replicate by meiosis, in which the genetic material is halved to allow reproduction. Meiosis generates genetic diversity, providing a richer source of material on which natural selection can act. Cell replication by mitosis is a precise, well-orchestrated sequence of events involving duplication of the genetic material (chromosomes), breakdown of the nuclear envelope, and equal division of the chromosomes and cytoplasm into daughter cells.The essential difference between mitotic and meiotic replication is that a single DNA duplication step is followed by only one cell division in mitosis but two cell divisions in meiosis (four daughter cells). Consequently, daughter cells contain only half of the chromosome content of parent cells. Thus a diploid (2n) parent cell becomes a haploid(n) gamete. Other major differences between mitosis and meiosis are outlined inTable 64.2. Research has shown that small RNA molecules (small RNAs), including small interfering RNAs (siRNAs), microRNAs (miRNAs), and piwi-interacting RNAs (piRNAs), are important regulators of gene germ cell expression at the post-transcriptional or translation level (He et al., 2009;Tolia and Joshua-Tor, 2007).

Spermatogenesis begins with type B spermatogonia dividing mitotically to form primary spermatocytes within the adluminal compartment.Primary spermatocytes are the first germ cells to undergo meiosis (Kerr and de Kretser, 1981). In this process, a meiotic division is followed by a typical mitotic reduction division, resulting in daughter cells with a haploid chromosome complement. In addition, as a consequence of chromosomal recombination, each daughter cell contains different genetic information. The resultant cell is the Sa spermatid (seeFig. 64.11).

Chromosomal recombination, the defining feature of mammalian meiosis, ensures that haploid gametes differ genetically from their adult precursors and is the real engine of genetic diversity and evolution. During meiotic prophase, formation of a synaptonemal complex with pairing of homologous (maternal and paternal) chromosomes occurs, along with physical interaction and exchange of DNA through reciprocal sites of crossing over(chiasmata) between homologs. Recent research has shown that defects in the fidelity of recombination within human male germ cells can cause azoospermia and male infertility (Walsh et al., 2009).In one study, 10% of nonobstructive azoospermic men had significant defects in recombination compared with men with normal spermatogenesis (Gonsalves et al., 2004). In addition, among men with maturation arrest pattern on testis biopsy, faulty recombination was observed in about half of cases, providing evidence that faulty recombination is linked to poor sperm production (Gonsalves et al., 2004). Variations in recombination also have implications for sperm aneuploidy, because alterations in crossover position are risk factors for chromosomal nondisjunction.Indeed, evidence suggests that the correlation of faulty recombination and sperm aneuploidy in azoospermic men is strong enough to explain the higher rate of chromosomal abnormalities in offspring conceived with in vitro fertilization (IVF) and intracytoplasmic sperm injection (ICSI) (Sun et al., 2008).

Definition

Meiosis is defined as the cellular and nuclear processes that reduce the chromosomal content per nucleus from two sets to one set. In most organisms, two sets of chromosomes (diploid) are reduced to one set (haploid) (see Chromosome Pairing, Synapsis). When the haploid cell becomes involved in the process of fertilization, it is referred to as a ‘gamete.’ If a cell with one set of chromosomes goes on to proliferate, it is called a ‘gametophytic generation.’ This occurs in many fungi, ferns, and, for a few divisions, in plants. Many variations in the meiotic process have evolved that are of particular adaptive value to specific organisms. The products of meiosis in organisms with three or four sets of chromosomes are usually unbalanced because of difficulties in the segregation and assortment of chromosomes. Some of the mechanics of meiosis are presented in the articles on Chiasma, and Synaptonemal Complex.

Meiosis

Meiosis describes the process of cell division for reproductive cells that will result in sperm or egg cells with half the number of chromosomes. This is in contrast to the cell division that takes place in nongerm cells called mitosis, which results in daughter cells that are identical. Figure 1 represents meiosis in females and males. Reproductive cells contain 23 pairs of chromosomes prior to meiosis, with one set maternally derived and the other set paternally derived. During the first stage of meiosis, each homologous pair of chromosomes line up (maternal chromosome 1 with paternal chromosome 1, and so on), duplicate themselves, and then between homologous chromosomes an exchange of DNA segments of equal length at the identical location occurs, called crossing-over or recombination. In the second phase of meiosis the reproductive cell divides twice more to produce four germ cells with half the complement of 23 chromosomes. In females, one reproductive cell undergoing meiosis results in one viable egg cell and three cells that are not functional, whereas in males a single reproductive cell undergoing meiosis results in four sperm cells. The resulting chromosomes in the germ cells, egg and sperm, contain DNA from maternally and paternally derived chromosomes due to crossing-over in the first phase of meiosis, ensuring added genetic variation. The segments of DNA that are exchanged during the crossing-over process is random for any meiotic division, thus it is next to impossible to recreate the exact combination of genes in germ cells produced in a single individual.