What functions do the chromosomes have?
The chromosome, as the organizational unit of our genetic material, serves primarily to ensure an even distribution of the duplicated genetic material to the daughter cells during cell division. For this purpose, it is worthwhile to take a closer look at the mechanisms of cell division or the cell cycle: The cell spends the majority of the cell cycle in interphase, i.e. the entire period in which the cell is not in the process of dividing. This in turn is divided into G1, S and G2 phase.
The G1 phase (G as in gap) follows immediately after cell division. Here the cell increases in size again and carries out general metabolic functions. From here it can also change into the G0 phase.
This means that it changes into a stage that is no longer capable of division and normally also undergoes major changes in order to fulfil a very specific function (cell differentiation). In order to fulfil these tasks, certain genes are read more intensively, while others are read less or not at all. If a section of DNA is not needed for a long time, it is often located in those parts of the chromosomes that have been densely packed for a long time (see chromatin).
On the one hand, this saves space, but is also an additional protection against accidental reading, in addition to the other mechanisms of gene regulation. However, it has also been observed that under very specific conditions differentiated cells from the G0 phase can re-enter the cycle. The G1 phase is followed by the S phase, the phase in which new DNA is synthesized (DNA replication).
Here, all DNA must be present in its loosest form, i.e. all chromosomes are completely unspiraled (see structure). At the end of the synthesis phase, the entire genetic material is present twice in the cell. However, since the copy is still attached to the original chromosome via the centromere (see structure), one does not speak of a duplication of chromosomes.
Each chromosome now consists of two chromatids instead of one, so that it can later take on the characteristic X-shape in mitosis (X-shape applies strictly speaking only to metacentric chromosomes). In the subsequent G2 phase, the immediate preparation for cell division takes place. This also includes a detailed check for replication errors and strand breaks, which can be repaired if necessary.
There are basically two types of cell division: mitosis and meiosis. All cells of an organism, with the exception of germ cells, are created by mitosis, whose sole purpose is the formation of two genetically identical daughter cells. Meiosis, on the other hand, has the very purpose of producing genetically different cells: In a first step, the chromosomes that correspond to each other (homologous) but are not identical are divided.
Only in the next step are the chromosomes, which consist of two identical chromatids, separated and distributed to two daughter cells each, so that in the end four germ cells, each with a different genetic material, are created from a precursor cell. The shape and structure of the chromosomes are essential for both mechanisms: Special “protein threads”, the so-called spindle apparatus, attach to the highly condensed chromosomes and pull the chromosomes in a finely regulated process from the middle plane (equatorial plane) to the opposite poles of the cell in order to ensure an even distribution. Even small changes in the microstructure of the chromosomes can have serious consequences here.
In all mammals, the ratio of the sex chromosomes X and Y also determines the sex of the offspring. Basically, it only depends on whether the sperm, which unites with the egg cell, carries an X or a Y chromosome. Since both forms of sperm are always formed to exactly the same extent, the probability is always equal for both sexes.
This random system therefore guarantees a more even gender distribution than would be the case, for example, with environmental factors such as temperature. It is now known that the inheritance of characteristics occurs via genes, which are stored in the form of DNA within the cells. These in turn are divided into 46 chromosomes, to which the 25000-30000 human genes are distributed.
Besides the trait itself, which is called phenotype, there is also the genetic equivalent, which is called genotype. The place where a gene is located on a chromosome is called locus. Since humans have each chromosome twice, each gene also occurs twice.
The only exception to this are the X chromosomal genes in men, as the Y chromosome carries only a fraction of the genetic information found on the X chromosome. Different genes located on the same locus are called alleles. Often there are more than two different alleles on one locus.
This is called polymorphism. Such an allele can simply be a harmless variant (standard variant), but also pathological mutations which can be the trigger for a hereditary disease. If the mutation of a single gene is sufficient to change the phenotype, this is called monogenic or Mendelian inheritance.
However, many hereditary traits are inherited through several interacting genes and are therefore much more difficult to study. Since in a Mendelian inheritance, the mother and father each pass on one of their two genes to the child, there are always four possible combinations in the next generation, although these can also be the same in relation to one trait. If both alleles of an individual have the same effect on the phenotype, the individual is homozygous with respect to this trait and the trait is therefore fully developed.
Heterozygotes have two different alleles that can interact with each other in different ways: If one allele is dominant over another, it completely suppresses the expression of the latter and the dominant trait becomes visible in the phenotype. The suppressed allele is called recessive. In a codominant inheritance, both alleles can express themselves uninfluenced by each other, whereas in an intermediate inheritance a mixture of both traits is present. A good example of this is the AB0 blood group system, in which A and B are codominant among each other but 0 is dominant over each other.