What are chromosomes?

Chromosomes are the highly organized structures inside biological cells, made up of DNA and other proteins, and where most of an individual’s genetic information resides. They have a definite 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 they are found in pairs, usually in identical numbers for all individuals of the same species. Depending on the number of chromosomes they have (chromosomal load), cells can be diploid (2n) or haploid (1n). The chromosomal number of the human species is 46 pairs.

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 tinctures used to observe them (from the Greek:  chroma, “color,” and soma, “body”).

But it was not until the 20th century that the role of chromosomes in heredity and genetic transmission was understood: we had to wait for Mendel’s laws and the first research on DNA.

In the cells of eukaryotic beings (that is, provided with a cell nucleus ), chromosomes are made up of chromatin, the substance that makes up DNA, RNA, and other proteins, some basic called histones, and others non-histones. All of this composes nucleosomes, forming inactive groups of DNA that make up the chromosomes themselves.

What are chromosomes, Structure, Function, Types
What are chromosomes, Structure, Function, Types

Structure of chromosomes

Genes are located in each of the “arms” of a chromatid.

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 the same position concerning their counterpart (remember that they are X-shaped), in compartments called loci  ( loci in the plural).

As chromosomes are made up of a centromere that divides each chromatid into two arms: a short one (parm) and a long one (q arm), depending on the location of the centromere, one 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 offset from the center, but not too much. That forms inaccurate and asymmetrical arms, clearly distinguishable.
  • Acrocentric chromosomes. The centromere is at one end, forming largely different arms. 

On the other hand, eukaryotic chromosomes have telomeres at their ends: composed of highly repetitive, non-coding DNA regions that fulfill the function of providing structural stability to the entire chromosome.

In prokaryotic organisms (without a cell nucleus), on the other hand, the chromosomes do not have telomeres, since they are circular.

Chromosome function

The function of chromosomes could not be more important: they are responsible for transmitting the genetic information contained in the DNA of the mother cell to the descendants, allowing cell replication and for 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 genetic content and prevent (where possible) it from being damaged or lost.

Chromosome Types

Prokaryotic chromosomes have a single strand of DNA.

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 DNA strand and are located within the nucleoids scattered in the cytoplasm of cells.
  • Eukaryotic chromosomes. Considerably larger, they have a linear double strand of DNA (double helix).

However, the chromosomes of eukaryotic living beings can also be distinguished according to their specific role 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 autosomally, they give the individual his non-sexual characteristics, that is, those that define him 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: males have 23 pairs of type XY chromosomes, while females have type XX.
What are chromosomes, Structure, Function, Types
What are chromosomes, Structure, Function, Types

What are chromosomes?, Structure, Function, Types