Metaphase is the third of the five phases of biological cell division, or more specifically, the division of what is inside that cell's nucleus. In most instances, this division is mitosis, which is the means by which living cells duplicate their genetic material (DNA, or deoxyribonucleic acid, in all life on Earth) and split into two identical daughter cells. The other phases are, in order, prophase, prometaphase (this part is omitted from many sources), anaphase and telophase. Mitosis is in turn one part of the overall cell life cycle, most of which is spent in interphase. Metaphase might best be conceived of as a step in which the elements of the soon-to-divide cell arrange themselves into a neat formation, like a tiny military platoon.
Most cells of the body are somatic cells, meaning that they do not play a role in reproduction. Almost all of these cells undergo mitosis, supplying new cells for growth, tissue repair and other day-to-day needs. On the other hand, gametes, also called germ cells, arise from a process of cell division called meiosis, which is divided into meiosis I and meiosis II. Each of these in turn includes its own metaphase, appropriately named metaphase I and metaphase II. (Tip: When you see any of the phases of cell division followed by a number, your source is describing meiosis rather than mitosis.)
DNA and the Basis of Genetics
Before discussing the specifics about a particular step in the division of a cell's genetic material, it is useful to step back and review what takes place inside cells to even reach this point.
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DNA is one of two nucleic acids, the other being ribonucleic acid (RNA). Although DNA might be considered the more fundamental of the two, DNA is used as the template for making RNA. On the other hand, RNA is more versatile and comes in a number of subtypes. Nucleic acids consist of long monomers (repeated elements identical in structure) of nucleotides, each of which includes three elements: a five-carbon sugar in ring form, a phosphate group and a nitrogen-rich base.
These nucleic acids differ in three key ways: DNA is double-stranded, while RNA is single-stranded; DNA contains the sugar deoxyribose, whereas RNA contains ribose; and while each DNA nucleotide has as its nitrogenous base either adenine (A), cytosine (C), guanine (G) or thymine (T), in RNA, uracil (U) takes the place of thymine. It is this variation in bases between nucleotides that produces the differences between individuals, and also what permits the genetic "code" put to use by all organisms. Every three-nucleotide base sequence holds the code for one of 20 amino acids, and amino acids are assembled elsewhere in the cell into proteins. Every strip of DNA that includes all of the code needed for a single unique protein product is called a gene.
Overview of Chromosomes and Chromatin
DNA in cells exists in the form of chromatin, which is a long, linear substance consisting of about one-third DNA and two-thirds protein molecules called histones. These proteins serve the vital function of compelling DNA to coil and twist in on itself to such a remarkable extent that a single copy of all of your DNA in each cell, which would reach 2 meters in length if stretched end to end, can be squeezed into a space only one- or two-millionths of a meter wide. Histones exist as octamers, or groups of eight subunits. The DNA winds its way around each histone octamer in the manner of thread wrapping around a spool approximately twice. Under a microscope, this gives chromatin a beady appearance, with "naked" DNA alternating with DNA enclosing histone cores. Each histone and the DNA around it makes up a structure called a nucleosome.
Chromosomes are nothing more than distinct pieces of chromatin. Humans have 23 different chromosomes, 22 that are numbered and one that is a sex chromosome, either X or Y. Every somatic cell in your body contains a pair of each chromosome, one from your mother and one from your father. Paired chromosomes (e.g., chromosome 8 from your mother and chromosome 8 from your father) are called homologous chromosomes, or homologs. These look very much identical under a microscope, but differ greatly in terms of their nucleotide sequences.
When chromosomes replicate, or make copies of themselves in preparation for mitosis, the template chromosome remains joined to the new chromosome at a point called a centromere. The two identical joined chromosomes are called chromatids. Chromosomes are usually asymmetrical along their long axis, meaning that there is more material on one side of the centromere than on the other. The shorter segments of each chromatid are called p-arms, while the longer pair's are called q-arms.
The Cell Cycle and Cell Division
Prokaryotes, most of which are bacteria, replicate their cells via a process called binary fission, which resembles mitosis but is considerably simpler owing to the less complex structure of bacterial DNA and cells. All eukaryotes, on the other hand – plants, animals and fungi – undergo both mitosis and meiosis.
A newly made eukaryotic cell begins a life cycle that includes the following phases: G1 (first gap phase), S (synthetic phase), G2 (second gap phase) and mitosis. In G1, the cell makes duplicates of every component of the cell with the exception of the chromosomes. In S, which takes about 10 to 12 hours and consumes roughly half of the life cycle in mammals, all of the chromosomes replicate, forming the sister chromatids as described above. In G2, the cell essentially checks its work, scanning its DNA for errors resulting from replication. The cell then enters mitosis. Clearly, the main function of every cell is to replicate precise copies of itself, genetic material especially, and this moves the whole organism toward both survival maintenance and reproduction.
