Sometime in high school, biology students learn about cell division, and one of the first things most of them are taught is that cell division assumes two basic forms, called mitosis and meiosis. The former is typically referred to as non-sexual reproduction of cells, while the latter is framed as a necessary component of sexual reproduction.
While these characterizations are accurate, many students are only just getting a handle on the essential concepts and the critical differences between mitosis and meiosis when the science course marches on to the next topic. The two kinds of cell division have just enough overlap to make keeping them clear in your head somewhat cumbersome. But given the right kind of attention, not only is understanding these processes not so daunting after all, it can also be downright fun.
What Are Cells?
Cells are the smallest, simplest objects known that contain all of the properties associated with life itself. These properties can be slotted into five basic capabilities:
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- Detecting and responding to changes in their environment.
- Physical growth and maturation.
- Maintenance of homeostasis, a constant internal environment.
- A complex chemistry.
Despite the vast "macro" differences in appearance between organisms, at the "micro" level, things are far more similar. A human cell, for example, does not look terribly different from a plant cell, as both of them have nuclei, cytoplasm and well-defined boundaries.
Prokaryotes vs. Eukaryotes
Prokaryotes, which include bacteria and a domain of similarly uncomplicated organisms called archaea, are almost all unicellular, do not reproduce sexually and essentially divide by growing larger and splitting in half, a process called binary fission.
Eukaryotes, which include all other living things (i.e., animals, plants and fungi), are virtually all multicellular – your own body has over 30 trillion cells – and reproduce sexually, that is, by combining the genetic material of two parent organisms. Their complexity requires that mitosis and cytokinesis replace the role of binary fission, and sexual reproduction hinges on the diversity and preservation of chromosome number guaranteed by meiosis.
The Cell Cycle
Eukaryotic cells undergo a cell cycle that describes the arc of their short life spans, which vary widely but are typically on the order of hours to a day or so.
Interphase refers to the period immediately after a daughter cell arises from a mitotic cell division, when the cell is already preparing for its next division but is not yet ready to divide in two. It includes the G1, S and G2 phases. In G1, (first gap phase), the cell enlarges and replicates its contents except for its chromosomes, which contain the organism's DNA, or genetic material. In S (synthesis phase), the cell replicates all of its chromosomes. In G2 (second gap phase), the cell assembles the structures it will need for mitosis and checks its previous work for errors.
Interphase is followed by the M phase, another term for mitosis, which itself has five phases, described in a subsequent section. Here, the cell's nucleus divides in two, separating the replicated chromosomes into two identical daughter nuclei. Immediately after the M phase, the cell undergoes cytokinesis, the division of the cell as a whole into a pair of daughter cells.
A eukaryotic organism's DNA is packaged into chromatin, which is a blend of DNA and support proteins called histones. This chromatin is divided into discrete chromosomes, with the number varying between species; humans have 46. These consist of 23 paired homologous chromosomes, one from each parent. 22 of these are autosomes, numbered 1 through 22, while the other is a sex chromosome, either X or Y.
Chromosome 1 from your mother looks exactly like chromosome 1 from your father on gross microscopic examination, and so on for the other 21 numbered autosomes. The sequence of nucleotides that make up a DNA strand, however, are not the same in homologous chromosomes.
Females have inherited an X chromosome from each parent, whereas males have received an X from their mother and a Y from their father. The unique process of meiosis 1 (the first half of meiosis) is the step where which sex chromosome will be passed on is determined, as detailed in a later section.
Mitosis vs. Meiosis
The ability to properly describe the stages of cell division is essential not only to keeping them apart, but to gaining an understanding of biology in general.
Mitosis is a straightforward replication of the contents of a nucleus. It is analogous to binary fission in prokaryotes. Mitosis and meiosis start in the same place: With 46 duplicated chromosomes for a total of 92 individual chromosomes. After chromosomes replicate in the S phase of the cell cycle, replicated chromosomes remain attached at a junction called the centromere, and these identical molecules are called sister chromatids.
- At this stage, homologous chromosomes, or simply homologs, have no physical association with each other. Be careful to distinguish between sister chromatids and homologous chromosomes.
The Phases of Mitosis
The five phases of mitosis are prophase, prometaphase, metaphase, anaphase and telophase.
- Prophase: In this step, the nuclear membrane dissolves, individual chromosomes become condensed in the nucleus, and the mitotic spindle, which ultimately pulls sister chromatids apart, begins to form on opposite poles, or sides, of the cell.
- Prometaphase: Here, chromosomes begin to migrate to the center of the cell.
- Metaphase: Chromosomes arrange themselves in a line through the midline of the cell (the metaphase plate), perpendicular to the spindles at the poles. One sister chromatid lies on each side of this plate.
- Anaphase: Sister chromatids are pulled apart and toward the poles by the mitotic spindle fibers, creating identical daughter nuclei.
- Telophase: This phase is in many ways reversal of prophase; new nuclear membranes form around the new daughter nuclei, and chromosomes begin to become more diffuse.
Mitosis is immediately followed by cytokinesis, and each daughter cell begins a new cell cycle.
The Two Stages of Meiosis
Meiosis is a rare event in terms of the overall number of cell divisions in the body, and occurs only in cells of the gonads (testes in males, ovaries in females). The entire process includes two cell divisions, called meiosis 1 and meiosis 2, that create four non-identical daughter cells, each with only 23 chromosomes, called gametes, or sex cells (sperm in males and eggs in females).
Each meiotic division has substages corresponding to those seen in mitosis.
In prophase of meiosis 1 (i.e., prophase 1), the replicated homologous chromosomes find each other in the nucleus and join each other side by side, forming bivalents, or tetrads. In a process called recombination or crossing over, the male-derived and female-derived homologs exchange portions of DNA with each other.
In metaphase 1, the bivalents line up along the metaphase plate, as in mitosis. However, whether the male-derived or the female-derived part of the tetrad winds up on a given side of the plate is completely random, meaning that when the cell goes on to divide in two during anaphase 1, the number of possible combinations of daughter cells produced is 223, or almost 8.4 million.
The daughter cells of meiosis 1 are clearly not identical, and they consist of paired chromatids, since the division line of meiosis 1 runs between the homologs, not through either of the centromeres present on either side. The chromatids are closely related, but they have been altered by recombination.
The 23 paired chromatids of each non-identical daughter cell then each undergo a division that creates two daughter cells, now called gametes, with a single copy of all 23 tricked-up, intentionally flipped-around chromosomes.
- The sperm that happen to land a Y chromosome go on to produce a male offspring if they wind up fusing with an egg cell in fertilization, while those containing an X can only contribute to a future daughter, since all egg cells contain an X chromosome.
A Final Note on Meiosis and Genetic Diversity
To avoid undue confusion about meiosis, which is often an admittedly tricky concept for most students, it is useful to step back and realize that meiosis 2 is simply a mitotic division. All of the processes of recombination and independent assortment in meiosis represent a one-two punch that forms the entire basis for the unique features of this form of cell division, and for the vast genetic diversity observed in eukaryotes.