The term chromosome comes from the Greek words for color (chroma) and body (soma), due to the staining of these structures with colorful dyes in research. Chromosomes are thread-like structures found in the nucleus of cells. They are comprised of a single molecule of DNA wrapped around spool-like proteins called histones. The complex of DNA and proteins is called chromatin. The packaging of DNA involves specialized proteins that bind and fold DNA into a series of coils and loops that allow DNA molecules to fit inside cells. If the DNA inside each human cell were stretched out, it would measure about 2 meters in length, yet chromosomal DNA fits inside cells that have a diameter of about 6 μm.
The genomes of eukaryotes are arranged on linear chromosomes so each gene is localized to a specific chromosome and region on that chromosome. Chromosomes are duplicated when cells divide and are passed on from parents to offspring. The human genome has approximated 3.2 x 109 nucleotide bases distributed over 24 different chromosomes. Each human cell contains two copies of each chromosome, one inherited from each parent, making 46 chromosomes. twenty-two pairs are common to both males and females. In addition, females carry two X chromosomes and males carry one X and one Y chromosome. Bacteria, or prokaryotes, carry their genes on a single DNA molecule that is usually circular. The bacterial chromosome does not have the same structure as eukaryotic chromosomes.
A karyotype is a collection of one individual’s chromosomes and also refers to the laboratory technique that produces the image of a chromosome set. A karyotype is used to look for abnormal numbers or structures of chromosomes, such as an extra chromosome 21 in Down syndrome. Karyotypes are prepared from mitotic or dividing cells that are arrested at the metaphase or prometaphase part of the cell cycle when chromosomes are in the most condensed state. Stains are used to bind DNA and generate characteristic banding patterns. Individual chromosomes are arranged and numbered in a standardized format, which makes it easier to identify abnormalities.
The above image is a human male karyotype from the National Human Genome Institute (NIH) website.
The naming of sex chromosomes X and Y predates the numbering system of the other chromosomes. In 1891, German biologist Hermann Henking studied the firebug sperm nuclei and noted an additional large chromatin element, which he identified as X. In the early 1900s, insect biologists Nettie Stevens and Edmund Beecher Wilson figured out how unequal-sized chromosomes relate to sex differences in insects, and in 1909 Wilson referred to the large chromosome as X and the smaller one as Y. By 1917, scientists were using X and Y to identify human sex chromosomes. In 1960, an international panel concluded that there are 22 human autosomes that should be numbered in descending order of length, and that the sex chromosomes should continue to be referred to as X and Y.
The constricted region of chromosomes is called the centromere, which functions as an attachment site to keep chromosomes aligned when they are copied during cell division. Chromosomes that are duplicated and still attached at the centromere have an X shape. The duplicated chromosomes are called sister chromatids. Attachment of kinetochore microtubules at the centromeres pull the sister chromatids to opposite poles of the dividing cell.
The above image shows duplicated chromosomes in the X shape during cell division (mitosis). (Image attribution: LadyofHats, Public domain, via Wikimedia Commons)
Telomeres are stretches of DNA that are repetitive in their DNA sequence. Telomeres protect the ends of chromosomes. Many cell types lose some of their telomeres each time the cell divides. Once the telomere is gone, the cell cannot replicate and dies. Other cell types, such as white blood cells, have the capacity to divide frequently and have a special enzyme called telomerase that elongates telomeres and prevents the loss of telomeres.
During cell division, chromosomes condense into distinct bodies. During the non-dividing state of interphase, chromosomes are less uniform, fill up the nuclear space, and are difficult to distinguish. During interphase, chromosomes each occupy a spatially limited, roughly elliptical domain called a chromosome territory. Chromosome territories are arranged radially around the nucleus in a manner that is dependent on cell-type and tissue-type. Gene-rich chromosomes occupy interior positions, and gene-poor chromosomes tend to be located in the periphery. Genes are able to relocate from the periphery towards the interior when transcriptionally active (turned on) or move in the other direction when switched off.
The above figure from Rada-Iglesias (2018) depicts chromosomes occupying distinct territories in the nucleus. Each territory is divided into subdomains that are transcriptionally active, transcriptionally inert (constitutive heterochromatin), and transcriptionally repressed (polycomb compartment).
