Chromosomes are strands of deoxyribonucleic acid containing so-called histones (basic proteins within a nucleus) and other proteins; the mixture of DNA, histones, and other proteins is also called chromatin. They contain genes and their specific genetic information. Histones are basic proteins that not only serve to package DNA, but are also essential for the expression of some genes encoded on them. The occurrence of chromosomes is limited to eukaryotes (living organisms with a cell nucleus), where they are located in the cell nucleus. Living organisms such as bacteria, which do not have a nucleus, have circular DNA molecules (plasmids), which in eukaryotes is similar to circular DNA within mitochondria or chloroplasts, as well as a “chromosome-like” circular bacterial chromosome. During the process of cell division, most of a chromosome is a long double helix molecule. However, during chromosome replication, an X-like form called a metaphase chromosome occurs. The number of chromosomes varies greatly in individual animals or plants. The adder’s tongue plant, a genus of ferns in the adder’s tongue family, has species with up to 1260 chromosomes, while the roundworm parasite has only 2 chromosomes. In the human body, the number of chromosomes is 46, with 23 inherited from each parent. In sexual reproduction, the two homologous sets of chromosomes in a cell nucleus result from the union of one maternal and one paternal haploid chromosome set at fertilization. Chromosomes are divided into autosomes (body chromosomes) and gonosomes (sex chromosomes). The chromosome pairs 1 to 22 are autosomes, the 23rd chromosome pair is the sex chromosomes (XY in male and XX in female). The order of the autosomes is based on the number of base pairs: chromosome 1 has the most base pairs and after that there is a steady decrease of base pairs from chromosome to chromosome until the last autosome 22. Hereditary disorders within autosomes are called autosomal hereditary disorders. In the following some explanations about the sex chromosome or gonosome (sometimes also called heterochromosome, heterosome or allosome): The male has one X chromosome, which he inherits from the mother and one Y chromosome, the female has two X chromosomes, one inherited from the father and the other from the mother. Chromosomal inheritances within gonosomes are differentiated into X-chromosomal and Y-chromosomal inheritances, respectively. The X chromosome has more than twice as many base pairs as the Y chromosome. The X- and Y- chromosome have partly very similar sequences, which is due to the evolutionary development of the Y-chromosome from the X-chromosome. There are also the so-called pseudoautosomal regions, which are areas at the ends of the gonosomes where, as in the case of the autosomes, several genes are duplicated in both sexes. There, X and Y chromosome recombination is possible during meiosis. There can be changes in chromosomes, which occur when there are double digit breaks in a DNA double helix, usually resulting in DNA repair. If during DNA repair the wrong DNA strands are joined together, a chromosomal abnormality, also called chromosomal mutation, occurs. These are clearly microscopically visible structural changes of the chromosome (= structural chromosomal aberrations), e.g. a deletion (loss of one or more base pair(s) of a DNA sequence), insertion (new gain of one or more base pair(s) of a DNA sequence), duplication (doubling of a specific sequence) or also a translocation (change in the location of a chromosome segment). Chromosomal mutations can cause hereditary diseases, which are chromosomal mutations during meiosis, or tumor diseases. To be distinguished from chromosomal mutations are numerical chromosomal anomalies, which are clearly microscopically visible numerical changes of the chromosome, i.e. a much larger change of the genetic material. They result from defective meiosis. Examples of numerical chromosomal abnormalities are aneuploidy – e.g. monosomy (the unique presence of a chromosome that actually exists twice) or trisomy and polyploidy (e.g. triploidy).An example of a monosomy is Turner syndrome (synonym: Ullrich-Turner syndrome): girls/women with this peculiarity have only one functional X chromosome instead of the usual two. A trisomy is an extra copy of a chromosome that is normally present in duplicate. More common trisomies are Klinefelter syndrome (XXY) and trisomy 21 (Down syndrome; chromosome 21 supernumerary). In triploidy, three complete sets of haploid chromosomes (69 chromosomes instead of 46) are present. Triploidy usually leads to an abortion (miscarriage). Here, the tendency is that the probability of miscarriage is lower for trisomies that have chromosomes with relatively few base pairs. Note: Diploidy refers to the presence of two homologous sets of chromosomes in one nucleus. This results in sexual reproduction from the union of one maternal and one paternal haploid chromosome set at fertilization. Typically, the chromosome sets of oocytes (eggs) and spermatozoa (sperm) are haploid, i.e. here the genome (genetic material) is present only once. This is the only way to ensure an “even” division during fertilization, resulting in a diploid zygote.