When chromosomes are not actively dividing, they exist as loosened forms of themselves, becoming diffuse, rather like tiny hairballs. Only at the onset of mitosis do they condense into the shapes familiar to anyone who as looked at a micrograph of the interior of a cell nucleus taken during cell division.
Summary of Mitosis
The G1, S and G2 phases are collectively termed interphase. The rest of the cell cycle is concerned with cell division – mitosis in somatic cells, meiosis in the specialized cells of gonads. The stages of mitosis and meiosis are collectively called the M phase, potentially introducing confusion.
In any case, in the prophase portion of mitosis, which is the longest of the five mitotic stages, the nuclear envelope disintegrates and the nucleolus within the nucleus disappears. A structure called the centrosome divides, and the two resulting centrosomes move to opposite sides of the cell, in a line perpendicular to that along which the nucleus and cell will soon divide. The centrosomes extend protein structures called microtubules toward the chromosomes that have condensed and are aligning near the middle of the cell; these microtubules collectively form the mitotic spindle.
In prometaphase, the chromosomes line up through their centromeres along the line of division, also called the metaphase plate. The microtubule spindle fibers connect to the centromeres at a location called the kinetochore.
Following metaphase proper (discussed in detail shortly) is anaphase. This is the shortest phase, and in it, the sister chromatids are pulled apart by the spindle fibers at their centromeres and drawn toward the oppositely positioned centrosomes. This results in the formation of daughter chromosomes. These are indistinguishable from sister chromatids aside from no longer being joined by the centromere.
Finally, in telophase, a nuclear membrane forms around each of the two new aggregations of DNA (which, remember, consists of 46 single daughter chromosomes per forming cell). This completes nuclear division, and the cell itself then divides in a process called cytokinesis.
Summary of Meiosis
Meiosis in humans occurs in specialized cells of the testes in men and the ovaries in women. Whereas mitosis creates cells identical to the original to replace dead cells or contribute to the growth of the whole organism, meiosis generates cells called gametes designed to fuse with gametes from the opposite sex for the purpose of creating offspring. This process is called fertilization.
Meiosis is divided into meiosis I and meiosis II. Like mitosis, the onset of meiosis I is preceded by all 46 of a cell's chromosomes replicating. In meiosis, however, after the nuclear membrane is dissolved in prophase, the homologous chromosomes pair off, side by side, with the homolog derived from the organism's father on one side of the metaphase plate and that from the mother on the other. Importantly, this assortment about the metaphase plate occurs independently – that is, 7 male-supplied homologs could wind up on one side and 16 female-supplied homologs on the other, or any other combination of numbers adding up to 23. In addition, the arms of the homologs now in contact trade material. These two processes, independent assortment and recombination, assure diversity in offspring and hence in the species as a whole.
When the cell divides, each daughter cell has one replicated copy of all 23 chromosomes, rather than the daughter chromosomes created in mitosis. Meiosis I, then, does not involve pulling chromosomes apart at their centromeres; all 46 centromeres remain intact at the onset of meiosis II.
Meiosis II is essentially a mitotic division, as each of the daughter cells from meiosis I splits in a way that sees sister chromatids migrate to opposite sides of the cell. The result of both parts of meiosis is four daughter cells in two different identical pairs, each with 23 single chromosomes. This allows for the preservation of the "magic" number 46 when male gametes and female gametes fuse.
Metaphase in Mitosis
At the onset of metaphase in mitosis, the 46 chromosomes are more or less lined up with one another, with their centromeres forming a fairly straight line from the top of the cell to the bottom (taking the positions of the centrosomes to be the left and right sides). "More or less" and "fairly," however, are not precise enough for the symphony of biological cell division. Only if the line through the centromeres is exactly straight will the chromosomes divide precisely into two identical sets, thereby creating identical nuclei. This is accomplished by opposing microtubules of the spindle apparatus playing a sort of tug-of-war contest, until each is applying sufficient tension to hold in place the specific chromosome each microtubule is handling. This does not happen for all 46 chromosomes at once; the ones fixed early oscillate slightly around their centromere until the last one falls into line, setting the table for anaphase.
Metaphase I and II in Meiosis
In metaphase I of meiosis, the dividing line runs between paired homologous chromosomes, not through them. At the end of metaphase, however, two other straight lines can be visualized, one passing through the 23 centromeres on one side of the metaphase plate and one passing through the 23 centromeres on the other.
Metaphase II resembles the metaphase of mitosis, except that each participating cell has 23 chromosomes with non-identical chromatids (thanks to recombination) rather than 46 with identical chromatids. After these non-identical chromatids are properly lined up, anaphase II follows to pull them to opposite ends of the daughter nuclei.