Chromosomes

Definition – What are chromosomes?

The genetic material of a cell is stored in the form of DNA (deoxyribonucleic acid) and its bases (adenine, thymine, guanine and cytosine). In all eukaryotic cells (animals, plants, fungi) this is present in the cell nucleus in the form of chromosomes. A chromosome consists of a single, coherent DNA molecule, which is connected to certain proteins.

The name chromosome is derived from the Greek and could be translated roughly as “colour body”. This name comes from the fact that scientists succeeded very early in the history of cytology (1888) in staining it with special basic dyes and identifying it under the light microscope. However, they are only really visible at a certain point in the cell cycle, mitosis (meiosis in germ cells), when the chromosome is present in a particularly dense (condensed) form.

How are chromosomes structured?

If the entire DNA double helix of a cell, i.e. around 3.4 x 109 base pairs, were to be connected to each other, the result would be a length of over one metre. However, the total length of all chromosomes added together is only about 115 μm. This difference in length is explained by the very compact structure of the chromosomes, in which the DNA is wound or spiralled several times in a very specific way.

Histones, a special form of protein, play an important role in this process. In total there are 5 different histones: H1, H2A, H2B, H3 and H4. Two of the last four histones form a cylindrical structure, the octamer, around which the double helix winds about twice (= superhelix).

H1 attaches itself to this structure to stabilize it. This complex of DNA, octamer and H1 is called nucleosome. Several of these nucleosomes now lie “pearl-chain-like” at relatively short intervals (10-60 base pairs) one behind the other.

The sections between the chromosomes are called spacer DNA. The individual nucleosomes now come into contact again via H1, resulting in further spiralisation and thus also compression. The resulting strand in turn is in loops that are stabilized by a backbone of acidic non-histone proteins, also known as Hertones.

These loops are in turn present in spirals stabilized by proteins, which results in the final stage of compaction. However, this high degree of densification only occurs during cell division in mitosis. In this phase, the characteristic shape of the chromosomes, which is made up of two chromatids, can also be seen.

The place where these are connected is called centromere. It divides each metaphase chromosome into two short and two long arms, also called p and q arms. If the centromere is located approximately in the middle of the chromosome, it is called a metacentric chromosome; if it is located at one of the ends, it is called an acrocentric chromosome.

Those in between are called submetacentric chromosomes. These differences, which can already be seen under the light microscope, together with the length of the chromosomes, allow an initial division of the chromosomes. Telomeres are the ends of the chromosomes that contain repeating sequences (TTAGGG).

These do not carry any relevant information, but serve to prevent the loss of more relevant DNA sections. With every cell division, a part of the chromosome is lost due to the mechanism of DNA replication. The telomeres thus act in a way as a buffer, delaying the moment when the cell loses important information through division.

If the telomeres of a cell fall below a length of approximately 4,000 base pairs, programmed cell death (apoptosis) is initiated. This prevents the spread of defective genetic material in the organism. A few cells possess telomerases, i.e. enzymes that are able to extend the telomeres again.

In addition to stem cells, from which all other cells are derived, these are germ cells and certain cells of the immune system. Furthermore, telomerases are also found in cancer cells, which is why one speaks of immortalisation of a cell in this context. Chromatin is the entire content of a cell nucleus that can be stained with a basic stain.

Therefore, the term includes not only DNA but also certain proteins, e.g. histones and hertones (see structure), as well as certain RNA fragments (hn- and snRNA). Depending on the phase in the cell cycle or the genetic activity, this material is present in varying densities. The denser form is called heterochromatin.

For easier understanding, it could therefore be regarded as a “storage form” and here again a distinction is made between constitutive and facultative heterochromatin. Constitutive heterochromatin is the densest form, which is present in all phases of the cell cycle in its highest condensation stage. It makes up about 6.5% of the human genome and is mainly located near the centromeres and the ends of the chromosome arms (telomeres) in small proportions but also at other locations (mainly chromosome 1,9,16,19 and Y).

Furthermore, the majority of constitutive heterochromatin is located near the nuclear membrane, i.e. at the edges of the cell nucleus. Thus the space in the middle is reserved for the active chromatin, the euchromatin. Facultative heterochromatin is slightly less dense and can be activated and deactivated as required or depending on the stage of development.

A good example is the second X chromosome in female karyotypes. Since basically one X chromosome is sufficient for the survival of the cell, as it is ultimately also sufficient in males, one of the two is deactivated in the embryonic phase. the deactivated X chromosome is known as Barr’s body.

Only during cell division, during mitosis, does it condense completely, reaching its highest density in the metaphase. However, since the various genes are read off at different frequencies – after all, not every protein is required in the same quantity at all times – a distinction is also made here between active and non-active euchromatin. Haploid (gr.

haploos = single) means that all chromosomes of a cell are present individually, i.e. not in pairs (diploid) as is usually the case. This is the natural state of all egg and sperm cells in which the two identical chromatids are not separated for the time being during the first maturation division of meiosis, but rather all chromosome pairs are separated first. As a result, after the first division in humans, the daughter cells have only 23 instead of the usual 46 chromosomes, which corresponds to half of the haploid set of chromosomes.

However, since these daughter cells still have an identical copy of each chromosome consisting of 2 chromosomes, the second division is required, in which the two chromatids are separated from each other. A polytene chromosome is a chromosome consisting of many genetically identical chromatids. Since such chromosomes are already easily recognizable at low magnification, they are sometimes called giant chromosomes. A prerequisite for this is endoreplication, in which the chromosomes are multiplied several times within the cell nucleus without cell division taking place.