Although meiosis is inextricably entwined with sexual reproduction and its advantages and disadvantages, it is important to separate the questions of the evolution of meiosis and the evolution of sex, because early meiosis may have been advantageous for different reasons than it is now. Thinking outside the box and imagining what the early benefits from meiosis might have been is one approach to uncovering how it may have evolved.
Meiosis and mitosis share obvious cellular processes and it makes sense that meiosis evolved from mitosis. The difficulty lies in the clear differences between meiosis I and mitosis. Adam Wilkins and Robin Holliday 1 summarized the unique events that needed to occur for the evolution of meiosis from mitosis. These steps are homologous chromosome pairing, crossover exchanges, sister chromatids remaining attached during anaphase, and suppression of DNA replication in interphase.
They argue that the first step is the hardest and most important, and that understanding how it evolved would make the evolutionary process clearer. They suggest genetic experiments that might shed light on the evolution of synapsis.
There are other approaches to understanding the evolution of meiosis in progress. Different forms of meiosis exist in single-celled protists. Comparing the meiotic divisions of different protists may shed light on the evolution of meiosis.
Marilee Ramesh and colleagues 2 compared the genes involved in meiosis in protists to understand when and where meiosis might have evolved. Although research is still ongoing, recent scholarship into meiosis in protists suggests that some aspects of meiosis may have evolved later than others. This kind of genetic comparison can tell us what aspects of meiosis are the oldest and what cellular processes they may have borrowed from in earlier cells.
Sexual reproduction requires that diploid organisms produce haploid cells that can fuse during fertilization to form diploid offspring. As with mitosis, DNA replication occurs prior to meiosis during the S-phase of the cell cycle. Meiosis is a series of events that arrange and separate chromosomes and chromatids into daughter cells.
During the interphases of meiosis, each chromosome is duplicated. In meiosis, there are two rounds of nuclear division resulting in four nuclei and usually four daughter cells, each with half the number of chromosomes as the parent cell. The first separates homologs, and the second—like mitosis—separates chromatids into individual chromosomes. During meiosis, variation in the daughter nuclei is introduced because of crossover in prophase I and random alignment of tetrads at metaphase I.
The cells that are produced by meiosis are genetically unique. Meiosis and mitosis share similarities, but have distinct outcomes. Mitotic divisions are single nuclear divisions that produce daughter nuclei that are genetically identical and have the same number of chromosome sets as the original cell.
Meiotic divisions include two nuclear divisions that produce four daughter nuclei that are genetically different and have one chromosome set instead of the two sets of chromosomes in the parent cell.
The main differences between the processes occur in the first division of meiosis, in which homologous chromosomes are paired and exchange non-sister chromatid segments. The homologous chromosomes separate into different nuclei during meiosis I, causing a reduction of ploidy level in the first division. The second division of meiosis is more similar to a mitotic division, except that the daughter cells do not contain identical genomes because of crossover. Skills to Develop Describe the behavior of chromosomes during meiosis Describe cellular events during meiosis Explain the differences between meiosis and mitosis Explain the mechanisms within meiosis that generate genetic variation among the products of meiosis.
Meiosis I Meiosis is preceded by an interphase consisting of the G 1 , S, and G 2 phases, which are nearly identical to the phases preceding mitosis.
Prophase I Early in prophase I, before the chromosomes can be seen clearly microscopically, the homologous chromosomes are attached at their tips to the nuclear envelope by proteins. The chromosomes are bound tightly together and in perfect alignment by a protein lattice called a synaptonemal complex and by cohesin proteins at the centromere. The result is an exchange of genetic material between homologous chromosomes. Prometaphase I The key event in prometaphase I is the attachment of the spindle fiber microtubules to the kinetochore proteins at the centromeres.
Metaphase I During metaphase I, the homologous chromosomes are arranged in the center of the cell with the kinetochores facing opposite poles. In this case, there are two possible arrangements at the equatorial plane in metaphase I. The total possible number of different gametes is 2 n , where n equals the number of chromosomes in a set.
