Whether or not you have a background in molecular biology or any branch of life science, you've almost certainly heard the term "DNA" in some context, be it a police drama (or actual court trial), a casual discussion about inheritance or a reference to the basic microscopic "stuff" that makes each of us structurally unique. If you've heard of deoxyribonucleic acid (DNA), the name given to the molecule that stores genetic information in all living things, then you're at least indirectly familiar with the idea of chromosomes. These, with a few minor caveats, are the result of divvying up the complete copy of DNA in the nucleus of every cell in your body into 23 parts. A complete copy of an organism's genetic code is called its genome, and chromosomes are the individual puzzle pieces of that genome. Humans have 23 distinct chromosomes, while other species have more or fewer; bacteria possess only a single, circular chromosome.
Chromosomes have a structural hierarchy that relates to the life-cycle phase of the cell in which those chromosomes sit. Virtually all cells have the capacity to divide into two daughter cells, a process that is required for growth of the whole organism, repairing damaged tissues and replacing old, worn-out cells. As a rule, when a cell in your body divides, it makes two genetically identical copies of itself; when chromosomes make copies of themselves to prepare for this division, the result is a pair of identical chromatids.
DNA: The Root of It All
DNA is one of two nucleic acids in nature, the other being RNA (ribonucleic acid). DNA is the genetic material of every living thing on Earth. Bacteria, which account for virtually every species in the world of eukaryotes, have a relatively small amount of of DNA arranged in a single circular chromosome (more on these to come). In contrast, eukaryotes such as plants, animals and fungi have a far greater complement of DNA, as befits complex, multicellular organisms; this is broken into a large number of chromosomes (humans have 23 pairs per cell).
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DNA consists of monomeric units called nucleotides. Each of these in turn consists of a five-carbon sugar bonded to a phosphate group at one carbon and a nitrogen-heavy base at a different carbon. This base can be either adenine (A), cytosine (C), guanine (G) or thymine (T), and it is this variation that allows DNA to vary from person to person and along the same DNA molecule. Every nucleotide "triplet" (e.g., A-A-A, A-A-C, etc.) in a strand of DNA codes for one of 20 amino acids, the monomers that make up all proteins. All of the sequential nucleotides that code for a single protein product is called a gene. DNA actually delivers its codes to the protein-manufacturing machinery of a cell via messenger RNA (mRNA).
Regarding DNA's relationship to chromatids and chromosomes, DNA is, rather notoriously in science circles, double-stranded and helical in shape, like the side of a spiral staircase. When DNA is being replicated (that is, copied) or transcribed into mRNA, local portions of the molecule unwind to allow the enzyme proteins that assist in these processes room to wander in and do their jobs. DNA, in life, is found in the nuclei of eukaryotic cells, and in the cytoplasm of bacteria, in the form of chromatin.
Chromatin is an unequal mixture of DNA and proteins, with the protein component making up about two-thirds of the mass of the structure. While DNA is the direct carrier of coded information for making proteins (and whole organisms), without the proteins, called histones, it could not possibly exist in the compressed form it must exist in to fit inside a cell nucleus. To offer an idea of just how compressed your DNA is, the complete copy that sits inside every one of your cells would reach 2 meters (roughly 6 feet) if stretched end to end, yet each cell is on the order of one- or two-millionths of a meter across.
Adding mass to save space may seem counterintuitive, but without the positively charged histone proteins, or something much like them, binding to (and thus largely controlling the arrangement of) the negatively charged DNA molecule, the DNA would have no physical impetus to compress itself. Histones in chromatin exist as eight-molecule entities consisting of four pairs of subunits. The DNA molecule winds its way around each one of these histones approximately twice, like thread around a spool creating a structure called a nucleosome. These nucleosomes in turn form stacks, like rolls of pennies; these stacks themselves form ring structures, and so on.
Chromatin is found in a relatively relaxed (though still very much looped and coiled) state when cells are not dividing. This allows processes such as replication and mRNA transcription to occur more easily. This loosened form of chromatin is called euchromatin. Chromatin that is condensed and resembles the material seen in micrographs of cell division is known as heterochromatin.
It simplifies the distinction between chromatin and chromosomes to know that chromosomes are nothing more than the body's chromatin divided into distinct physical structures. Each chromosome includes one long DNA molecule along with all of the histones required to package and compact it.
Your own chromatin is divided into 23 chromosomes, 22 of which are numbered chromosomes (autosomes, or somatic chromosomes) and the remaining one being a sex chromosome, either X or Y. Most cells (gametes are the exception) contain two copies of each chromosome, one from the mother and one from the father. While the sequence of bases on the nucleotides of your father's chromosomes differs from that of your mother's chromosomes, chromosomes with the same number look virtually identical under a microscope. Modern analytical methods make formally distinguishing chromosomes from each other a fairly straightforward exercise, but even a basic visual examination allows for a considerable level of identification if the eyes are expert ones.
