We explain what chromosomes are and how their structure is composed. Also, their main functions and types of chromosomes.
What are chromosomes?
Chromosomes are called highly organized structures inside biological cells composed of DNA and other proteins, and where most of an individual's genetic information resides. They have a defined X shape, which is perfectly observable during the stages of cell division or replication (meiosis or mitosis).
Each chromosome has a characteristic shape and size, and are found in pairs generally in identical numbers for all individuals of the same species. Depending on the number of chromosomes they have (chromosome load), cells can be diploid (2n) or haploid (1n). The chromosome number of the human species is 46 pairs.
the chromosomes were discovered in plant cells at the end of the 19th century by the scientists Karl Wilhelm von Nägeli (Switzerland) and Edouard Van Benenden (Belgium), independently, and their name comes precisely from the dyes used to observe them (from the Greek: chroma“color”, and soma“body”).
But until the 20th century, the role of chromosomes in inheritance and genetic transmission was not understood: we had to wait for Mendel's laws and the first research on DNA.
In the cells of eukaryotic beings (that is, those provided with a cell nucleus), chromosomes are made up of chromatin, the substance that makes up DNA, RNA and other proteins, some basic proteins called histones and others non-histones. All of this makes up nucleosomes, forming inactive groups of DNA that make up the chromosomes themselves.
See also: Mutation
Chromosome structure
the chromosomes have a double structure composed of two structures parallel to each other and joined by a centromere, called chromatids. In each of the “arms” of a chromatid, the genes are located, in identical position with respect to their counterpart (remember that they are X-shaped), in compartments called locus (loci in plural).
like chromosomes consist of a centromere that divides each chromatid into two arms: a short one (p arm) and a long one (q arm), depending on the location of the centromere we can speak of:
- Metacentric chromosomes. The centromere is almost in the middle of the structure, forming arms of very similar length.
- Submetacentric chromosomes. The centromere is displaced from the center, but not too much. This forms inaccurate and asymmetrical arms, clearly distinguishable.
- Acrocentric chromosomes. The centromere is at one end, forming long different arms.
On the other hand, eukaryotic chromosomes have telomeres at their ends: composed of regions of highly repetitive, non-coding DNA, which fulfill the function of providing structural stability to the entire chromosome.
In prokaryotic organisms (without a cell nucleus), however, chromosomes do not have telomeres, as they are circular in shape.
Chromosome function
The function of chromosomes could not be more important: are responsible for transmitting the genetic information contained in the DNA of the mother cell to the descendants allowing cellular replication and the growth of organisms, the replacement of old or damaged cells, and the creation of reproductive cells (as well as new individuals during sexual reproduction). These are biological structures that preserve the genetic content and prevent (as far as possible) it from being damaged or lost.
Types of chromosomes
As has already been seen, there are different chromosomes for eukaryotic and prokaryotic cells, differentiable in form and function.
- Prokaryotic chromosomes They have a single strand of DNA and are located within nucleoids dispersed in the cytoplasm of cells.
- Eukaryotic chromosomes Considerably larger, they have a double strand of linear DNA (in a double helix).
However, the chromosomes of eukaryotic living beings can also be distinguished according to their specific function in the constitution of the total genome of the individual, something that is extremely important when a new individual of the species is being created through sexual reproduction:
- Somatic chromosomes Also called autosomal, they give the individual its non-sexual characteristics, that is, those that define it in the rest of the non-reproductive aspects.
- Sex chromosomes Known as allosomes, they are the chromosomes that determine the sexual characteristics of the individual and therefore can be differentiated according to biological gender: men have a 23rd pair of XY type chromosomes, while women have type XX.
