In theory, all students learn about cell division at some point in their first exposure to biology, Comparatively few, however, have the chance to learn why the basic task of reproduction must be combined with a means of increasing genetic diversity in order for organisms to have a maximal chance of surviving whatever challenges their environment throws their way.
You perhaps already understand that cell division, in the majority of contexts that term is used, refers simply to a process of duplication: start with one cell, allow time for the growth of whatever is important in each cell, split the cell in half, and you now have double the number you had before.
While this is true of mitosis and binary fission, and does indeed describe the overwhelming majority of cell divisions that occur in nature, it omits meiosis -- both the critical nature of the process and the unusual, highly coordinated microscopic symphony it represents.
Cell Division: Prokaryotes vs Eukaryotes
Prokaryotes: All life on Earth can be divided into prokaryotes, which includes the Bacteria and the Archaea, almost all of which are single-celled organisms. All cells have a cell membrane, cytoplasm and genetic material in the form of DNA (deoxyribonucleic acid)
Prokaryotic cells, however, lack organelles, or specialized membrane-bound structures within the cytoplasm; they therefore have no nucleus, and the DNA of a prokaryote usually exists as a small, ring-shaped chromosome sitting in the cytoplasm. Prokaryotic cells reproduce themselves, and hence the whole organism in most cases, by simply growing larger, duplicating their one chromosome and splitting into two identical daughter nuclei.
Eukaryotes: Most eukaryotic cells divide in a manner similar to binary fission, except that eukaryotes have their DNA allotted among a higher number of chromosomes (humans have 46, with 23 inherited from each parent). This everyday type of division is called mitosis, and, like binary fission, it produces two identical daughter cells.
Meiosis combined the mathematical practicality of mitosis with the coordinated chromosome shake-ups required to generate genetic diversity in subsequent generations, as you'll soon see.
The genetic material of eukaryotic cells exists in these cells' nuclei in the form of a substance called chromatin, which consists of DNA combined with a protein called histones that allow for supercoiling and very dense compaction of the DNA. This chromatin is divided into discrete chunks, and these chunks are what molecular biologists call chromosomes.
Only when a cell is actively dividing are its chromosomes easily visible even under a powerful microscope. At the onset of mitosis, each chromosome exists in a replicated form, since replication must follow every division to preserve chromosome number. This gives these chromosomes the appearance of an "X," because the identical single chromosomes, known as sister chromatids, are joined at a point called the centromere.
As noted, you get 23 chromosomes from each parent; 22 are autosomes numbered 1 through 22, while the remaining one is a sex chromosome (X or Y). Females have two X chromosomes, while males have an X and a Y. "Matching" chromosomes from the mother and the father can be determined using their physical appearance.
The chromosomes that make up these sets of two (e.g., chromosome 8 from the mother and chromosome 8 from the father) are called homologous chromosomes, or simply homologs.
Recognize the difference between sister chromatids, which are individual chromosome molecules in replicated (duplicated) set, and homologs, which are pairs in a matched but not non-identical set.
The Cell Cycle
Cells begin their lives in interphase, during which the cells grow larger, replicate their chromosomes to create 92 total chromatids from 46 individual chromosomes and check their work. The subphases that correspond to each of these interphase processes are called G1 (first gap), S (synthesis) and G2 (second gap).
Most cells then enter mitosis, also known as the M phase; here, the nucleus divides in a series of four steps, but certain germ cells in the gonads that are destined to become gametes, or sex cells, enter meiosis instead.
Meiosis: Basic Overview
Meiosis has the same four steps as mitosis (prophase, metaphase, anaphase and telophase) but includes two successive divisions that result in four daughter cells instead of two, each with 23 chromosomes instead of 46. This is enabled by the markedly different mechanics of meiosis 1 and meiosis 2.
The two events that set meiosis apart from mitosis are known as crossing over (or genetic recombination) and independent assortment. These occur in prophase and metaphase of meiosis 1, as described below.
Steps of Meiosis
Rather than merely memorize the names of the phases of meiosis 1 and 2, it is helpful to gain enough of an understanding of the process apart from it specific labels to appreciate both its similarities to everyday cell division and what makes meiosis unique.
The first decisive, diversity-promoting step in meiosis is the pairing up of homologous chromosomes. That is, the duplicated chromosome 1 from the mother pairs with the duplicated chromosome 1 from the father, and so on. These are called bivalents.
The "arms" of the homologs trade small bits of DNA (crossing over). The homologs then separate, and the bivalents line up along the middle of the cell randomly, so that the maternal copy of a given homolog is as likely to wind up on a given side of the cell as the paternal copy.
The cell then divides, but between the homologs, not through the centromeres of either duplicated chromosome; the second meiotic division, which is really just a mitotic division, is when this occurs.
Phases of Meiosis
Prophase 1: Chromosomes condense, and the spindle apparatus forms; homologs line up side by side to form bivalents and exchange bits of DNA (crossing over).
Metaphase 1: The bivalents align randomly along the metaphase plate. Because in humans there are 23 paired chromosomes, the number of possible arrangements in this process is 223, or almost 8.4 million.
Anaphase 1: The homologs are pulled apart, producing two daughter chromosome sets that are not identical because of crossing over. Each chromosome still consists of chromatids with all 23 centromeres in each nucleus intact.
Telophase 1: The cell divides.
Mitosis 2 is simply a mitotic division with the steps labeled accordingly (prophase 2, metaphase 2, etc), and serves to separate the not-quite-sister chromatids into distinct cells. The end result is four daughter nuclei that contain a unique blend of slightly altered parental chromosomes, with a total of 23 chromosomes.
This is required because these gametes fuse with other gametes in the process of fertilization (sperm plus egg), bringing the chromosome number back of to 46 and giving each chromosome a fresh homolog.
Chromosome Accounting in Meiosis
A meiosis diagram for humans would show the following information:
Beginning of meiosis 1: 92 individual chromosome molecules (chromatids) in one cell, arranged in 46 duplicated chromosomes (sister chromatids); the same as in mitosis.
End of prophase 1: 92 molecules in one cell arranged in 23 bivalents (duplicated homologous chromosome pairs), which each contain four chromatids in two pairs.
End of anaphase 1: 92 molecules split into in two non-identical (thanks to independent assortment) daughter nuclei, each with 23 similar but non-identical (thanks to crossing over) chromatid pairs.
Beginning of meiosis 2: 92 molecules split into in two non-identical daughter cells, each with 23 similar but non-identical chromatid pairs.
End of anaphase 2: 92 molecules split into four mutually non-identical daughter nuclei, each with 23 chromatids.
End of meiosis 2: 92 molecules split into four mutually non-identical daughter cells, each with 23 chromatids. These are gametes, and are called spermatozoa (sperm cells) if produced in the male gonads (testes) and ova (egg cells) if produced in the female gonads (ovaries).