When your chromosomes replicate between cell divisions – that is, when each DNA molecule and the histones that bind to that molecule make complete copies of themselves – the result is two identical chromosomes. These chromosomes remain physically linked at a point of highly condensed chromatin called a centromere, and the two identical chromosomes it joins are referred to as chromatids (often, sister chromatids). The centromere is the same distance from corresponding ends of sister chromatids, meaning that the nitrogenous bases in the DNA joined to either side of the centromere are the same. However, the centromere, its name notwithstanding, need not be in the middle of the chromatids, and in fact usually is not. The two paired shorter sections of chromatid on one side of the centromere are called the p-arms of the chromosome, while the longer portions on the opposite side of the centromere are called the q-arms.
Chromatids vs. Homologous Chromosomes
It is important for a full understanding of cellular genetics, and in particular cell division, to understand the difference between homologous chromosomes and chromatids. Homologous chromosomes are simply the two chromosomes you have with the same number, one from each parent. Your paternal chromosome 11 is the homolog of your maternal chromosome 11, and so on. They are not identical, any more than any two automobiles of the same year, make and model are identical except at the level of construction; they all have different wear levels, mileage totals, repair histories and so on.
Chromatids are two identical copies of a given chromosome. Thus, after chromosome replication but before cell division, the nucleus of each of your cells has two identical chromatids in each homologous but non-identical chromosome, for a total of four chromatids, in two identical sets, associated with every chromosome number.
Chromatids in Mitosis
When bacterial cells divide, the entire cell divides and makes two complete copies of itself that are identical to the parent bacteria and hence to each other. Bacterial cells lack nuclei and other membrane-bound cell structures, so this division merely requires the lone circular chromosome sitting in the cytoplasm to replicate before the cell splits neatly in half. This form of asexual reproduction is called binary fission, and because bacteria are single-celled organisms, fission is equivalent to reproducing the whole organism.
In eukaryotes, most cells of the body undergo a similar process when they divide, called mitosis. Because eukaryotic cells are more complex, containing more DNA arranged into multiple chromosomes and so on, mitosis is more elaborate than fission, even though the result is the same. At the start of mitosis (prophase), the chromosomes assume their compact form and start to migrate toward the middle of the cell, and two structures called centrioles move to opposite sides of the cell, along a line perpendicular to the one along which the cell ultimately divides. In metaphase, all 46 chromosomes line up along the dividing line, now called a metaphase plate, in no particular order but with one sister chromatid on each side of the plate. By this time, microtubules extend from the centrioles on either side of the plate to attach to the sister chromatids. In anaphase, the microtubules function as ropes and physically separate the chromatids at their centromeres. In telophase, both the nucleus of the cell and the cell itself complete their division, with new nuclear membranes and cell membranes closing off these new daughter cells in the proper places.
Because the chromosomes align along the metaphase plate in such a way as to ensure that one sister chromatid in each pair lies on each side of the dividing line, the DNA in the two daughter cells is precisely identical. These cells are used in growth, tissue repair and other maintenance functions, but not in reproduction of the whole organism.
Chromatids in Meiosis
Meiosis involves the formation of gametes, or germ cells. All eukaryotes reproduce sexually and hence make use of meiosis, including plants. Using humans as an example, the gametes are spermatocytes (in males) and oocytes (in females). Each gamete has only one copy of each of the 23 chromosomes. This is because the ideal fate of a gamete from one sex is to fuse with a gamete from the opposite sex, a process called fertilization. The resulting cell, called a zygote, would have 92 chromosomes if each gamete had the usual 46 chromosomes. It makes sense that gametes would not have both a mother's copy of a chromosomes and a father's copy, since gametes themselves are parental contributions to the next generation.
Gametes result from a unique division of specialized cells in the gonads (testes in males, ovaries in females). As with mitosis, all 46 chromosomes replicate and prepare to line up along a line through the middle of the cell. However, in the case of meiosis, homologous chromosomes line up next to each other so that the line of ultimate division runs between homologous chromosomes but not between replicated chromatids. The homologous chromosomes exchange some DNA (recombination), and the chromosome pairs sort themselves along the division line randomly and independently, meaning that a maternal homolog has as much chance of a paternal one of landing on the same side of the line for all 23 chromosome pairs. Thus when this cell divides, the daughter cells are not identical to either each other or the parent, but contain 23 chromosomes with two chromatids each. Owing to recombination, these are no longer sister chromatids. These daughter cells then undergo a mitotic division to yield four daughter cells, each with one chromatid for each of the 23 chromosomes. The male gametes are packaged into spermatozoa (sperm with "tails") while the female gametes become egg cells that are released from the ovary about once every 28 days in humans.