In this example, there are four possible genetic combinations for the gametes. Anaphase I In anaphase I, the microtubules pull the linked chromosomes apart. Telophase I and Cytokinesis In telophase, the separated chromosomes arrive at opposite poles. Link to Learning Review the process of meiosis, observing how chromosomes align and migrate, at Meiosis: An Interactive Animation. Meiosis II In some species, cells enter a brief interphase, or interkinesis , before entering meiosis II. Prophase II If the chromosomes decondensed in telophase I, they condense again.
Prometaphase II The nuclear envelopes are completely broken down, and the spindle is fully formed. Metaphase II The sister chromatids are maximally condensed and aligned at the equator of the cell.
Anaphase II The sister chromatids are pulled apart by the kinetochore microtubules and move toward opposite poles. In prometaphase I, microtubules attach to the fused kinetochores of homologous chromosomes, and the homologous chromosomes are arranged at the midpoint of the cell in metaphase I. In anaphase I, the homologous chromosomes are separated. In prometaphase II, microtubules attach to the kinetochores of sister chromatids, and the sister chromatids are arranged at the midpoint of the cells in metaphase II.
In anaphase II, the sister chromatids are separated. In telophase I, the separated chromosomes arrive at opposite poles. In some organisms, the chromosomes decondense and nuclear envelopes form around the chromatids in telophase I.
Then cytokinesis, the physical separation of the cytoplasmic components into two daughter cells, occurs without reformation of the nuclei. In nearly all species of animals and some fungi, cytokinesis separates the cell contents via a cleavage furrow constriction of the actin ring that leads to cytoplasmic division.
In plants, a cell plate is formed during cell cytokinesis by Golgi vesicles fusing at the metaphase plate. This cell plate will ultimately lead to the formation of cell walls that separate the two daughter cells. Two haploid cells are the end result of the first meiotic division. The cells are haploid because at each pole there is just one of each pair of the homologous chromosomes.
Therefore, only one full set of the chromosomes is present. Although there is only one chromosome set, each homolog still consists of two sister chromatids.
Learning Objectives Describe the stages and results of meiosis I. During prophase I, the homologous chromosomes condense and become visible as the x shape we know, pair up to form a tetrad, and exchange genetic material by crossing over. In metaphase I, the tetrads line themselves up at the metaphase plate and homologous pairs orient themselves randomly.
In anaphase I, centromeres break down and homologous chromosomes separate. In telophase I, chromosomes move to opposite poles; during cytokinesis the cell separates into two haploid cells. Meiosis is a type of cell division that reduces the number of chromosomes in a parent cell by half to produce four reproductive cells called gametes.
In humans, diploid cells contain 46 chromosomes, with 23 chromosomes inherited from the mother and a second similar set of 23 chromosomes inherited from the father. Pairs of similar chromosomes are called homologous chromosomes. During meiosis, the pairs of homologous chromosome are divided in half to form haploid cells, and this separation, or assortment, of homologous chromosomes is random.
This means that all of the maternal chromosomes will not be separated into one cell, while the all paternal chromosomes are separated into another. Instead, after meiosis occurs, each haploid cell contains a mixture of genes from the organism's mother and father.
As in the case of mitosis, the chromosomes replicate during an interphase resting period when they are an undifferentiated mass. At the beginning of the first stage of meiosis, prophase I , the chromosomes contract and become visible. These doubled chromosomes then come together in homologous pairs. Breakage of the ends of chromatids usually occurs at this time, followed by crossing-over , which results in chromosomes with new combinations of genes.
During metaphase I , all of the doubled homologous chromosome pairs line up along the midline of the cell between the two centrioles. M etaphase I 1st Division Reduction Division. During anaphase I , the homologous chromosome pairs separate and are pulled to opposite poles of the cell by spindle fibers attached to the centrioles.
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