Meiosis: Cellular Processes and Life Cycles
Cellular Process
Overview of meiosis. The interphase is duplicated genetic material, and the phenomenon of recombination (represented by red and blue chromosomes). In meiosis I, the homologous chromosomes are divided into two daughter cells. In meiosis II, as in mitosis, each chromatid migrates to one pole. The result is four haploid daughter cells (n).
The preparatory steps that lead to meiosis are identical in pattern and name to the interphase of the mitotic cell cycle. The interphase is divided into three phases:
- G1 Phase: Characterized by increased cell size due to the accelerated manufacture of organelles, proteins, and other cellular materials.
- S Phase (Synthesis): Replicates the genetic material, i.e., the DNA is replicated, giving rise to two new chains joined by the centromere. Chromosomes, which until now had only one chromatid, now have two. 98% of the DNA is replicated; the remaining 2% is not replicated.
- G2 Phase: The cell continues to increase its biomass.
The interphase is immediately followed by meiosis I and II. Meiosis I consists of the segregation of each of the homologous chromosomes, then the diploid cell divides into two diploid cells but with half the chromosomes. Meiosis II consists of unpairing each of the chromatids of the chromosome, which are segregated one to each pole, so that after division, it produces four haploid cells. Meiosis I and II are divided into prophase, metaphase, anaphase, and telophase, similar in purpose to their analogous subphases in the mitotic cell cycle. Therefore, meiosis encompasses the interphase (G1, S, G2), meiosis I (prophase I, metaphase I, anaphase I, telophase I), and meiosis II (prophase II, metaphase II, anaphase II, telophase II).
Meiosis and Life Cycle
Sexual reproduction is characterized by the fusion of two sex cells (haploids) to form a zygote (diploid). Therefore, in a sexual life cycle, meiosis must occur before the gametes can be replicated.
In animals and a few other organisms, meiosis immediately precedes the formation of gametes. Somatic body cells of an individual organism multiply by mitosis and are diploid; the only haploid cells are gametes. These are formed when some germinal line cells undergo meiosis. The formation of gametes is called gametogenesis. Male gametogenesis, called spermatogenesis, results in the formation of four haploid sperm per cell that enters meiosis. In contrast, female gametogenesis, called oogenesis, produces one egg per cell that enters meiosis. This is done by a process that assigns virtually all the cytoplasm to one of two nuclei in each meiotic division. At the end of the first meiotic division, one nucleus is retained; the other, called the first polar body, is excluded from the cell and eventually degenerates. In general, at the end of the second division, one nucleus becomes the second polar body, and the other nucleus survives. Thus, a haploid nucleus becomes the recipient of most of the cytoplasm and accumulated nutrients of the original meiotic cell.
However, while meiosis takes place somewhere in sexual life cycles, it does not always directly precede the formation of gametes. Many simple eukaryotes (including some fungi and algae) remain haploid (their cells divide by mitosis) most of their lives, and individuals can be unicellular or multicellular.
Two haploid gametes (produced by mitosis) are merged to form a diploid zygote, which undergoes meiosis to return to the haploid state.
The more complex life cycles are found in plants and some algae. These cycles, which are characterized by alternation of generations, consist of a multicellular diploid stage, called the sporophyte generation, and a multicellular haploid stage, called the gametophyte generation. Spore-bearing plant diploid cells undergo meiosis to form haploid spores, each of which is divided in mitotic form to produce a multicellular haploid gametophyte. The gametophytes produce gametes by mitosis. The male and female gametes (egg and sperm) then fuse to form a diploid zygote, which is split mitotically to produce a multicellular diploid sporophyte.